Book - Buchanan's Manual of Anatomy including Embryology 15

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Frazer JE. Buchanan's Manual of Anatomy, including Embryology. (1937) 6th Edition. Bailliere, Tindall And Cox, London.

Buchanan's Manual of Anatomy: I. Terminology and Relative Positions | II. General Embryology | III. Osteology | IV. Bones of Trunk | V. Bones of Head | VI. Bones of Upper Limb | VII. Bones of Lower Limb | VIII. Joints | IX. The Upper Limb | X. Lower Limb | XI. The Abdomen | XII. The Thorax | XIII. Development of Vascular Systems | XIV. The Head and Neck | XV. The Nervous System | XVI. The Eye | XVII. The Ear | Glossary
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Talk:Book - Buchanan's Manual of Anatomy including Embryology 15 Jump to:navigation, search CHAPTER XV THE NERVOUS SYSTEM[edit] The nervous system is arranged in two main divisions, cerebro-spina and autonomic; this last contains the sympathetic and the parasym pathetic.

The sympathetic system consists of two gangliated cords situate* on either side of the vertebral column, and three main prevertebra plexuses: the cardiac situated in the thorax; the epigastric or solar and the hypogastric plexus, the latter two being situated in th abdomen; subsidiary plexuses are associated with these.

The parasympathetic system includes cranial and sacral outflows leaving the cerebro-spinal axis through certain nerves without joininj the sympathetic cords.

The cerebro-spinal nervous sytem or axis consists of the encephaloi and the spinal cord, the former being situated within the crania cavity, and the latter within the spinal canal. The continuity betwee; these two divisions is established through the foramen magnum.

The cerebro-spinal axis is central in position, and is connecte* with the various parts of the body by the cranial and spinal nerves It is composed of two kinds of nervous matter, white and grey. Th white matter consists chiefly of nerve-fibres, and the grey matter c nerve-cells, with their axis-cylinder processes or axons and dendrites the pervading supporting tissue in each case being called neuroglk In the spinal cord the white matter is disposed externally, whilst th grey matter is situated in the interior. In the brain there is the sam arrangement of grey matter in the centre, surrounded by white mattei but a third and more modern layer of cortical grey matter has bee added to the surface of the white, a layer which is unrepresented i the spinal cord.

The cerebro-spinal axis is surrounded by three membranes, c meninges , which, from without inwards, are named the dura matei arachnoid membrane, and pia mater.


THE SPINAL CORD.

Membranes of the Spinal Cord. —The membranes are three: th dura mater, the arachnoid membrane, and the pia mater.

Dura Mater. —This is the most external covering of the cord. I forms a dense fibrous tube, known as the theca, which extends fror the margin of the foramen magnum of the occipital bone to the lowe level of the second sacral vertebra. Inferiorly, where it has becom

1410 1


THE NERVOUS SYSTEM


1411


Frontal Lobe


Temporal Lobe

/. Basilar Artery Vertebral Arteries


[■six


j -— Superior Cervical Sympathetic / Ganglion


1>. Cervical Plexus

■j -Vertebral Artery

i-Middle Cervical Ganglion

Brachial Plexus ist Thoracic Ganglion


4th Intercostal Nerve Anterior Spinal Artery


U_ Thoracic Sympathetic Cord




. Spinal Ganglion


Subcostal Nerve


L-. Lumbar Sympathetic Cord


Lumbar Plexus


Pelvic Sympathetic Cord


^— Sacral Plexus


Ganglion Impar


Eig. 859.—The Cerebrospinal and Sympathetic Systems (Anterior

View) (Hirschfeld and Leveille).



















1412


A MANUAL OF ANATOMY


tapering behind the second sacral vertebra, the theca is perforate by the filum terminale, and from this level the spinal dura mater i prolonged downwards around the filum terminale, with which it blend: Finally, it is attached, along with the filum terminale, to the bac of the first coccygeal vertebra, where it is incorporated with th periosteum. In this situation the spinal dura mater is firmly fixed.

The theca surrounds the spinal cord very loosely, and it is separate from the wall of the spinal canal by an interval, called the extra-dur; space, which is occupied by venous plexuses and loose areolar tissue.

Opposite the intervertebral foramina of each side the theca has series of openings, which are arranged in two parallel rows. Th openings constituting each pair are placed side by side, but are distinc from each other, and they transmit the ventral or anterior and dors; or posterior roots of the spinal nerves. Each of these roots, as it make its exit, receives a tubular sheath from the margin of the corresponds thecal opening, and these sheaths remain distinct as far as the spin; ganglion of the dorsal root. After this the neighbouring sheath form one which blends with the sheath of the corresponding spin; nerve.

The spinal dura mater is maintained in position by several cor nections. (i) Superiorly it is fixed to the margin of the forame magnum of the occipital bone. (2) Opposite the body of the ax: it is firmly attached anteriorly ’ to the posterior occipito-axial ligamen (3) Below the level of the axis it is loosely connected anteriorly wit the posterior longitudinal ligament of the bodies of the vertebrae b fibrous bands. (4) Laterally it is connected with the sheaths of th spinal nerves by means of the tubular sheaths which it gives to th ventral and dorsal nerve-roots. (5) Inferiorly it blends with th periosteum over the back of the first coccygeal vertebra through th filum terminale. Posteriorly it is quite free from connections.

The spinal dura mater differs from the cranial dura mater in th following respects: (1) It is destitute of an outer or periosteal layei (2) it does not send septa into the spinal cord; and (3) it does nc contain venous sinuses.

Blood-supply of Spinal Dura Mater.—The arteries are derive from (1) the spinal branches of the vertebral, intercostal, and lumba arteries; and (2) the lateral sacral arteries, which are branches of th internal iliac artery.

Nerve-supply.—The nerves are partly spinal and partly sympatheth

Lymphatic Vessels.—There are no lymphatic vessels, their plac being taken by perivascular lymph-spaces in connection with th arteries.

Structure. —The spinal dura mater consists of fibrous tissue and some elast: tissue disposed in parallel longitudinal bundles. Its internal and extern; surfaces are covered by endothelial cells.

Subdural Space.—Between the spinal dura mater and the arachnoi there is a narrow cleft-like interval, which is known as the subduri space. It contains a small amount of fluid, and communicates freel


THE NERVOUS SYSTEM


T 4 X 3


Dura Mater (Theca)


—-Arachnoid


h the lymph-spaces or clefts in the sheaths of the spinal nerves, has, however, no communication with the subarachnoid space. Spinal Arachnoid Membrane.—This is a delicate transparent mbrane which loosely surrounds the spinal cord between the theca ernally and the pia mater internally. It is separated from the

ca by the subdural space, and from the pia mater by the sub.chnoid space. Superiorly it is continuous with the cranial arachd, and inferiorly it encloses the cauda equina. On either side it ms sheaths for the processes of the ligamentum denticulatum as as the inner surface of » theca. The ventral and

sal roots of the spinal ves also receive sheaths m it, which accompany un through the openings the theca, but soon cease.

It is of practical importce to remember that the ichnoid membrane usually Is at the lower level of the iond sacral vertebra, and ver extends lower than the ird.

Subarachnoid Space (Ca- , m Subarachnoidale).—This l ice, which is wide, is uated, as stated, between s arachnoid and pia mater, contains cerebro-spinal

id, and its dorsal part is

ntinuous superiorly with Fig - 86 °- Portion of the Spinal Cord,

- rerebello rnednllarv cis- showing the Membranes, Ligamenta o LEiEDEiio-iiiEuuiiaiy Lis Denticulata, and Roots of the Spinal

"na of the cranial sub- Nerves.

ichnoid space, which com micates with the fourth ventricle by the ‘ foramen of Magendie,’ e median aperture of the roof.

The subarachnoid space is partially divided into two compartmts, ventral and dorsal, by ligamenta denticulata, which form inmplete lateral septa. The ventral roots of the spinal nerves traverse e ventral compartment, and the dorsal roots the dorsal compartsnt. The dorsal compartment is partially subdivided into two •rtions, right and left, by means of a third incomplete septum, called e posterior septum. This partition extends from the pia mater as crosses the dorsal median fissure of the spinal cord to the dorsal -rt of the arachnoid at the median line. All the compartments of


Ligamentum Denticulatum

-Anterior Nerve-Root

Posterior Nerve-Root Pia Mater


_ Spinal Cord


Anterior Nerve-Root (cut)


e subarachnoid space communicate freely with each other.

The subarachnoid space has no communication with the subdura ace.













1414


A MANUAL OF ANATOMY


Structure of the Arachnoid Membrane. —The arachnoid consists of fine fibrou tissue arranged in interlacing bundles, the intervals between these bundle being occupied by delicate cellular membranes. Several such layers, intimatel blended together, form the membrane.

Beneath the arachnoid, and constituting a part of it, there is a reticulum c subarachnoid trabeculce. These trabeculae consist, as in the case of the arachnoi proper, of fine fibrous tissue, but the intertrabecular spaces, instead of bein occupied by cellular membranes, contain cerebro-spinal fluid. The trabecula reticulum connects the arachnoid with the subjacent pia mater, and varie greatly in density in different parts.


Spinal Pia Mater.—This is the deepest membrane of the spins cord. It is definitely fibrous and very vascular, and closely invest

the cord. Superiorly it is con


A] Dura Mater (Theca)


. Linea Splendens


Ligamentum Denticulatuir.


Pia Mater


J_L \nterior Nerve-Root


Fig. 86i.—A Portion of the Spinal Cord (Anterior View).

The theca has been laid open, and the arachnoid membrane removed.


tinuous with the cranial pi mater, and inferiorly it is pro longed from the conus medul laris over the upper half of th intrathecal part of the filur terminate. On either side i forms tubular sheaths for th ventral and dorsal roots of th spinal nerves, which blend wit] the sheaths of the nerves.

Along the course of th ventral median fissure of th cord it sends a vascular fob into that fissure. Along th course of the dorsal medial fissure, over which it passes the neuroglial septum occupy ing that fissure is attached t< it. From the deep orifice o the pia mater several septa ar


prolonged into the cord, which carry with them portions of the glia sheath.

The pia mater is separated from the arachnoid by the subarach noid space, and opposite the dorsal median fissure of the cord it i connected with the dorsal part of the arachnoid by the posterio subarachnoid septum.

Blood-supply.—The pia mater derives its arteries from the anterio and posterior spinal arteries, and the neural branches of the latera spinal arteries.

Nerve-supply.—The nerves are derived from the sympathetic system


Structure. —The spinal pia mater consists of two layers—outer and innei The outer layer consists of fibrous tissue, which is disposed for the most part i] parallel longitudinal bundles. The inner layer consists of areolar tissue containing a great many bloodvessels, and its outer and inner surfaces are covered by endcj thelial cells. Between the two layers there are narrow cleft-like lymphati spaces, which communicate with the subarachnoid space, and with lymphati clefts around the arteries of the pia mater.









THE NERVOUS SYSTEM


1415


The spinal pia mater differs from the cranial pia mater in being icker and more adherent to the nervous matter. The greater thick:ss is due to the presence of the outer layer, the cranial pia mater presenting the inner layer of the spinal pia mater.

Linea Splendens.—The pia mater at times presents a glistening >pearance immediately in front of the ventral median fissure. This ea is known as the linea splendens, and it extends along the entire ngth of the cord and along the conus medullaris on to the filum rminale.

Ligamentum denticulatum is a band of pia mater which extends ong the spinal cord on each side opposite the corresponding lateral >lumn. It lies between the ventral and dorsal roots of the spinal irves, and extends from the margin of the foramen magnum to the wer end of the cord. It lies within the subarachnoid space, and


Dura Mater


Fig. 862._Diagram of a Transverse Section of the Spinal Cord

and its Membranes.


A.M.F. Anterior Median Fissure, with Process of Pia Mater P.M.F. Posterior Median Sulcus A.C. Anterior Column L.C. Lateral Column P.C. Posterior Column


A.R. Anterior Nerve-Root P.R. Posterior Nerve-Root G. Spinal Ganglion S.P. Spinal Nerve A.P.D. Anterior Primary Ramus P.P.D. Posterior Primary Ramus


irtially divides that space into two compartments—ventral and orsal

Internally it forms an uninterrupted band which is attached to le pia mater along the lateral column of the cord. Laterally it is enticulated. The denticulations (about twenty-one m number) carry ith them sheaths from the arachnoid, and their pointed outer ends re attached to the inner surface of the theca in the intervals between ich pair of openings for the exit of the ventral and dorsal nerve-roots, he topmost denticulation is at the foramen magnum, lying between le eleventh nerve and the vertebral artery; the lowest is e ween tie last thoracic and first lumbar nerves, at the first lumbar vertebral

iygl

The two ligamenta denticulata act as lateral supports to the spinal °rd. ,. ,

Structure— Each ligamentum denticulatum consists of fibrous tissue, which

continuous with the outer layer of the pia mater.




1416 A MANUAL OF ANATOMY

External Characters of the Spinal Cord. — The spinal cord, o medulla spinalis, which is somewhat cylindrical, is that division o the cerebro-spinal axis which is situated within the spinal canal. I extends from the lower margin of the foramen magnum in the occipita bone to about the level of the disc between the bodies of the firs and second lumbar vertebrae, and it is about 18 inches in length Superiorly it is continuous with the medulla oblongata, and inferiorh it terminates in a tapering portion, called the conus medullaris. Fron the lower end of this cone a slender, glistening thread, called tb

filum terminale, about io inche long, is continued downward between the bundles of lumbar sacral, and coccygeal nerve of either side, which constitut the cauda equina, to be attache< to the back of the first coccy geal segment.

The spinal cord is of smalle dimensions than the spina canal, and is therefore relieve* from pressure during the ordi nary movements of the verte bral column. It is surrounde* by the three membranes alread; described—the dura mater, th archnoid membrane, and th pia mater. Within its theca o dura mater the cord is sus pended by means of the liga menta denticulata, and th nerve - roots as they emerg through the openings in th theca.

The spinal cord varies ii shape in different regions. L the cervical region, as seen ii transverse section, it is trans versely oval, and is slights flattened from before backwards. In the thoracic region it is almos circular, but the transverse diameter exceeds the antero-posterior In the lumbar region it is still more circular than in the thoraci region.

The cord has two swellings, which are known as the cervical an* lumbar enlargements, and are associated with the numerous larg< nerve-trunks destined for the upper and lower limbs. The cervica enlargement extends from near the upper end of the cord to the secon* thoracic vertebra, and its breadth is greatest opposite the sixth cervica vertebra. The lumbar enlargement, which is less conspicuous thai


Fig. 863. —Lumbar and Sacral Portions of Spinal Canal, showing Lateral View of Conus Medullaris, Filum Terminale, and Theca (Testut).





THE NERVOUS SYSTEM


1417


he cervical, extends from the level of the tenth thoracic vertebra o the conus medullaris, and its breadth is greatest opposite the twelfth horacic vertebra.

Filum Terminale.—This delicate glistening thread lies in the nedian line between the lumbar, sacral, and coccygeal nerves of either side, which constitute the cauda equina, and it extends from the apex )f the conus medullaris to the back of the first coccygeal segment.

[t is about 10 inches in length. As low as the back of the body of

he second or third sacral segment it is situated within the theca, Dut at that level it pierces the theca, from which it receives an investment, md then passes to be attached to the back of the first coccygeal segment, where it blends with the periosteum. The intrathecal portion is known as the filum terminale internum, and the extrathecal portion as the filum terminale externum.

Structure. —The filum terminale internum in its upper half consists of pia mater prolonged from the conus medullaris of the spinal cord. This encloses grey matter, within which, over about the upper third, there is a continuation of the central canal of the cord. The lower half consists chiefly of connective tissue. The filum terminale externum is a mere fibrous filament invested by a prolongation of the theca which blends with it. It is also composed of pia mater prolonged downwards from the conus medullaris, and reinforced by fibres derived from the lower portions of the ligamenta denticulata and linea splendens.

Its lower part is purely fibrous. Fig . 864 ._the terminal

Cauda EQuina.—This is situated witmn Part of the Spinal Cord,

the lower part of the theca. It consists and the Cauda Equina of the roots of the lumbar, sacral, and ~

coccygeal nerves of each side, which are arranged m the form o a leash, and the filum terminale lies in the median line between the two nerve-leashes. On account of the high origins of the individual nerves, relatively to the positions of the intervertebral foramina through which they pass, the direction of the nerves is almost vertical until they reach the level of their respective foramina o exi .

Fissures of the Spinal Cord.—The spinal cord, which is somewhat flattened in front and behind, is incompletely divided into two symmetrical halves by two median formations, anterior and posterior. The anterior median Assure extends into the cord for one-third of its thickness from before backwards, and it contains a o c o le pia



_ Linea Splendens


Cauda Equina Conus Medullaris


If


_Filum Terminale















1418


A MANUAL OF ANATOMY


mater. At the bottom of the fissure the transverse band of nervefibres, called the anterior white commissure , crosses between the two halves of the cord. The posterior median septum is not an actual fissure like the anterior, and does not contain a fold of the pia mater, but is a septum of neuroglia, which extends into the cord for about half its thickness from before backwards. The posterior grey commissure lies at the bottom of the septum. The anterior median fissure is a definite depression, but it is not so deep as the posterior septum.

The posterior septum is marked on the surface by a median sulcus.

Each half of the cord presents a groove along the line of entrance of the fasciculi of the posterior nerve-roots, called the postero-lateral sulcus, but there is no similar groove along the line of emergence of the fasciculi of the anterior nerve-roots, these being spread over an area of some breadth. By means of the postero-lateral sulcus on the one hand, and the area corresponding to the emergence of the fasciculi of the anterior nerve-roots on the other, each half of the spinal cord is divided superficially into three white columns—anterior, lateral, and posterior.

The anterior white column is situated between the anterior median fissure and the most lateral fasciculi of the anterior nerve-roots; the lateral white column is the area between the most lateral fasciculi of the anterior nerve-roots and the postero-lateral sulcus; and the posterior white column lies between the postero-lateral sulcus and the posterior median sulcus. Practically the anterior column represents the region in front of the anterior nerve-roots, the lateral column the region between the anterior and posterior nerve-roots, and the posterior column the region behind the posterior nerve-roots. According to some authorities there are only two columns—namely, antero-lateral and posterior, the former extending from the anterior median fissure to the postero-lateral sulcus, and representing the combined anterior and lateral columns.

In the cervical region the surface of each posterior white column presents a slight groove which is situated nearer the posterior median sulcus than the postero-lateral sulcus. This groove is called the posterior intermediate or paramedian furrow . It contains a septum of pia mater, and in this manner the posterior column of the cord is marked off into two tracts. The medial and smaller tract is called the fasciculus gracilis (postero-median column of Goll), and the lateral and larger is called the fasciculus cuneatus (Burdach’s column, posterolateral column). These two columns extend throughout the cord, but it is only above the level of the mid-thoracic region that they are separated from each other by a septum of pia mater, known as the posterior intermediate septum.

Origin of the Spinal Nerves.—There are thirty-one pairs of spinal nerves, which arise from the sides of the spinal cord. They are arranged in five groups on either side as follows: cervical, eight in number; thoracic, twelve ; lumbar, five ; sacral, five ; and coccygeal, one. Each spinal nerve is attached superficially to the cord by two roots, anterior


THE NERVOUS SYSTEM


1419


nd posterior, the posterior root being the larger of the two. The ortion of the cord from which each pair of spinal nerves arise is spoken f as a segment of the cord. Each root is ensheathed by tubular probations of the coverings of the cord—namely, the pia mater, arachnid, and dura mater, in this order from within outwards—and these heaths ultimately blend with the perineurium. The roots are separated rom each other by the lateral column of the cord and the ligamentum enticulatum, and they pass through separate openings in the theca of

ura mater. The anterior roots are composed of efferent or motor fibres, and their asciculi emerge from the cord in an irregular manner, being spread ver an area corresponding in breadth to the caput of the anterior

A

2


?IG. 865.—Two Segments of the Spinal Cord, showing the Attachments of the Anterior and Posterior Nerve-Roots, and the Spinal Ganglia.

A, superior view; B, anterior view.

1, 1. Anterior Median Fissure 5 > 5 - Spinal Ganglion

2, 2. Posterior Median Sulcus 6, 6. Spinal Nerve

a, a. Anterior or Motor Nerve-Root 7 , 7 - Anterior Primary Ramus

4, 4. Posterior or Sensory Nerve-Root 8, 8. Posterior Primary Ramus

9, 9. Medial and Lateral Branches of Posterior Primary Ramus

horn of the grey matter in the interior. The posterior roots are composed of afferent or sensory fibres, and their fasciculi enter the cord in a straight line along the course of the postero-lateral sulcus. Each posterior root presents an oval swelling, called the spinal ganglion. These ganglia are for the most part situated in the intervertebral foramina, and immediately beyond each ganglion the anterior and posterior roots unite to form a spinal nerve, which is necessarily a mixed nerve, inasmuch as it is composed of afferent and efferent fibres.

Each spinal nerve breaks up into an anterior and a posterior primary yamus

The upper cervical nerve-roots are short, and pass almost horizontally outwards. The succeeding nerve-roots, however, gradually increase in length, and incline downwards as they pass outwards,











1420


A MANUAL OF ANATOMY


This downward inclination goes on increasing until it becomes almost vertical in the case of the lumbar, sacral, and coccygeal nerves, which constitute the cauda equina. From this disposition it follows that in the majority of cases the superficial origins of the spinal nerves are on a higher level than the intervertebral foramina through which they emerge from the spinal canal.

Relation of the Spines of Vertebrae to the Bodies and to the Origins of the Nerves. —In the case of the cervical and the eleventh and twelfth thoracic vertebra the extremities of the spinous processes correspond to the lower margins of the bodies of the respective vertebrae. In the case of the thoracic vertebra, from the first to the tenth inclusive, the extremity of each spinous process corresponds to some part of the body immediately below. In the case of the lumbar vertebrae the extremity of each spinous process corresponds to the centre of the body of its own vertebra.

Each cervical spinous process is nearly opposite the lower fasciculi of the roots of the nerve below. The spinous process of the seventh cervical vertebra (vertebra prominens) is opposite the roots of the first thoracic nerve. From the third to the tenth thoracic vertebrae the spinous processes correspond to the second root below. The eleventh thoracic spine corresponds to the first and second lumbar nerves. The twelfth thoracic spine corresponds to the third, fourth, and fifth lumbar nerves. The first lumbar spine corresponds to the first, second, and third sacral nerves. (Gowers, from an original investigation.)


Mode of distinguishing the Anterior and Posterior Surfaces of the Spinal Cord.—These surfaces may be recognized by attending to the following points:


Anterior Surface.

1. Linea splendens in median line,

especially in lower part.

2. Anterior spinal artery in

median line.

3. Fasciculi of anterior nerve roots spread over a wide area.

4. Presence of an anterior median

sulcus which can be opened.


Posterior Surface.

1. Ganglion on each posterior nerve root.

2. Arterial anastomotic chain behind and

in front of the posterior nerve-roots.

3. Fasciculi of posterior nerve-roots lie

in a straight line, and enter through postero-lateral sulcus.

4. Presence of a posterior median sulcus

which cannot be opened.

5. Presence of gracile and cuneate fasci culi in upper part.


Internal Structure of the Spinal Cord.—The spinal cord, as seen in transverse section, consists of a central portion composed of grey matter, and an external portion composed of white matter.

Grey Matter.—This is arranged in the form of two irregular crescents, the concavities of which are directed outwards, and the convexities inwards, the latter being connected across the middle line by the grey commissure. The arrangement has been likened to the letter )-(. The grey commissure lies at the bottom of the posterior median sulcus, and presents about its centre the minute opening of the central canal of the cord. The part of the commissure in front of this canal is known as the anterior grey commissure, and the part behind as the posterior grey commissure. In front of the anterior grey commissure there is a transverse band of white matter, called the anterior white


THE NERVOUS SYSTEM


1421


immissure, which lies at the bottom of the anterior median fissure, ach crescent of grey matter consists of two horns, anterior and osterior, the former being in front of, and the latter behind, the grey mnmissure. The anterior horn is broad and blunt, and it stops short f the surface of the cord, being separated from the surface by white latter which is traversed by the fasciculi of the anterior nerve-roots, he blunt extremity of the anterior horn is called the caput cornu, and le portion adjoining the grey commissure, which is slightly constricted,

called the cervix cornu. The posterior horn is for the most part mg, narrow, and tapering, and its pointed extremity almost reaches tie surface of the cord at the bottom of the postero-lateral sulcus, 'his pointed extremity is called the apex cornu, and it contains a transient substance, known as the substantia gelatinosa (of Rolando), diich forms the cap for the caput cornu posterioris. It contains a


Central Canal


?i G . 866._ Transverse Section of the Spinal Cord in the Upper Thoracic

Region, showing the Arrangement of the Grey Matter and Cells (Semi-diagrammatic) (after Poirier).

small amount of neuroglia, and numerous nerve-cells. The portion idjoining the grey commissure, which is slightly constricted, is called the cervix cornu, and the portion contiguous to the cervix, w 11c is slightly enlarged, is called the caput cornu. The part between the

two cornua is called the body. ' ,

About the centre of the concavity of the body crescent the grey

matter projects into the lateral column in the form of processes arranged in a reticular manner and enclosing white matter. This network is sailed the processus reticularis, and it is most conspicuous m the cervical region. In the thoracic region, more particular y in 1 s uppei part, the grey matter of each crescent forms a triangular projection which extends laterally for a short distance immediately m front of the processus reticularis, and adjacent to the junction o e an erior

ornu with the grey commissure. This projection is known as the







1422


A MANUAL OF ANATOMY


lateral horn. When followed into the lower cervical and into the lumbar regions it blends with the anterior horn, the thickness of which it increases, but it is again present above the level of the fourth cervical vertebra.

The grey matter has been described, so far, as it would be seen on looking at transverse sections through the cord; under such conditions the use of the term ' horn ’ or ' cornu * is quite appropriate. As it exists in the complete cord, however, the grey matter is in the form of a continuous column , and in considering it in such a way it should be described as possessing anterior, posterior, and lateral ' columns,’ rather than ‘ horns.’

The grey matter varies in amount in different parts of the cord. It is present in largest quantity in the lumbar enlargement, where the large nerve-trunks for the lower limbs arise, and next to this in the cervical enlargement, where the large nerve-trunks for the upper limbs arise.

The horns of the crescents of grey matter vary in shape, as seen on section, in different regions. In the cervical region the anterior


A


Fig. 867. —Transverse Sections of the Spinal Cord in Different

Regions.

A, cervical region; B, mid-thoracic region; C, lumbar region; D, conus

medullaris.

horns are short, broad, and blunt, and the posterior horns are long, narrow, and pointed. In the thoracic region both horns are narrow, though the posterior is more so than the anterior. In the lumbar region both are broad, though the anterior is more so than the posterior. These differences render sections of the spinal cord in the cervical, thoracic, and lumbar regions easily recognizable. As stated, the lateral horn is also a characteristic of the cord in the thoracic region, more particularly in its upper part.

Central Canal.—This minute canal is situated about the centre of the grey commissure, and extends throughout the entire length of the spinal cord. Superiorly it is continued into the lower half of the medulla oblongata, and it opens into the lower part of the fourth ventricle at the calamus scriptorius. Interiorly, near the apex of the conus medullaris it becomes enlarged, and assumes the shape of an inverted ±. This enlargement is known as the ventriculus terminalis. From this point it is prolonged for some distance into the filum terminale, and it ends in a closed extremity. In the cervical and thoracic regions the central canal is nearer the anterior surface of the cord than the













THE NERVOUS SYSTEM


i 4 2 3


sterior, but in the lumbar region it occupies the centre. In the aus medullaris it is near the posterior surface.

The canal is lined with ciliated columnar epithelium, the columnar Is being known as ependymal cells.

The central canal represents the lumen of the neural tube of ectoderm from ich the spinal cord is developed.

White Matter of the Spinal Cord.—The white matter forms the ter part of the cord, and is arranged in three columns—anterior, teral, and posterior. The anterior column is situated between the iterior median fissure and the anterior horn of grey matter, and tends as far as the most lateral fasciculi of the anterior nerve-roots, therefore includes the superficial coating of the anterior horn, where is traversed, over an area of some breadth, by the scattered fasciculi the anterior nerve-roots. The lateral column is situated between the iterior and posterior horns of grey matter, in the concavity of the escent. Its superficial limits are the most lateral fasciculi of the iterior nerve-roots and the fasciculi of the posterior nerve-roots at e postero-lateral sulcus. The posterior column is situated between e posterior median sulcus and the posterior horn of grey matter, its perficial limit being the fasciculi of the posterior nerve-roots at the >stero-lateral sulcus.

The white matter increases in quantity from below upwards, and pta of pia mater and neuroglia fibres pass into it at various points.


Jhief Distinguishing Characters of the Spinal Cord in Different Regions, as

seen in Transverse Sections.


Cervical Region.

Transversely oval.

Anterior Horn, short, broad, and blunt.

Posterior Horn, long, narrow, and tapering.

Formatio Reticularis, well marked.

Lateral Horn, merged into anterior, except above fourth cervical vertebra.

White Matter, large in amount.


Central Canal, nearer the ventral than the dorsal surface.

Postero - intermediate Sulcus and Septum of pia mater, well marked.


Thoracic Region.

1. Circular.

2. Anterior and Posterior

Horns, both narrow, posterior more so than anterior.

3. Formatio Reticularis,

not very distinct.

4. Lateral Horn, con spicuous, especially in upper part

5. White Matter, less in

amount, but large in proportion to Grey Matter.

6. Central Canal, nearer

the ventral than the dorsal surface.

7. Postero - intermediate

Sulcus, absent, but Septum of pia mater recognizable.


Lumbar Region.

1. Almost circular.

2. Anterior and Posterior

Horns, both broad, anterior more so than posterior.

3. Formatio Reticularis,

absent.

4. Lateral Horn, merged

into anterior.


5. White Matter, small

in amount, and Grey Matter, large.

6. Central Canal, in the

centre.


7. Postero - intermediate Sulcus and Septum of pia mater, absent.


I 4 2 4


A MANUAL OF ANATOMY


Minute Structure of the Spinal Cord—Grey Matter.—The grey matter consists of nerve-cells, nerve-fibres, and neuroglia, and is very vascular.

The nerve-cells are present in great numbers, and are multipolar. Each cell sends off at various points several protoplasmic processes, one of which becomes the axis-cylinder of a nerve-fibre, and is called the axis-cylinder process, or axon. The other processes are known as the protoplasmic processes {of Deiters), or dendrites, and, after successive branchings, they terminate in free extremities. There are no anastomoses between the dendrites of the same cell, nor between those of contiguous cells. A multipolar nerve-cell, with its axon and dendrites, constitutes a neuron. The multipolar cells form longitudinal columns of various lengths, and, as seen in transverse sections of the cord, they are arranged in groups which occupy particular regions. These

Central Canal


Fig. 868.— Transverse Section of the Spinal Cord in the Upper Thoracic Region, showing the Arrangement of the Grey Matter and Cells (Semi-diagrammatic) (after Poirier).

cell-columns or groups are three in number—namely, anterior or ventral, in the anterior horn of grey matter; lateral, in the lateral horn of grey matter; and posterior, constituting the thoracic nucleus (or posterior vesicular column of Lockhart Clarke), and being very conspicuous in the medial portion of the cervix of the posterior grey horn in the thoracic region. Besides these main columns or groups, other nerve-cells are present, which are scattered irregularly throughout the other portions of the grey matter.

The anterior or ventral cell-column is situated, as stated, in the anterior horn of grey matter, and extends throughout the whole length of the spinal cord. Its cells are of large size and very conspicuous, and their axons, which are at first non-medullated, become medullated, and then constitute the fasciculi which emerge to form the anterior nerve-roots. These cells are therefore the sources from which the






THE NERVOUS SYSTEM


4 25

Trent or motor nerve-fibres proceed, and the ventral column is nsequently spoken of as the motor column. The ventral or motor 11 s of this column are arranged in two groups, medial and lateral, re medial group occupies the medial part of the anterior grey horn, id the lateral group is situated in its outer part. In the cervical and mbar enlargements of the cord the cells of the lateral group are very imerous, and are arranged in two sub-groups, ventro-lateral and )rso-lateral.

The intermedio-lateral cell-column is situated, as stated, in the teral horn of grey matter, and the cells constitute a column known

the intermedio-lateral nucleus. The thoracic cell-column is situated in the medial part of the

rvix of the dorsal cornu of grey matter. This extends throughout le entire thoracic region of the cord, and for a short distance into le cervical and lumbar enlargements. The cells make an elongated jcleus (often termed Clarke’s column), and are of large size. This )lumn exists chiefly in the thoracic portion of the cord, whence the ime thoracic nucleus.

The cells of the grey matter differ as regards their axons, some having short, id others long, axons.

The cells with short axons have their axons confined to the grey matter, which they ramify not far from the parent-cells. They serve to bring contiguis cells into relation with one another.

The cells with long axons are partly root-cells and partly association-cells^ id their axons travel for some distance from the parent-cells. The axons of le root-cells leave the cord in the fibres of the ventral or motor nerve-roots, he axons of the association-cells constitute association-fibres, which are disused in two ways: (1) Some enter the white matter of the same side of the >rd, in which they divide into ascending and descending branches. Eventually iey re-enter the grey matter, and terminate in arborizations at some distance om the parent-cells. (2) Other association-fibres cross to the opposite side 1 the ventral or white commissure. Some of these end in arborizations around le cells of the grey crescent, whilst others enter the white matter, in which ley are disposed as on the side from which they have crossed.

Destination of Axons of Cells of Grey Matter.

Cells of Ventral Horn. —(1) Many axons become the axis-cylinder processes f the efferent fibres of the ventral nerve-roots. (2) Other axons constitute ssociation-fibres, which cross to the opposite side in the ventral 01 white comlissure. After crossing, some end in arborizations around the cells of the entral horn * others enter the white matter; and a few are regarded by some uthorities as entering the ventral nerve-roots of the side to which they have rossed

Cells Of Lateral Horn.—The axons of the cells of the intermedio-lateral nucleus ass to the efferent fibres of the ventral nerve-roots, and they are regarded as irnishing the white rami communicantes of the sympathetic system.

Cells Of Dorsal Horn. —The axons of the cells of the dorsal horn have various irections; (1) Some pass to the ventral horn and ventral 01 white commissure. >) The axons of the cells of the thoracic nucleus pass to the lateral column, and re usually regarded as entering the dorsal cerebellar and ventro-lateral cereellar tracts. (3) The axons of the cells of the substantia gelatmosa pass partly 3 the lateral column, adjacent to the dorsal horn, where they divide into ascenc ig and descending branches, and partly into the posterior marginal bun e o

90


1426


A MANUAL OF ANATOMY


Lissauer. (4) The axons of other cells in the lateral part of the cervix of tb dorsal horn pass to the lateral and ventral horns, the ventral or white commissure and the lateral column. (5) The axons of the cells of the caput cornu posteriori pass to the lateral column of the same side, and a few are regarded as passinj to the opposite side in the ventral or white commissure.

Dorsal or Grey Commissure. —This commissure lies at the bottoir of the dorsal median septum. It consists of (1) grey matter, con taining a few small nerve-cells, and (2) medullated nerve-fibres. Thes( fibres pass across from one side to the other, and later on diverge ir each grey crescent. They serve as association fibres which bring th( cells of opposite sides into relation with one another. This commissun contains the central canal of the cord, which for the most part is nearei the ventral portion of the commissure than the dorsal. The part o: the commissure surrounding the central canal is called the substantia gelatinosa centralis. It consists of neuroglia, a few nerve-cells, anc nerve-fibres; and it is invaded by processes derived from the deep end. c of the ciliated columnar epithelial cells which line the central canal.

Summary of the Gelatinous Substances of the Grey Matter. — These are

(1) The substantia gelatinosa, which forms a cap for the caput cornu posterioris :

(2) the substantia gelatinosa centralis, which surrounds the central canal ©f the cord; and (3) the substantia gelatinosa externa, which forms the glial sheath oj the cord beneath the pia mater.

White Matter. —The white matter of the cord consists of longitudinal medullated nerve-fibres, traversed by septa of the pia mater, and embedded in neuroglia. The fibres have no primitive sheath or neurilemma.

Ventral or White Commissure. —This commissure lies at the bottom of the ventral median fissure, and it is separated from the central canal of the cord by a part of the dorsal or grey commissure. It consists ol medullated nerve-fibres, destitute of a neurilemma, some of which pass transversely, but most of them decussate, entering the commissure ventrally on one side, and leaving it dorsally on the opposite side. The fibres, after crossing, enter the grey crescent and the ventral column. They are derived from (1) the anterior cerebro-spinal tract, (2) the processes of root-cells and of association cells, and (3) the fibres of the spino-thalamic tract, to be presently described.

Fibres of Roots of Spinal Nerves—Ventral or Anterior NerveRoots. —The fibres of the ventral nerve-roots arise within the cord from several sources. (1) Many of them are axons of the medial cells of the ventral horn of grey matter of the same side. (2) Some are axons of the lateral cells of the ventral horn. (3) Others are axons of the cells of the thoracic nucleus of the same side. (4) A few are axons of cells in the dorsal horn of grey matter of the same side. (5) A few are regarded as being axons of the medial cells of the ventral and intermediate grey matter of the opposite side , which cross in the ventral or white commissure. All the axons receive their medullary sheaths near the parent cells, and they form funiculi, which leave the white matter of the ventral column over an area corresponding to the caput


THE NERVOUS SYSTEM


1427


the ventral horn of grey matter, after which each fibre acquires its imitive sheath or neurilemma.

Most of the axons of the fibres of the ventral nerve-roots belong to the ntral (motor) cells of the ventral horn of grey matter of the same side.

Dorsal or Posterior Nerve-Roots.—The fibres of the dorsal nerveots arise from the unipolar (orginally bipolar) cells of the spinal nglia. The single pole or process of each of these cells is T-shaped, le half of the horizontal limb of the T is central, and enters the cord



' IG . 869.— Course of Nerve-Fibres in the Spinal Cord (from Halliburton’s

Handbook of Physiology ’ (after Schafer).

P. Cerebro-spinal Tract 1, 2, 3, 4. Anterior Cornual Cells K, A, A, A. Axons of Anterior Cornual Cells M. Muscular Fibre

G. Unipolar Cell of a Spinal Ganglion, giving Origin to a Fibre of a Posterior NerveRoot

B. Peripheral Branch of Fibre S. Skin . .

(lowerC) Central Branch of Fibre, passing into the Spinal Cord

. Descending Branch of Fibre in the Spinal Cord


D. Ascending Branch of Fibre in the Spinal Cord Pi, P2. Posterior Cornual Cells

C (upper C). Cell of Clarke’s Column or Thoracic

Nucleus .

5. Collateral, passing directly to arborize around an

Anterior Cornual Cell (2). >

6. Collateral, with an Intermediate Cell-Station in a

Posterior Cornual Cell (P 2 ).

7. Collateral, arborizing around a Cell ot Clarke s

Column (upper C). ,

8. Continuation of Main Ascending Branch of Fibre.


the dorso-lateral sulcus between the dorsal and lateral coiumns of ilte matter. The other half of the horizontal limb is peripheral, and

sses outwards in the course of the nerve. • ol K 11 n f

Within the cord a few lateral fibres enter the marginal bundle 3sauer, and the dorsal horn of grey matter, but most of them pass -o th e poslero-lateral column (Burdach) close to the dorsal horn of

y matter. Within this column the fibres divide into ^ branches lending and descending. The descending branches, after a short





1428


A MANUAL OF ANATOMY


course, enter the dorsal horn. These descending fibres are usualh regarded as forming the ‘ comma tract.’ The ascending branches are longer than the descending, and, at various levels, they also enter the dorsal horn. The ascending branches of the fibres of the dorsal roots of the lower spinal nerves enter the postero-medial column.

The ascending and descending branches give off numerous collateral fibrils, which enter the dorsal grey column. These collaterals have the following destinations: (1) The dorsal horn of the same side, and that of the opposite side through the dorsal or grey commissure; and (2) the ventral and lateral horns of the same side. In each case they

come into close relation with the corresponding nerve-cells_ e.g., the

cells of the dorsal horn, including the thoracic nucleus, the ventral or motor cells of the ventral horn, and the cells of the thoracic nucleus in the lateral horn.


Sensory Fibres entering Fasciculus Cuneatus (Burdach) Cells of Posterior Column and Thoracic Nucleus ' Posterior Ro<


Spinal Ganglia. — these are situated on the posterior roots of the spinal nerves in the intervertebral foramina, and outside the theca, though invested by a prolongation from it. Each ganglion is oval, and consists of unipolar nerve-cells. The single pole of each cell divides into two processes, one of which is centripetal and forms part of the posterior nerve-root, whilst the other is centrifugal and passes into the spinal nerve. The pole and its inward and outward processes resemble the letter T. In early life the cells are bipolar.

The fibres of the posterior nerve-roots have their deep origins in the unipolar cells of the spinal ganglia, and they grow into the spinal cord. On the other hand, the fibres of the anterior nerve-roots have their deep origins within the spinal cord, where they arise as the axons of the multipolar nerve-cells of the anterior column of grey matter, and they grow outwards.




THE NERVOUS SYSTEM


1429


Tracts of the Spinal Cord.

Posterior Column. —The tracts of this column are ascending and xending, and they are as follows:

Ascending Tracts. Descending Tracts

Fasciculus gracilis (Fig. 871, 1). Semilunar (comma) tract (Fig. 871, 9). Fasc. cuneatus (Fig. 871, 2). Septo-marginal bundle (Fig. 871, 10).

Postero-lateral tract (of Lissauer)

(Fig. 871, 3).

Lissauer’s tract also belongs to the lateral column.


19

Descending


Ascending.


F IG . 871.— The Tracts of the Spinal Cord.

Ascending.

Fasciculus gracilis (Goll’s column).

. Fasciculus cuneatus (Burdach’s column). Fasciculus postero-lateralis (Lissauer’s tract). _

|.. Posterior spino-cerebellar fasciculus (Flechsig’s tract).

5. Anterior spino-cerebellar fasciculus (Gower’s tract).

5 . Lateral spino-thalamic tract.

7. Anterior spino-thalamic tract.

1 . Intersegmental tract (fasciculus proprius: ground bundle).


Descending.

9. Semilunar (or comma) tract.

10. Septo-marginal bundle.

n. Lateral cerebro-spinal fasciculus (crossed pyramidal tract).

12. Anterior cerebro-spinal fasciculus (direct pyra midal tract).

13. Lateral intersegmental tract.

14. Anterior intersegmental tract.

15. Sulco-marginal tract.

16. Rubro-spinal tract (Monakow’s bundle).

17. Tecto-spinal tract.

18. Olivo-spinal tract (Helweg’s tract).

19. Vestibulo-spinal tract.


Ascending Tracts. —The fasciculus gracilis (tract of Goll) is situated ose to the posterior median septum. Its fibres are derived from the orsal roots of the coccygeal, sacral, lumbar, and lowei thoracic nerves, hey are at first contained in the cuneate tract, but as they ascend ley are gradually displaced medially, and so foim a special tract, he fibres terminate superiorly in connection with the cells of the

ucleus gracilis of the medulla oblongata.

The fasciculus cuneatus (tract of Burdach) is situated on the lateial ide of the tract of Goll next to the dorsal horn of grey mattei.











1430


A MANUAL OF ANATOMY


Above the mid-thoracic region it is separated from Goll’s tract by the dorsal intermediate or paramedian furrow and a septum of pia mater. Its fibres are derived from the dorsal nerve-roots. Above the mid-thoracic region they are derived from the dorsal roots of the upper thoracic and cervical spinal nerves, and these fibres terminate superiorly in connection with the cells of the nucleus cuneatus of the medulla oblongata. Below the mid-thoracic region the fibres are derived from the lower dorsal nerve-roots, and these, being displaced inwards into the gracile tract, terminate in connection with the cells of the nucleus gracilis.

The postero-lateral fasciculus (Lissauer’s tract) is close to the outer surface of the cord. It embraces the contiguous parts of the posterior and lateral columns, and occupies the region of the dorso-lateral sulcus where the funiculi of the dorsal nerve-roots enter the cord. It lies between the substantia gelatinosa and the surface of the cord. Its fibres are derived from the dorsal nerve-roots, and they ascend close to the substantia gelatinosa , in which they terminate at different levels.

Descending Tract.—The semilunar tract (comma) is situated in the cuneate fasciculus. Its fibres are usually regarded as being derived from the descending branches of the fibres of the dorsal nerve-roots, in which case they are exogenous. The other view, however, is that the fibres are intrinsic or endogenous, and spring from the cells of the dorsal cornu of grey matter.

Antero-lateral Column,—1 he tracts of this column are arranged into descending and ascending, and are as follows:


Descending Tracts.

1. Lateral cerebro-spinal (crossed pyra midal) tract (Fig. 871, 11).

2. Anterior cerebro-spinal (direct pyra midal) tract (Fig. 871, 12).

3. Intersegmental tract (Fig. 871, 13 and

14).

4. Rubrospinal tract (Fig. 871, 16).

5. Vestibulo-spinal tract (Fig. 871, 19).

6. Olivo-spinal tract (Fig. 871, 18).

7. Tecto-spinal tract (Fig. 871, 17).


Ascending Tracts.

1. Dorsal spino-cerebellar tract

(Fig. 871, 4).

2. Ventral spino-cerebellar tract

(Fig. 871, 5).

3. Anterior spino-thalamic tract

(Fig. 871, 7).

4. Lateral spino-thalamic tract

(Fig. 871, 6).

5. Spino-tectal tract (Fig. 871, 6).


Descending Tracts.—The crossed pyramidal or lateral cerebro-spinal tract (fasciculus spinalis lateralis, Fig. 871, 11) is a long descending tract of large size, which is situated deeply in the dorsal part of the lateral column directly in front of the dorsal cornu of grey matter. It is separated from the outer surface of the cord by the dorsal spinocerebellar (or direct cerebellar) tract. It diminishes in size as it descends, and in the lumbar region it becomes superficial. At about the level of the third sacral nerve it ends. The fibres of this tract have their origin in the pyramidal cells of the motor area of the cortex of the cerebral hemisphere of the opposite side. From this origin they descend through (1) the internal capsule of the corpus striatum, (2) the crus


THE NERVOUS SYSTEM


i43i


srebri, and (3) the pons. On leaving the pons they enter the pyramid f the medulla oblongata on the side from which they have arisen, t the lower part of the pyramid they cross to the opposite side and ike up their position deeply in the dorsal part of the lateral column f the spinal cord. The fibres of the crossed pyramidal tract of one de therefore come from the cerebral hemisphere of the opposite side, nd they form the inner and larger part of the pyramid of the medulla blongata, also of the opposite side. As this tract descends, fibres

ave it in each segment of the cord. These fibres enter the ventral orn of grey matter, and end in close relation with the ventral or lotor cells, the axons of many of which form the axis-cylinder prossses of the fibres of the ventral or motor nerve-roots.

The direct pyramidal or anterior cerebro-spinal tract is of small size, nd is situated in the anterior column, where it lies close to the ventral ledian fissure. It diminishes in size as it descends, and usually srminates about the centre of the thoracic region, but fibres have been raced as low as the fourth sacral nerve. The fibres of this tract, like lose of the crossed pyramidal tract, have their origin in the pyramidal ills of the motor area of the cortex of the cerebral hemisphere, but in his case of the same side. The fibres of the direct pyramidal tract of ne side therefore come from the cerebral hemisphere of the same side, hey pursue a similar downward path as low as the pyramid of the ledulla oblongata of the same side, of which they form the smaller art. They take no part, however, in the decussation of the pyramids, s do the fibres of the crossed pyramidal tract. Their course is directly ownwards into the corresponding half of the spinal cord, where most fthem take up their position in the anterior column close to the ventral ledian fissure. The fibres of the direct pyramidal tract, though they ike no part in the decussation of the pyramids, cross to the opposite ide at regular intervals as they descend in the anterior column of tie cord. The crossing takes place in the ventral or white commissure, nd, having entered the ventral horn of grey matter of the opposite ide, the fibres end, like those of the crossed pyramidal tract of that ide, in close relation with the ventral or motor cells, the axons of lany of which pass to the ventral or motor nerve-roots.

Most of the pyramidal fibres therefore cross from the side on which hey arise to the opposite side. In the case of the crossed pyramidal ract the crossing takes place in the lower part of the medulla obmgata. In the case of the direct pyramidal tract the crossing takes lace in the ventral median fissure of the spinal cord along the course f the tract.

The ground-bundle or intersegmental tracts (fasciculus proprius nterior, Fig. 871, 14; lateralis, Fig. 871, 13; and posterior , Fig. 871, 8) ontain association fibres linking together various parts of the cord, he fasciculus proprius anterior is continued up into the medulla as he posterior longitudinal bundle, but the ground-bundles, as a whole, re regarded as descending tracts.

The vestibulo-spinal tract (fasciculus vestibulo-spinalis, Fig. 871,


1432


A MANUAL OF ANATOMY


ig) is situated in the anterior column, where it lies superficially. It forms a communication between the vestibular structures, through Defiers’ nucleus, with the motor cells of the cord.

The prepyramidal or rubro-spinal tract (Fig. 871, 16) is situated in the lateral column on the ventral aspect of the crossed pyramidal tract. Its fibres are chiefly derived from the red nucleus of the tegmentum or dorsal part of the crus cerebri of the opposite side, and they are regarded as terminating in the dorsal part of the ventral grey matter.

The tecto-spinal tract (fasciculus tecto-spinalis , Fig. 871, 17) runs from the superior corpus quadrigeminum of the opposite side to the motor cells, and lies in front of the rubro-spinal tract.

The bulbo-spinal or olivo-spinal tract, or bundle of Helweg (Fig. 871) 18), is confined to the cervical region of the cord, and is triangular. Its fibres are regarded as arising in the medulla oblongata behind the olive, but their mode of termination is not known. They lie near the surface of the cord external to the anterior nerve-roots.

Ascending Tracts.—The dorsal spino-cerebellar tract, or direct cerebellar tract (of Flechsig) (Fig. 871, 4), is situated in the lateral column. It lies in front of the dorso-lateral sulcus, between the crossed pyramidal tract and the outer surface of the cord.. It commences in the lower part of the thoracic region, and superiorly it traverses the lower part of the medulla oblongata on its lateral aspect, after which it'enters the restiform body, by which it is conducted to the vermis of the cerebellum. Its fibres are usually regarded as being derived from the thoracic nucleus or column of Clarke.

The ventral spino-cerebellar tract, or tract of Gowers (Fig. 871, 5), is situated chiefly in the lateral column, in front of the dorsal cerebellar tract, close to the outer surface of the cord. It is comma-shaped in section, its dorsal part being broad, but as it extends forwards between the funiculi of the ventral nerve-roots it tapers and enters the ventral column superficially. It begins near the lumbar region of the cord. Superiorly it extends through the medulla oblongata and pons, and afterwards passes along the superior cerebellar peduncle into the cerebellum, terminating in the vermis. It therefore takes an indirect course as compared with that of the dorsal spino-cerebellar tract. Its fibres are crossed and are usually regarded as being derived from the thoracic nucleus and posterior horn of the opposite side. The ventral spino-cerebellar tract contains the spino-thalamic and spinotectal tracts.

The spino-thalamic tract (Fig. 871, 6) consists of fibres which arise as the axons of cells of the dorsal grey matter, around which cells the fibres of the dorsal nerve-roots have terminated. The spino-thalamic fibres cross to the opposite side in the ventral or white commissure, thus giving rise to a spinal inferior sensory decussation or spino-thalamic decussation, as distinguished from the superior sensory decussation in the bulb, called the decussation of the fillets, which is produced by the deep arcuate fibres which arise from the cells of the nucleus gracilis and


THE NERVOUS SYSTEM


1433


ucleus cuneatus. The spinothalamic fibres, having crossed in the entral white commissure, ascend in the tract of Gowers, and after raversing the bulb and pons they terminate in the optic thalamus f the side to which they have crossed as a cell-station. It is important d note that there are two sensory decussations —lower or spinal, and pper or bulbar. In unilateral lesions of the spinal cord there would nly be partial anaesthesia on the opposite side; whereas in unilateral

sions of the bulb, involving both the fillet-fibres and the spino-thalamic bres, there would be complete anaesthesia on the opposite side.

The spino-tectal tract (Fig. 871, 6) is also an ascending tract. Its bres are connected with the cells of the ventral cornu of grey matter, hey ascend in conjunction with the ventral spino-cerebellar tract, and ass through the formatio reticularis of the bulb and pons. After lis they decussate with those of the opposite side, forming the fountain ecussation (of Meynert), which lies between the two red nuclei, to duch nuclei the spino-tectal fibres furnish collaterals. After the ecussation the fibres of either side pass to the corresponding superior illiculus of the corpora quadrigemina.

The tracts of the antero-lateral column may be otherwise arranged s follows:


Ventral Column.

nterior cerebro-spinal tract (descending). (12).

nterior intersegmental (descending)

( 1 4 ) - . ;

ulco-marginal tract (descending)

( 15 ) estibulo-spinal tract (descending)

( T 9);

nterior spino-thalamic tract (ascending) ( 7 ).


Lateral Column.

Lateral cerebro-spinal tract (descending) (11).

Rubro-spinal tract (descending) (16). Tecto-spinal tract (descending) (17). Olivo-spinal tract (descending) (18). Dorsal cerebellar tract (ascending)

( 4 )-.

Anterior spino - cerebellar tract (Gowers, ascending) (5).

Posterior spino-thalamic and spinotectal tracts (ascending) (6). Lateral intersegmental (descending) ( 13 )

The spino-thalamic and spino-tectal tracts (ascending) are contained 1 the ventral spino-cerebellar tract. A part of the postero-lateral isciculus (Lissauer’s tract) lies superficially in the dorsal part of the iteral column, and it has been described in connection with the dorsal olumn.


Association Fibres of Antero-lateral Column—Intersegmental Fasciculi.— The

art of the antero-lateral column which is not occupied by the descending and scending tracts is adjacent to the grey matter, and it constitutes the antero,teral ground-bundle. It is divided into two parts—anterior and lateral.

The anterior intersegmental group is situated in the ventral column in front

the ventral cornu of grey matter, and has been already described. The lateral group occupies the lateral column ventral and medial to the crossed framidal tract.

The portion of the ventro-lateral ground-bundle adjacent to the grey matter, id almost surrounding it, is known as the limiting zone.

The fibres of the entire antero-lateral ground-bundle are association or ngitudinal commissural fibres, which serve to connect the grey matter of sue


I 434


A MANUAL OF ANATOMY


cessive segments of the spinal cord. They are derived from the cells of the gre matter of the same side, and also of the opposite side, the latter crossing in th ventral or white commissure.

Arteries of the Spinal Canal and Spinal Cord—Arteries of the Spina Canal.—These vessels enter the spinal canal through the intervertebra and sacral foramina. In the cervical region they are branches of th vertebral, deep cervical, and superior intercostal arteries; in th thoracic and lumbar regions they are derived from the dorsal branche of the intercostal lumbar and ilio-lumbar arteries; and in the sacra region they come from the lateral sacral arteries. Within the spina canal each spinal artery divides into three branches—neural or central and anterior and posterior parietal. The neural or central brand pierces the theca of the spinal cord. It supplies the coverings of th cord and the nerve-roots, and it anastomoses with the anterior anc posterior spinal arteries on the cord. This branch is sometimes spokei of as the lateral spinal artery. The parietal branches divide and joii again with one another in such a way that they form five anastomoti( chains in the spinal canal outside the dura mater; of these, one i: antero-median, two antero-lateral, and two postero-lateral.

Arteries of the Spinal Cord.—These are: (i) the anterior spina artery; (2) the posterior spinal arteries, right and left; and (3) th lateral spinal arteries, right and left (neural or central branches jus described in connection with the spinal canal).

The anterior spinal artery is formed by the union of the anterior spinal branches, right and left, of the vertebral arteries. It descend: along the front of the cord in the median line, and is reinforced at regular intervals by the lateral spinal arteries. In this manner ar anterior longitudinal anastomotic chain is formed, which descend: for some distance on the filum terminale.

The anterior spinal branches of the vertebral arteries are seldom of equa size, and often only one is present.

The posterior spinal arteries are two in number, right and left, anc each is a branch of the corresponding vertebral artery. Each vesse descends on the side of the cord in two branches, one being in front of and the other behind the posterior nerve-roots. These are reinforcec by branches from the lateral spinal arteries, and the lateral longitudinal anastomotic chains formed in this manner extend over the entire length of the cord. It will thus be seen that there are five anastomotic chains inside the dura mater in relation to the cord, though they have not quite the same distribution as the extradural; one is antero-median and two on each side postero-lateral. Of these two, one lies in front of and the other behind the posterior nerve-roots.

It is only under very favourable conditions that all these arteries are injected equally.

Veins of the Spinal Column and Spinal Cord—Veins of the Spina) Column.—These veins form two plexuses, extra- and intra-spinal, which for convenience are divided into five groups from behind forward:


THE NERVOUS SYSTEM


1435


) posterior extraspinal, (2) posterior intraspinal, (3) veins of the irtebral bodies, (4) anterior intraspinal, (5) anterior extraspinal.

The dorsal spinous venous plexus is situated deeply upon the supernal surface of the neural arches of the vertebrae under cover of the ultifidus spinae muscle. It receives its tributaries from the integuent and muscles of the back, and it communicates with the posterior ngitudinal intraspinal plexus by branches which pierce the ligamenta iva. In the neck the blood is conveyed away from the plexus by uns which open into the vertebral venous plexus around the vertebral tery of each side; in the thoracic region by veins which join the dorsal ■anches of the intercostal veins; and in the lumbar region by veins hich join the dorsal branches of the lumbar veins.

The veins of the bodies of the vertebrae (venae basis vertebrae) are mtained within the cancellated tissue of the vertebral bodies. They )mmunicate in front with the anterior extraspinal veins, and posteriorly iey terminate in two venous trunks which, emerging through the



Fig. 872. —Schematic Sections to show Positions of Longitudinal

Arterial and Venous Channels.

vo foramina on the posterior surface of each vertebral body, open ito the transverse communicating branch between the two anterior •ngitudinal intraspinal veins.

The anterior longitudinal intraspinal veins form two anastomotic lains, which are situated on the posterior surfaces of the bodies of le vertebrae, one on either side. They communicate with each other pposite the centre of each body by transverse branches which receive le terminal trunks of the venae basis vertebrae. These transverse ranches pass between the posterior longitudinal ligament and the odies of the vertebrae. Superiorly the anterior intraspinal veins )mmunicate with the vertebral and the transverse or basilar plexuses f veins, and laterally an offset passes outwards through each interertebral foramen, which, with that of the posterior intraspinal vein, )rms a plexus around the adjacent spinal nerve.

The posterior longitudinal intraspinal veins are situated in front of le laminae, one on either side, and they are connected at frequent itervals by transverse branches. They communicate with the dorsal




A MANUAL OF ANATOMY


!436

spinous venous plexus by branches which pierce the ligamenta flava. Superiorly they communicate with the marginal sinuses on either side of the foramen magnum and vermiform fossa, which by their union form the occipital sinus. With the marginal sinuses and the anterior intraspinal veins they form a venous ring at the foramen magnum. Laterally each vein sends outwards through the corresponding intervertebral foramen an offset, which, with that of the anterior intraspinal vein, forms a plexus around the adjacent spinal nerve.

The anterior and posterior intraspinal veins are situated between the theca of dura mater and the wall of the spinal canal.

The anterior extraspinal veins form a plexus along the anterior aspect of the bodies of the vertebrae, which is most copious in the neck. On either side it communicates with the vertebral plexus around the vertebral artery in the neck, the intercostal veins in the thoracic region, and the lumbar veins in the lumbar region. It is also connected with the venae basis vertebrae.

Veins of the Spinal Cord.—These vessels lie within the substance of the pia mater, and are disposed as venous chains, one being in front, one behind, and two on either side. The anterior vessel lies over the anterior median fissure beneath the anterior spinal artery; the posterior vessel is also medially placed; and the two lateral vessels are situated one in front of and the other behind the posterior nerve-roots (Fig. 872). Besides these principal chains the veins form a plexus on the surface of the cord. Laterally branches emerge through the intervertebral foramina, which, along with the offsets of the anterior and posterior intraspinal veins, form plexuses around the spinal nerves. From these plexuses the blood is conveyed on either side into the vertebral plexus and deep cervical vein in the neck, and into the intercostal and lumbar veins in the corresponding regions.

Lymphatics.—There are no lymphatic vessels in the spinal cord. Their place is taken by spaces in the outer coat of the arteries, called perivascular spaces, which are in communication with the subarachnoid space.

Development of the Spinal Cord.

The formation of the neural tube from the neural plate and groove is described on pp. 34 and 39. A short general account of the formation of the cord from the tube, and of the spinal nerves, is given on p. 53. Further details are given in the following paragraphs.

The proliferating cells of the early neural tube become confluent and form a syncytium, which is evident in the growing cord to a comparatively late stage. Exhibiting this syncytial character, the ectodermic cells of the wall of the tube undergo proliferation, the wall becomes thickened, and it consists of two kinds of cells—namely, (1) sustentacular or supporting cells, and (2) nervecells proper. The former make the ependyma and neuroglia of the spinal cord, and the latter give rise to the grey and white matter. The loosely arranged syncytial network is known as the myelospongium. This myelospongium becomes condensed internally and externally, and these condensed layers form the internal and external limiting membranes . The wall of the young neural tube is arranged in three layers or zones—namely, (1) inner or ependymal, (2) intermediate or mantle zone, and (3) outer or marginal zone.

The ependymal zone consists of a single layer of elongated cells, connected


THE NERVOUS SYSTEM


1437


th the internal limiting membrane. Their bases are directed towards the nen of the neural tube, and from their apices delicate radial fibres pass outrds to the external limiting membrane. Amongst them there are some concuous cells, called germinal cells. These lie close to the wall of the neural ?e, and by their proliferation they give rise to ependymal cells and neuroblasts, e latter migrate outwards into the mantle zone.

The mantle layer consists mainly of neuroblasts derived from the lining layer ependymal cells, but smaller neuroglial cells are scattered among- these, and ‘ whole is supported by a network of spongioplasm, in which, in fact, the


j. 873.—Three Sections from Different Levels of Cord in Embryo

of 4-9 Mm.

ft lower figure, under higher power, shows nerve-fibres leaving ventro-lateral

wall.

dei may be said to be embedded. Neuroglial fibres develop from the neuroil cells, and extend throughout the thickness of the cord, ramifying and joining Tin the spongy basis.

The marginal zone is the peripheral and outlying part of the spongy netrk, forming a definite layer superficial to the mantle zone. It is a region ich will be occupied by the tracts of nerve-fibres as these form, acting as scaffolding or support for them; it increases enormously in thickness as the r asion by fibres progresses.

As just said, the white matter of the cord is made by nerve-fibres growing


1438


A MANUAL OF ANATOMY


in the marginal zone, the grey matter is formed from the mantle zone, and th ependymal layer, when it has ceased to proliferate and give off the cells of th mantle zone, becomes the lining cell layer of the central canal. The cana itself is the remains of the ventral part of the original cavity of the neural tube

Neuroglial cells have many branches, and are spoken of as glia-cells or spidet cells. The neuroglial fibres are fibrillations of the peripheral protoplasm of th cells, from which they become differentiated.

The neuroblasts lie in groups within the mantle layer, and they give ris to the nerve-cells of the spinal cord. Each cell is primarily unipolar and pear shaped. It has a prominent nucleus, and the body is prolonged into a proces or pole, which represents the axon or axis cylinder process of a nerve-fibre. Sub sequently the pear shape is lost, due to the formation of secondary processes o dendrites, the cell being now multipolar.

Formation of the Cord. —The number of neuroblasts within the mantli zone increases rapidly, the multiplication being due to frequent division of th germinal cells in the ependymal zone.

The division of germinal cells is apparently very extensive and rapid There is doubt, however, as to further addition by division of the nucle within the mantle zone; if there is such division, it is probably amitotic as the occurrence of mitotic figures in this zone is very exceptional.

Whatever may be the origin of all the nuclei, they soon show a tendency t( gather more particularly in dorsal and ventral thickenings on each side. Thu:

there occur longitudinal bulgings on eacl side, showing not only on the oute: surface, but also markedly on the in ternal surface, making the prominence: known as the dorsal (or alar) and ventra (or basal) laminae (Fig. 874) which affeci the form of the contained cavity. Ai interlaminar sulcus runs down the sid( wall of the cavity between these tw( laminae.

A semidiagrammatic sectior across the cord of an embryo ai the end of the first month is given in Fig. 874 to illustrate thes* points. The main collections 0: neuroblasts in the mantle zone make the ventral (V) and dorsa' (D) laminae, separated by the sulcus (IL). A floor-plate, (F) connects the two sides and is composed of a thinner ependymal layer with a fairly thick marginal zone; a roof plate (R) is practically only ependymal. The neural crest, described on p. 53, lies beside the tube on each side, and is represented here by a mass of neuroblasts which will become the posterior root ganglion (G); the interganglionic parts of the neural crest (p. 54) have disappeared by this time, leaving the ganglionic masses in position. Differential disposition of neuroblasts in the mantle zone has begun already.

General Formation. —The neuroblasts of the basal lamina make the cells of the anterior grey column, and the fibres of the afferent roots pass out directly from them. Those of the dorsal lamina are utilized in forming the matter of the posterior grey column. The spinal ganglia send nerve-fibres (posterior roots) into the dorsal region of the cord, the ganglia, as seen, being outside the cord from the beginning. The marginal zone carries fibres from the neuroblasts, and thus increases in depth gradually and continuously; in this way the white matter of the cord is laid down round the grey substance. The cavity, becoming relatively smaller, remains only as the central canal', there is some reason to think


Fig. 874. —Section across Cord, Semi-diagrammatic, about End of First Month.

(Explanation in text.)




THE NERVOUS SYSTEM


J 439


at the dorsal portion of the original cavity is actively obliterated by fusion

tween its walls. The ependymal zone, after its germinal functions have ceased the end of the second month, becomes the ependymal lining of the canal.

The ventral lamina differentiates ore quickly than the dorsal part, id can be described first. In Fig.

74 it can be seen that a tract of ldei ( b ) is present, having a distinct irso-ventral direction and passing edial to the main ventral or ventroteral neuroblastic mass (a). This tract comes early. Its appearance ggests at first a dorso-ventral igration, but such migration is rtainly not present, and the arngement seems only due to the rection of early fibrils in this tract path, directed towards the floorate, where they cross to the other ie.

This early indication of decussation is of interest. For some fundamental but not very evident reason, the passage of impulses—afferent or efferent —to the opposite side seems to be of basic importance, and a glimpse at the drawings given already will make it clear that the floor-plate is the decussating region; the roof-plate does not seem to provide the necessary marginal zone, and is in fact stretched into a transparent cellular layer higher up, so that the commissural fibres have only the floor-plate for their passage. So far, then, as the primary neural tube extends, all commissural fibres pass ventrally, and the tract b might even be spoken of as a ‘ lateral commissure path ’; such a name, however, would not take account of certain other characters, which might be summed up perhaps in a ‘ path of least resistance,’ so that, for instance, vessels tend to enlarge and lie in this path.

Without labouring the matter further, it will be enough to direct attention to this ' path,’ to which reference will be made from time to time.


Grouping of Ventral Neuroblasts. — Fig. 875 gives tracings from different A els of the cord at 15 mm., showing the modifications found at this period the ventro-lateral group (a of Fig. 874). The groups are not so clearly marked, course, as indicated in the tracings, but are nevertheless quite evident; uC d mC are upper and middle cervical levels, uD and mD are upper and middle












1440


A MANUAL OF ANATOMY


thoracic, and iL and mL are first and middle lumbar levels. The ‘ commissun path ’ already mentioned is shown at b, and the grouping of neuroblasts seems i correspond well with the condition in the adult cord; hence the arrangements i the ventral grey column appear to be attained at an early stage. The group c, i the cervical and upper thoracic sections, increases in size as it is traced upward; It is composed of neuroblasts originating from the ependymal zone at the sam level of origin as. the a group, but separated from this last collection by the pat b ; this suggests that it might be looked on as of the same morphological value a the a group, but of later development. Whether this way of regarding it i justifiable or not, the group is responsible for supplying the hypoglossal nucleu and (possibly) that of the sixth nerve, while the a group, at the hypoglossal leve is apparently taken into the formation of the olive; this will be dealt with in th proper place.

Dorsal Lamina and Associated Formations. —The neuroblasts of the dorsa lamina increase and differentiate slowly, forming a massive but apparent! undifferentiated collection in the second month, when the ventral formation (Fig. 875) are evident. There is at first very little marginal zone over thi dorsal collection, but about the end of the first month the fibrils growing ii from the ganglionic mass, beside the cord, begin to collect as a small bundl (C, Fig. 874) on the dorso-lateral aspect of the neuroblastic mass. This bund! is the earliest sign of the posterior white column, and increases rapidly in size, a the same time extending medially. The bundle is to be identified with thi cuneate fasciculus. The medial extension no doubt helps to form the gracil fasciculus, but this may have some separate formation as well. The postero lateral tract (Lissauer’s zone) begins to form a little later, as the entering fibre; of the posterior root increase in number.

The method of elongation of the posterior horn is not clear; doubtless th< increasing depth of the surrounding white columns has something to do with it but the other factors are not apparent.

The deposition of fibrils within the marginal zone to make the white column! goes on, seemingly, throughout foetal life; they can be recognized in the firs' part of the second month at least, and perhaps earlier than this. It may b( assumed that the shorter fibres are formed first, and occupy the marginal zon< close to the neuroblasts; thus we get the short intersegmental fibres clothing the grey matter. The subsequently developed longer fibres are laid down or these short ones, and the latest developed would be the most superficial; thu; we find long fibres reaching the mid-brain and thalamus, and superficial to these although mixed with them to some extent, fibres running to the cerebellum, a later formation. This, in a general way, agrees with what is known about the position of such tracts in the cord.

The downward-running tracts doubtless follow a comparable regulation ir their disposal, but the matter of decussation is important here. The cerebrospinal fibres, for example, decussate for the most part immediately before entering the cord, and thus pass at once into the b path mentioned above; following this, they reach the space ventral to the posterior horn and Lissauer’s tract, in which they lie as the lateral cerebro-spinal or crossed pyramidal tract. The uncrossed fibres ultimately cross and also enter the b path, from which they reach the anterior horn.

Myelinization. —The tracts are at first made up of axis cylinders alone, and these acquire myelin sheaths subsequently. The time when this occurs differs in the various tracts. The process begins about the fourth or fifth month in the root fibres, and after this appears in the tracts more or less in the order of their formation as laid down above. The pyramidal fibres do not begin to develop their sheaths till about the time of birth, and the process is said to continue until after puberty.

Caudal End of Spinal Cord. —This undergoes certain modifications. It can be seen in Fig. 877 that a tail process, bent dorsally, represents the atrophied remnant, in the early part of the second month, of the large ‘ tail ’ of younger stages. This tail process contains a prolongation from the neural tube; it


THE NERVOUS SYSTEM


1441

smains up to about the 18 mm. stage, when the atrophied filament vanishes, arrying with it the included neural prolongation. Fig. 876 shows two median mgitudinal sections of the end of the cord in embryos of 16 and 35 mm. repectively, the tail remnant being present in the younger specimen, although rst about to disappear. The neural cell-layers in this remnant are continued ito a canal (c), the walls of which are continuous with the ependymal layers f the cord. A second canal (vc) is seen on its ventral side, the cavity of which pens into the central cavity of the cord (the continuity is not very clear in ledian section). The central cavity of the cord ends in a dilated ventricle, diich seems to be a normal condition at this stage. That part of the neural ibe which corresponds with the quondam tail is evidently disappearing, shows regular growth, and is represented by remnants.

In the 35 mm. embryo the tail has gone, and the caudal neural remnant shows coccygeal vestigial cyst {cyst) where the caudal portion has separated, the


toirqinal jone / (ye rtf. comm.)


16 mm


!f- -\coccyK

cyst/ v


Fig. 876. —Median Sagittal Sections of Ends of Cords in Sixth and

Ninth Weeks.

Ependymal tissue shown in black. Description in text.


yrst lying very near the surface. Some nerve-fibres have developed in connection ith this {%/) and pass to the cord itself. Remnants of this canal are seen irther forward, and the ventral canal is seen opening into the ventral part of

le terminal ventricle, as in the younger stage.

After this stage the cord does not grow in length at the same rate as the ertebral column, so that its caudal end gets farther and farther away from the Dccygeal region. Hence, the vestigial cyst remaining in situ with a superficial ttachment, the intervening cell-strands are drawn out in a lengthening conection The main cell masses caudal to the ventricle are drawn up with it, taking the nervous elements found in the upper end of the filum terminate; le rest of the filum is composed of drawn-out pia mater, the included and

fetched nerve-tissue having disappeared.

The coccygeal vestigial cyst enlarges somewhat and develops nerve-tissue )und it, but disappears during the later foetal months; it is a possible cause, y persistence, of certain congenital cysts found near the coccyx.


9i



1442


A MANUAL OF ANATOMY


Membranes of Spinal Cord. —The membranes—namely, pia mater, arachnoid and dura mater (theca)—are developed from the mesoderm which invests th neural tube.

Growth of Spinal Cord. —The cord originally occupies the entire length o the spinal canal of the vertebral column. The vertebral column, however grows more rapidly than the cord, so that at the period of birth the cord does no extend lower than the level of the third or fourth lumbar vertebra. Sub sequently its lower limit is the intervertebral disc between the bodies of th first and second lumbar vertebrae. This produces a change in the course o the lumbar, sacral, and coccygeal nerves. In order to reach the level of th intervertebral foramina through which they emerge from the spinal cana they descend almost vertically, and constitute the bundles of nerves known a the cauda equina.

A linear reconstruction of the coccygeal portion of the cord in a 15 mm embryo is given in Fig. 877. It shows the atrophying tail-remnant, with it included piece of neural tissue, but also shows, proximal to this, a portion o nerve-tube truly coccygeal in nature and position, from which take origii four (? or more) nerves behind the coccygeal nerve. These post-coccygea nerves, which have double roots, join with each other and with the coccygea


Fig. 877. —Linear Reconstruction of Caudal End (15 Mm.), showing thi Prolongation of Cord into the Tail Filament, and the Presenci of Four Nerves beyond the Coccygeal.

The vertebral levels of the spinal nerves are indicated.

nerve in a series of ill-defined loops. They emerge between the rudimentar] vertebrae caudal to the sacrum. The broken-up post-coccygeal portion of th( cord is in part carried up with the persisting coccygeal portion, and in pari left behind; the intermediate part is drawn out with the filum terminale. Som< nervous matter still persists at the upper end of this structure. The post coccygeal nerves atrophy and disappear, but Rauber has described remains of ganglia and nerves beside the upper part of the filum, which may repre sent remnants of the upper post-coccygeal nerves, drawn up with the cord.


THE ENCEPHALON.


The encephalon is the part of the cerebro-spinal axis which if contained within the cranial cavity. It is composed of the medulh oblongata, pons Varolii, cerebellum, and cerebrum. In the embryc it consists of three hollow vesicles.


Encephalon =

Prosencephalon or Fore-brain. Mesencephalon or Mid-brain. Rhombencephalon or Hind-brain.



THE NERVOUS SYSTEM


1443


The subdivisions of the prosencephalon are the telencephalon and he thalamencephalon or diencephalon; the mesencephalon remains mdivided; and the subdivisions of the rhombencephalon are the metncephalon and the myelencephalon.


Fore-brain or Prosencephalon = { ^alamencephalon or Diencephalon. Mid-brain or Mesencephalon = Mesencephalon.

Hind-brain or Rhombencephalon = { Mydenceplulon.


rhe various parts of the encephalon which are developed from these ubdivisions will be made evident from the following table:


Telencephalon


Thalamencephalon

or

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon



Cerebral Hemispheres.

Lateral Ventricles.

Anterior Part of Third Ventricle. Interventricular Foramina.

Olfactory Lobes.

Posterior Part of Third Ventricle.

Optic Thalami and Corpora Geniculata. Pineal Body.

Interpeduncular Structures.

Pituitary Body.

Optic Nerve and Retina.

Corpora Quadrigemina.

Crura Cerebri.

Aqueduct (of Sylvius).

Cerebellum.

Pons (Varolii).

Pontine Part of the Fourth Ventricle.


( Medulla Oblongata (or Bulb).

\ Bulbar Part of Fourth Ventricle.


General Description of the Base and Superior Surface of the

Encephalon.

The inferior aspect of the encephalon is known as the base. In the allowing general description of the parts which it presents the order pursued is, as nearly as possible, from behind forwards and upwards.

The medulla oblongata (or bulb) lies on the under aspect of the

erebellum in the median line, occupying the vallecula which separates the two cerebellar hemispheres. The surface exposed is the ventral surface, which presents (1) the anterior median sulcus, crossed at its tower part by the decussation of the pyramids; (2) the pyramid, on sither side of this sulcus; and (3) the olivary body, external to each pyramid.

The hemispheres of the cerebellum lie one on either side of the medulla oblongata, and they conceal from view the posterior parts of the cerebral hemispheres and the posterior part of the great longitudinal fissure. They are characterized by the laminated arrangement of their nervous matter, the laminae being curved and separated from



1444


A MANUAL OF ANATOMY


each other by fissures. Posteriorly the hemispheres are separated fror each other by the posterior notch. When the medulla oblongata i raised, and the cerebellar hemispheres slightly separated from each othei the vallecula is fully exposed, and the inferior vermis is seen lyini deeply in it, with the sulcus vallecula on either side of it.

The pons' (pons Varolii) forms a prominent elevation above th medulla oblongata, the surface exposed being the ventral surface. L the median line this surface presents a longitudinal groove, which i


Olfactory Bulb Olfactory Tract


Lateral Sulcus


Optic Nerve


Gyrus Rectus


Subst. Perfor Ant.


Tuber Cinere and Infundibi Corpus Mami


Crus Cerebri


Area Perforal Post.


Fifth Nerve Sixth Nerve

Facial Nerve Pars Intermedia


Auditory Nerve-' Glossopharyngeal Nerve Vagus Nerve

Accessory Nerve


Oblongata


Hypoglossal Nerve


Fig. 878. —The Base of the Encephalon, and the Cranial Nerves. 1, frontal lobe (orbital surface); 2, temporal lobe; 3, cerebellum.


occupied by the basilar artery. On either side the pons becomes th<

middle peduncle of the cerebellum, passing outwards and backwards into the cerebellar hemisphere.

The temporal lobes of the cerebrum are situated in front of the cerebellar hemispheres, and are conspicuous by their prominence. Each terminates anteriorly in a projecting extremity, called the

temporal pole.

The stem of the lateral sulcus lies immediately in front of the













THE NERVOUS SYSTEM


1445


mporal lobe, and is occupied by the middle cerebral artery. At the iner end of the stem of the fissure is the depression often referred to 5 the vallecula Sylvii or vallecula cerebri.

The frontal lobes of the cerebrum lie in front of the stem of the ,teral fissure. The exposed parts are the orbital surfaces, each of hich is separated from its fellow of the opposite side by the great ingitudinal fissure. Each orbital surface presents a straight fissure, died the olfactory sulcus, which is situated near the great longitudinal ssure, and is parallel to it. This sulcus is occupied by the olfactory •act and olfactory bulb.

The crura cerebri, or peduncles, right and left, appear at the upper order of the pons, and soon diverge from each other as they pass awards and upwards to sink into the cerebral hemispheres.

The optic tract of each side winds round the outer and ventral spects of the corresponding crus cerebri. Its course is forwards and lwards towards its fellow of the opposite side.

The optic commissure, or chiasma, connects the two optic tracts fter their convergence.

The optic nerves, right and left, leave the front of the commissure, nd pass forwards and outwards to the optic foramina.

The interpeduncular space is situated in front of and above the ons. It is somewhat diamond-shaped, and its boundaries are as dlows: posteriorly, the divergence of the crura at the upper border f the pons; anteriorly, the optic commissure; and, laterally, the crus erebri and optic tract from behind forwards. The following parts e within this space, in the order named, from behind forwards: (1) the rea perforata posterior; (2) the corpora albicantia or mamillaria; nd (3) the tuber cinereum, with the infundibulum. The structures ccupying the interpeduncular space form for the most part the floor f the third ventricle.

The area perforata posterior or posterior perforated substance

Drresponds to the posterior median angle of the diamond-shaped intereduncular space, and it lies in a deep depression, called the intereduncular fossa (or fossa Tarini). The grey matter which forms it is erforated by openings for the passage of the postero-medial branches f the posterior cerebral arteries.

The corpora mamillaria are situated directly in front of the area erforata posterior, and present the appearance of small, white, peake bodies lying close to the median line.

The tuber cinereum extends from the mammillary bodies to the ptic commissure, and is composed of grey matter. The infundibulum

connected with the tuber cinereum close behind the optic comlissure, and passes downwards to the posterior part of the pituitary ody.

The area perforata anterior or anterior perforated substance of each de coincides with the vallecula at the inner end of the stem of the iteral fissure. It lies outside the interpeduncular space, close to the uter aspect of the optic commissure. It consists of grey matter, which


I446


A MANUAL OF ANATOMY


is perforated by openings for the passage of a few antero-medial branches of the anterior cerebral artery, and numerous antero-lateral branches of the middle cerebral artery. These branches are destined for the nucleus caudatus and nucleus lenticularis of the corpus striatum, the grey matter of which nuclei comes to the surface of the brain at the anterior perforated substance.

The medulla oblongata and pons occupy the basilar groove of the interior of the base of the skull; the cerebellar hemispheres occupy the cerebellar fossae of the occipital bone; the temporal lobes of the cerebrum sink deeply into the lateral divisions of the middle fossa of the base of the skull; the orbital surfaces of the frontal lobes occupy the lateral divisions of the anterior fossa; the stem of the lateral fissure faces the posterior border of the small wing of the sphenoid; the optic commissure lies above the olivary eminence and optic groove of the sphenoid; and the olfactory bulb rests upon one half of the cribriform plate of the ethmoid bone. The olfactory bulb and olfactory tract, essential parts of the brain, occupy the olfactory sulcus on the orbital surface of the frontal lobe near the great longitudinal fissure; and the olfactory filaments pass through the foramina of the cribriform plate of the ethmoid bone on their way from the olfactory cells of the olfactory mucous membrane to the olfactory bulb. Posteriorly the olfactory tract divides into two roots, medial and lateral. The medial root curves inwards behind the ‘ area of Broca ’ to the callosal gyrus. The lateral root passes backwards and laterally across the outer part of the area perforata anterior. The triangular area of grey matter, which is situated between the diverging roots of the olfactory tract, is called the trigonum olfactorium. It is sometimes spoken of as the middle or grey root of the olfactory tract. The area of Broca is situated in front of the medial (inner) root of the olfactory tract, and is continuous with the callosal gyrus.

Superficial Origins of the Cranial Nerves.

The first or olfactory nerve is represented by the filaments which, as has been seen already, have their superficial origin from the lower surfaces of the olfactory bulbs and pass through the cribriform plate.

The second or optic nerve is connected with the lateral extremity of the front part of the optic commissure.

The third or oculo-motor nerve emerges through the oculo-motor sulcus on the inner aspect of the crus cerebri, just above or in front of the pons, and close to the posterior perforated substance.

The fourth or trochlear nerve, having emerged from the upper part of the superior medullary velum, makes its appearance in the interval between the crus cerebri internally and the temporal lobe externally.

The fifth or trigeminal nerve consists of two roots, which emerge close together from the lateral aspect of the ventral surface of the pons. The sensory root is large, and the motor root, which is small, lies above and slightly medial to the sensory root.


THE NERVOUS SYSTEM


1447


The sixth or abducent nerve appears at the lower border of the pons ust lateral to the pyramid of the medulla oblongata.

The seventh or facial nerve emerges at the lower border of the pons n front of the restiform body of the medulla oblongata.

The eighth or auditory nerve likewise appears at the lower border of

he pons in front of the restiform body of the medulla oblongata, [t lies on the outer side of the facial nerve.

The N. intermedins is a small nerve which appears between the facial md auditory nerves. It is regarded as the sensory root of the facial lerve.

The ninth or glosso-pharyngeal nerve emerges, in the form of about fix fasciculi, from the postero-lateral sulcus of the medulla oblongata, Detween the olivary body and the restiform body, immediately below the facial nerve.

The tenth or vagus nerve lies directly below the glosso-pharyngeal nerve, and emerges by several fasciculi from the postero-lateral sulcus Df the medulla oblongata in front of the restiform body.

The eleventh or accessory nerve has several roots which lie below the fasciculi of the vagus nerve. These rise (a) from the medulla nblongata and ( b ) the upper part of the lateral column of the spinal

ord as low as the level of the fifth cervical nerve. The first is the zranial origin of the nerve, the second its spinal root. They lie below the fasciculi of the vagus nerve, and external to, or in front of, the posterior roots of the adjacent cervical spinal nerves.

The twelfth or hypoglossal nerve emerges by several fasciculi through the antero-lateral sulcus of the medulla oblongata between the pyramid and the olivary body. These fasciculi lie in line with the sixth nerve superiorly.

Arteries at the Base of the Encephalon. —The arteries which supply the brain are the two vertebral and the two internal carotid arteries.

The vertebral arteries incline medially as they ascend on the ventral aspect of the medulla oblongata, and at the lower border of the pons they unite to form the basilar artery. The branches of each vertebral artery to be noted are as follows: (1) the posterior spinal branch , which arises from the main vessel immediately after it has pierced the dura mater, and descends upon the side of the medulla oblongata to the spinal cord; (2) the anterior spinal branch , which arises higher up than the preceding, and passes downwards and inwards on the ventral aspect of the medulla oblongata to unite with its fellow and form the anterior spinal artery ; and (3) the posterior inferior cerebellar branch, of large size, which arises from the main vessel near the pons, and passes backwards round the medulla oblongata to enter the vallecula of the cerebellum.

The basilar artery extends from the lower border of the pons to the upper border, occupying the basilar groove on its ventral surface. It is formed by the union of the two vertebral arteries, and terminates by dividing into the two posterior cerebral arteries. The branches of the basilar artery to be noted on either side are as follows. (1) the


1448 A MANUAL OF ANATOMY

transverse arteries of the pons ; (2) the internal auditory artery , which accompanies the auditory nerve through the meatus auditorius internus; (3) the anterior inferior cerebellar artery , which arises from the basilar about its centre, and passes backwards to the inferior surface of the cerebellar hemisphere; (4) the superior cerebellar artery , which arises from the basilar near its termination, and passes laterally close to the


upper border of the pons, and then round the outer side of the crus cerebri to the superior surface of the cerebellar hemisphere; and (5) the posterior cerebral artery , which arises from the termination of the basilar, and passes laterally parallel to the superior cerebellar artery, and then round the crus cerebri to the inferior surface of the occipital lobe. The posterior cerebral and superior cerebellar arteries are separated from


}




THE NERVOUS SYSTEM


1449


iach other by the third and fourth cranial nerves. The branches of

he posterior cerebral artery are (1) postero-medial, which pass to the posterior perforated substance; (2) postero-lateral, which pass round

he crus cerebri; and (3) posterior choroidal, which pass to the upper Dart of the choroidal fissure.

The internal carotid artery of each side appears at the vallecula

erebri, and there divides into the anterior and middle cerebral arteries. Near its termination it gives off the posterior communicating artery, which passes backwards to join the posterior cerebral artery. It also pves off the anterior choroidal artery, which passes backwards and Dutwards between the crus cerebri and the uncinate gyrus to the lower and anterior part of the choroidal fissure.

The anterior cerebral artery passes forwards and inwards between the optic nerve and the medial root of the olfactory tract, and enters the great longitudinal fissure.

As it is about to enter that fissure it is connected with its fellow of the opposite side by the anterior communicating artery, which is short, but of fairly large size.

Amongst other branches the following are to be noted arising from the anterior cerebral artery: (1) anteromedial, few and inconstant; and (2) antero-lateral, both of which pass to the anterior perforated substance.

The middle cerebral artery, of large size, sinks into the lateral fissure, which it traverses in an outward ‘direction. Before disappearing into the fissure antero-lateral ganglionic branches are to be noted arising from it, which are arranged in two sets, medial and lateral striate, for the corpus striatum and internal capsule.

Circulus Arteriosus.—This is an important communication between the vertebral and internal carotid arterial systems at the base of the brain, which is situated around the interpeduncular space. It is not actually a circle, though so named, but is a heptagon—that is to say, it has seven angles and seven sides.

Beginning at the median line posteriorly, and proceeding forwards on either side to the median line in front, at the great longitudinal fissure, the component arteries of the circle are: (1) the basilar, (2) the posterior cerebral, (3) the posterior communicating, (4) the internal

arotid, (5) the anterior cerebral, and (6) the anterior communicating. These communications serve to insure a uniform supply of arterial fiood to the brain in cases of obstruction to one or other of the principal irterial trunks. The communications also serve to equalize the circu

1. Internal Carotid

2. Middle Cerebral

3. Anterior Cerebral

4. Anterior Communicating

5. Posterior Communicating

6 . Posterior Cerebral

7. Basilar

8 . Superior Cerebellar

9. Transverse Pontine xo. Internal Auditory xr. Anterior Inferior Cerebellar

12. Posterior Inferior Cerebellar

13. Vertebral

14. Anterior Spinal

15. Posterior Spinal

16. Anterior Choroid

17. Posterior Choroid

18. Cential or Ganglionic

19. Central or Ganglionic

20. Central or Ganglionic (Postero-mesial)

‘21. Central or Ganglionic (Postero-lateral)

Fig. 880. —The Arteries at the Base of the Brain, and the Circulus Arteriosus.


13 15 14



1450


A MANUAL OF ANATOMY


lation of blood through the different parts of the brain, an arrangement which, though doubtless advantageous, cannot be essential, since one or both of the posterior communicating arteries are often very small and sometimes absent.

Superior Surface of the Brain. —The brain is ovoid superiorly, its greatest breadth corresponding to the positions of the parietal eminences of the parietal bones. In the median line it presents a deep cleft, called the longitudinal fissure, which extends from the front to the


Great Longitudinal Fissjarc


Fig. 88i.—The Cerebral Hemispheres (Superior View). Fissure of Rolando—central fissure.


back, and divides it into two hemispheres, right and left. This fissure is occupied by a process of the dura mater, called the falx cerebri, and the corpus callosum lies at its deep part. In front of the corpus callosum the fissure extends down to, and is visible on, the base of the brain, but behind the corpus callosum it only extends to the level of the tentorium cerebelli, which separates the cerebellum from the posterior parts of the cerebral hemispheres. The fissure, therefore, in this situation is not visible inferiorly until the cerebellum and the tentorium cerebelli have been removed.


/



THE NERVOUS SYSTEM


1451


Each hemisphere is semi-ovoid, its medial surface being flat. The anterior and posterior extremities are rounded, the former being the thicker of the two. The anterior extremity is known as the frontal pole, and the posterior extremity forms the occipital pole. The surface of each hemisphere consists of grey matter, which is spoken of as the cerebral cortex. Superiorly and externally it is convex in adaptation to the concavity of the vault of the cranium. It is broken up into a number of tortuous eminences, called gyri or convolutions, and these are separated from each other by clefts, called sulci or fissures. The surfaces of the gyri which bound the sulci are covered with grey matter, like their exterior. The pia mater closely covers the gyri, and also dips into the sulci, so as to cover the opposed surfaces of the gyri. The arachnoid membrane, however, does not dip into the sulci, but passes over them. The sulci are of various depths, but the average depth is about J inch.

RHOMBENCEPHALON.

1. The Medulla Oblongata.

The medulla oblongata (or bulb) is continuous with the spinal cord, and extends from the lower margin of the foramen magnum of the occipital bone to the lower border of the pons. Its direction is upwards and forwards, and it measures 1 inch in length, f inch in breadth at the widest part, and fully \ inch in thickness. Interiorly its girth corresponds with that of the spinal cord, but it widens superiorly, so that it is somewhat pyramidal. Its ventral surface faces the basilar groove of the occipital bone, and its dorsal surface is directed towards the vallecula of the cerebellum.

The bulb is composed of two symmetrical halves, its bilateral symmetry being indicated superficially by upward prolongations of the ventral or anterior sulcus and dorsal or posterior median septum of the spinal cord. The anterior median fissure extends as high as the lower border of the pons, where it expands slightly and forms a blind recess, called the foramen ccecum. In its lower part this fissure is interrupted and crossed by bundles of nerve-fibres, which are derived from the inner three-fourths of each pyramid, the decussation thus formed being known as the decussation of the pyramids, or motor decussation. The posterior median septum only extends along the lower half of the bulb, and it terminates superiorly at the point of divergence of the margins of the fourth ventricle.

Each half of the bulb presents two grooves. The antero-lateral sulcus is situated between the pyramid and the olivary body, and along this sulcus the roots of the hypoglossal nerve emerge in line with the ventral roots of the spinal nerves. Whilst, however, the latter are spread over a certain area, the hypoglossal roots emerge along a straight line corresponding to the ventro-lateral sulcus of the bulb. This sulcus is not represented on the surface of the spinal cord. The posterolateral sulcus lies on the dorso-lateral aspect of the olivary body. Along


1452


A MANUAL OF ANATOMY


this sulcus, in order from above downwards, there are (i) the roots of the glosso-pharyngeal nerve, (2) the funiculi of the vagus nerve, and (3) the funiculi of the bulbar part of the accessory nerve.

The bulb in its lower half contains a prolongation of the central canal of the spinal cord. This part of the bulb is spoken of as the closed part , and it extends as high as the level of the lower point of the ventricle. In the upper half of the bulb the central canal opens out at this level into the ^fourth ventricle, and the dorsal aspect of the bulb forms the lower,.or bulbar half of the floor of the fourth ventricle. The upper half of. the bulb is therefore spoken of as the open part.


Optic Commissure

Optic Nerve Infundibulum, - ~

Tuber Cinereum '■» * # „ . „

C Optic Tract

Mamillary Body . /

Third Nerve

Mesial Root of Optic Tract '

Lateral Root of Optic Tract Lateral Geniculate Body


Post.Perforated Subst.


Fourth Nerve


Sixth Nerve


Root of Fifth Nerve Root of Fifth Nerve


Facial Nerve

Pars Intermedia Auditory Nerve - Glosso-pharyngeal Nerve -Vagus Nerve Superficial Arcuate Fibres—

Accessory Nerve-"

First and Second Cervical Nerves;^


Middle Peduncle of Cerebellum

Inferior Peduncle - — Hypoglossal Nerve ..Anterior Median Fissure

Decussation of the Pyramids


Fig. 882. —The Medulla Oblongata, Pons, and Interpeduncular Region. C.C., crus cerebri; P., pyramid; O.B., olivary body.


The surface of each half of the bulb is divided into three areas by the above-mentioned sulci, with the corresponding nerve funiculi. These surface areas are ventral, lateral, and dorsal.

Ventral or Anterior Area.—This superficial area is situated between the median and the antero-lateral sulcus, along which the funiculi of the hypoglossal nerve emerge. It constitutes the pyramid of the bulb. The two pyramids, right and left, represent the motor tracts of the bulb. As regards position, the pyramid is like the anterior column of the spinal cord, and it consists of bundles of nerve-fibres disposed longitudinally. Inferiorly it is somewhat narrow, but it widens superiorly. At the lower border of the pons it undergoes a slight constriction, after which it sinks into the pons. As it traverses the pons its funiculi become separated into several strata, and these are gathered together at the upper border of the pons into the crus cerebri of the corresponding side.





THE NERVOUS SYSTEM


1453


Inferiorly each pyramid is disposed in two parts—medial and lateral. The medial portion represents as a rule the inner three-fourths, and its fibres cross to the opposite side in the lower part of the ventral median sulcus of the bulb. Thereafter they sink deeply into the dorsal part of the lateral column of the spinal cord on the side to which they have crossed, where they constitute the crossed pyramidal or lateral cerebro-spinal tract. The intercrossing of fibres which takes place in the lower part of the ventral median fissure of the bulb is called the decussation of the pyramids, or the motor decussation, and, as stated, it usually involves the fibres of the inner three-fourths of the pyramid.

The lateral portion of the pyramid represents as a rule the lateral fourth, and its fibres take no part in the decussation. The path of most of them is downwards into the anterior column of the spinal cord of the same side, where they lie close to the anterior median fissure of the cord and constitute the direct pyramidal or anterior cerebro-spinal tract. A few of them, however, descend into the lateral column of the same side, and constitute the uncrossed lateral pyramidal tract.

The pyramid of the bulb, therefore, only corresponds topographically with the anterior column of the spinal cord. The direct cerebro-spinal tract of the anterior column of the cord forms the greater part of the lateral fourth of the corresponding pyramid of the bulb; and the crossed cerebro-spinal tract of the lateral column of the cord forms the medial three-fourths of the pyramid of the opposite side. The remainder of the anterior column of the cord sinks deeply into the bulb and lies on the dorsal aspect of the pyramid.

The ventral surface of each pyramid is crossed above the level of the decussation of the pyramids by the anterior superficial arcuate fibres , which emerge from the ventral median fissure and take an arched course outwards and then backwards to the inferior cerebellar peduncle.

The sixth cranial nerve emerges close to the lower border of the pons, immediately lateral to the pyramid, and in line with the funiculi of the hypoglossal nerve as these leave the ventro-lateral sulcus.

Lateral Area of the Medulla Oblongata.— This superficial area is situated behind the funiculi of the hypoglossal nerve. Superiorly the oval eminence, called the olive, is included in it. Inferiorly it has the appearance of being a prolongation of the lateral column of the spinal cord, but this is not the case. The crossed cerebro-spinal tract of the lateral column of the cord sweeps obliquely across to the opposite side, where it forms the greater part of the pyramid of that side. The parts, therefore, of the lateral column of the cord which form the lateral area of the bulb below the olive are (1) the dorsal or direct spinocerebellar tract, (2) the ventral spino-cerebellar tract, and (3) the lateral intersegmental bundle. The dorsal spino-cerebellar tract, as it ascends, soon inclines obliquely backwards to join the inferior peduncle. The ventral spino-cerebellar tract and intersegmental bundle ascend until they reach the lower end of the olive. They then in part sink deeply, and ascend to the pons on the dorsal or deep aspect of the olive. Most


T 454


A MANUAL OF ANATOMY


of the cerebellar fibres, however, remain on the surface, and ascend in the small interval which lies between the outer part of the olive and the funiculi of the glosso-pharyngeal and vagus nerves.


Fig. 883.—A Sketch to show the Disposition of Spinocerebellar Fibres in Lateral Region of Medulla.


The lateral area of the bulb below the olive thus represents the dorsal or direct spino-cerebellar tract, ventral spino-cerebellar tract, and, deeply, the lateral ground-bundle of the lateral column of the spinal cord of the same side (Fig. 883).

Superiorly, as stated, the lateral area presents an oval eminence, called the olive. It lies between the funiculi of the hypoglossal nerve on the one hand, and the funiculi of the glosso-pharyngeal and vagus nerves on the other, with the intervention of some ascending fibres belonging to the ventral spino-cerebellar tract. Its long axis is placed vertically, and in this direction it measures about J inch. Superiorly it is separated from the pons by a deep transverse groove, and interiorly the anterior superficial arcuate fibres arch over its lower part.

At the lower border of the pons, lateral to the upper end of the olive, the facial and auditory nerves make their appearance. The facial nerve is in line


The dorsal fibres (interrupted lines) run to inferior peduncle, therefore have a dorsal tendency as they ascend, covering in the spinal root of fifth nerve, which is making a slight prominence, the tuberculum gelatinosum (T). The ventral fibres (Gowers’ tract) are dotted. The arrows indicate many fibres from other parts (olives, etc.), helping to complete the peduncle. C, G, cuneate and gracile tubercles ; P.R.O., position of pallidorubro-olivarv tract.


closed and open part of the lower and upper.


with the roots of the glosso-pharyngeal nerve. The auditory nerve appears lateral to the facial nerve, and between the two is the small pars intermedia (of Wrisberg ).

Dorsal or Posterior Area of the Medulla Oblongata.—This superficial area is limited in front by the sulcus containing the funiculi of the glosso-pharyngeal, vagus, and bulbar part of the spinal accessory nerves. Posteriorly its lower half extends as far as the dorsal median fissure, and its upper half extends only as far as the lateral boundary of the lower or bulbar half of the floor of the fourth ventricle. Inasmuch as this area belongs to both the bulb, it will be considered in two sections—


Lower Portion of Posterior Area. —This, it has been shown, is limited behind by the dorsal median fissure, and it is in direct continuity with the dorsal column of the spinal cord of the same side, which is composed






THE NERVOUS SYSTEM


455

>f the gracile and cuneate columns. It presents three longitudinal minences—namely, the funiculus gracilis, funiculus cuneatus, and uniculus gelatinosus.

The funiculus gracilis is a prolongation of the column of the spinal ord, and lies close to the dorsal median fissure. The funiculus cuneatus 3 a prolongation of the column of the cord, and lies lateral to the uniculus gracilis, from which it is separated by an upward continuation >f the dorsal intermediate or paramedian furrow of the cord.

At the lower level of the ventricle each of these two funiculi )ecomes enlarged and terminates in a prominence or bulb. The enargement formed by the funiculus

racilis is called the clava, or gracile ubercle, and that formed by the funi:ulus cuneatus is termed the cuneate ubercle. The two clavae, right and left, ie on either side of the lower angle of he fourth ventricle, and as the bulb >pens out dorsally at this level to form he lower or bulbar half of the floor of he fourth ventricle each clava is dis)laced laterally. An angular interval low separates the two clavae, and the irolongation of the central canal of the pinal cord through the lower or closed >art of the bulb opens into the fourth ventricle in the angle between the two

lavae. The funiculus gracilis, with its tu>ercle, and the funiculus cuneatus, with ts cuneate tubercle, are to a large exent produced by the collections of grey natter which they contain—namely, the mcleus gracilis and nucleus cuneatus.

The funiculus gelatinosus is situated >n the outer side of the funiculus cuneatus, between it and the funi:uli of the bulbar part of the spinal accessory nerve. It is produced >y the substantia gelatinosa (of the spinal cord), which is close to the urface in the lower or closed part of the bulb. Interiorly the funiculus s narrow, but it widens as it ascends, and superiorly it terminates n an enlarged extremity, called the spinal tract of the trigeminal or uberculum gelatinosum.

The funiculus and tubercle are covered by a thin layer of longiudinal nerve-fibres which represent the spinal or descending sensory oot of the fifth cranial nerve.

Upper Portion of Posterior Area.— This belongs to the upper or open >art of the bulb, and extends as far as the lateral boundary of the ower or bulbar half of the floor of the fourth ventricle. It presents l prominent round tract, called the restiforrn body , which is situated


Fig. 884.—Posterior View of Medulla.

G, C, gracile and cuneate tubercles ; g, c, corresponding tracts; F, gelatinous tubercle; O, obex.







A MANUAL OF ANATOMY


1456

between the lower half of the floor of the fourth ventricle and the funiculi of the vagus and glosso-pharyngeal nerves. Its direction is upwards, outwards, and backwards, and it enters the corresponding hemisphere of the cerebellum. It is otherwise known as the

inferior cerebellar peduncle.

The inferior peduncle succeeds to the funiculus gracilis and funiculus cuneatus of the lower portion of the posterior area of the bulb, but it is quite distinct from these funiculi, and receives no fibres from them. The sources of its fibres will be given in connection with the internal structure of the bulb (see p. 1463). Meanwhile, it is


Fig. 885.—Section through Medulla just above Decussation of Pyramids: Shows the Prominence of Spinal Tract of Fifth Nerve.

F is the dorsal spino-cerebellar tract immediately ventral to this, and G is the

ventral tract.

clear that it constitutes the great tract of connection between the cerebellar hemisphere, the bulb, and the spinal cord.

The restiform body becomes conspicuous above the level of the cuneate tubercle, and forms the lateral boundary of the lower or bulbar half of the floor of the fourth ventricle.

Internal Structure of the Medulla Oblongata. —Each half of the bulb is composed of grey nervous matter and tracts of white nervous matter.

Grey Matter. —The grey matter lies largely in the interior. Over the dorsal aspect of the upper or open part of the bulb, however, it comes to the surface, and covers the lower or bulbar half of the floor of the fourth ventricle.



THE NERVOUS SYSTEM


1457


As compared with the grey matter of the spinal cord, it presents mportant modifications, and its component parts are as follows:

1. Substantia or formatio reticularis.

2. A thick layer of grey matter around the central canal in the

lower or closed part of the bulb.


Fig. 886.—The Decussation of the Pyramids: Scheme representing the Passage of the Various Tracts from the Spinal Cord to the Medulla (L. Testut’s ‘ Anatomie Humaine ’).


a. Pons

b. Medulla Oblongata (anterior aspect)

c. Decussation of the Pyramids

d. Section of the Cervical Spinal Cord

1. Anterior Cerebro-spinal Tract

2. Lateral Cerebro-spinal Tract

3. Sensory Tract


3'. Nucleus Gracilis et Nucleus Cuneatus

4. Antero-lateral Intersegmental Tract

5. Anterior Pyramid

6. Fillet or Lemniscus

7. Posterior Longitudinal Bundle

8. Ventral Cerebellar Tract

9. Dorsal Cerebellar Tract


3. A thick layer of grey matter over the floor of the fourth

ventricle in the upper or open part of the bulb.

4. Substantia gelatinosa (nucleus of spinal tract, N. V.).

5. Nuclei of grey matter.

The modifications undergone by the grey matter of the bulb in ts lower or closed part are brought about by the decussation of the

92


























A MANUAL OF ANATOMY


1458

pyramids. The nerve funiculi of the lateral cerebro-spinal tract of the spinal cord, on one side as they are traced upwards, pass through the base of the ventral grey column of that side, and then cross in the lower part of the ventral median fissure of the bulb to the pyramid of the opposite side, of which they form the inner and larger part. The nerve funiculi of the tract of the other side are disposed in a similar manner. The ventral grey column of either side is thus broken up by the corresponding crossed pyramidal tract. Its basal part remains on the ventral and lateral aspects of the central canal, but its caput is detached and displaced laterally by the pyramid and olive of the same side (see Fig. 887).

The dorsal horn of grey matter is gradually displaced laterally and ventralwards, in the lower or closed part of the bulb, by the funiculus gracilis and funiculus cuneatus. Its basal part remains on the dorsal and lateral aspects of the central canal; its cervix is broken up into a network by intersecting nerve-fibres; and its caput is thereby detached.


Fig. 887.— Schematic Sections showing Decussation of Pyramids with the Destruction of Base of Ventral Grey Column (Testut).

The caput lies close to the detached caput of the ventral grey matter, but does not blend with it.

Substantia or Formatio Reticularis. —The grey matter of the detached caput of the ventral grey cornu is broken up into a network by intersecting nerve-fibres, which run longitudinally and transversely. This reticulum, augmented by the network formed in the cervix of the dorsal grey cornu, constitutes the substantia or formatio reticularis of the bulb. It lies deeply within the bulb, dorsal to the olive and pyramid of the same side, and it consists of grey matter, longitudinal and transverse nerve-fibres, and some nerve-cells.

The funiculi of the hypoglossal nerve, as they pass forwards to the ventro-lateral sulcus of the bulb, divide the formatio reticularis into two parts—lateral and medial (Fig. 891). The lateral portion is situated behind the olive, and is called the formatio reticularis grisea, from the large amount of grey matter, with nerve-cells, which it contains. The medial portion is situated behind the pyramid, and is












THE NERVOUS SYSTEM


1459


illed the formatio reticularis alba. It contains little grey matter nd few nerve-cells.

Central Grey Matter. —The grey matter which surrounds the jntral canal in the lower or closed part of the bulb is derived from le basal portions of the ventral and dorsal grey columns of the upper art of the spinal cord. In the upper or open part of the bulb this mtral grey matter spreads out and forms a thick layer over the wer or bulbar part of the floor of the fourth ventricle. The medial %rt of this layer represents the basal part of the ventral grey horn, id it contains the hypoglossal nucleus. The lateral part represents le basal part of the dorsal grey horn, and it contains vagus, glossoharyngeal, and vestibular nuclei.

The hypoglossal nucleus is frequently spoken of as ' morphologically continuous with ’ or ‘ representing ’ the ventral grey column above the cervical nerves. This continuity, however, is not an actual anatomical fact; it exists only in the site of ependymal zone origin of the neuroblasts concerned in forming the nuclei. The ordinary motor cells of the ventral grey column in the cord have been derived from the lower part of the ependymal zone, from which they have migrated to form the ventral portion of the marginal zone. Later, when the collections of neuroblasts in the ventral horn have already settled into something approaching their final arrangements, a secondary output of neuroblasts frees itself from the ependymal zone in the same region, but does not migrate any further; this, then, might be looked on as of the same ependymal or original value as the ventral cells, although not anatomically continuous with them. It is from this secondary formation, which is found in the cervical and hind-brain regions, that the hypoglossal nucleus is formed; possibly the sixth nucleus owns a like origin, but this cannot be said with certainty. The other nuclei mentioned in the preceding paragraph are not concerned in this development in any way.

Substantia Gelatinosa (Fig. 885).—This caps the detached and splaced caput of the dorsal horn of grey matter. Having increased

amount owing to the presence of root-fibres of the fifth nerves id lying close to the surface, it gives rise to the tuberculum gelatinosum, metimes referred to simplv as the ‘ spinal tract of the fifth nerve.

Nuclei of Grey Matter.— The nuclei, which will be considered in is place, are as follows:

1. Nucleus gracilis.

2. Nucleus cuneatus.

3. Olivary nuclei.

4. Arcuate nucleus.

5. Nucleus lateralis.

The nucleus gracilis is a collection of grey matter within the funiclis gracilis. For the most part it is connected with the grey matter


A MANUAL OF ANATOMY


1460

on the dorsal and lateral aspects of the central canal, and it ma] be regarded as being in large part an extension from the basal par of the dorsal grey cornu. It is elongated, and increases in size a: it ascends. It gives rise to the prominence of the funiculus gracilis and to the clava, and the fibres of the funiculus gracilis, as they ascend terminate at intervals around the cells of the nucleus (see Fig. 885).

The nucleus cuneatus is a collection of grey matter within th< funiculus cuneatus. It is a direct extension from the basal part 0


Fig. 888.—The Formatio Reticularis of the Medulla Oblongata, showi by a Horizontal Section passing through the Middle of the Olivary Body (Demi-schematic) (L. Testut’s ‘ Anatomie Humaine ').


1. Anterior Median Fissure

2. Fourth Ventricle

3. Formatio Reticularis 3'. Reticularis Alba

3". Reticularis Grisea

4. Raphe

5. Anterior Pyramid

6. Lemniscus

7. Inferior Olive with the two

Accessory Nuclei


7'. Peduncle of Olivary Body

8. Hypoglossal Nerve 8'. Hypoglossal Nucleus

9. Vagus Nerve

g'. Terminal Nucleus of Vagus Nerve

10. External Dorsal Vestibular Nucleus

xx. Nucleus Ambiguus

12. Nucleus Gracilis


13. Nucleus Cuneatus

14. Caput of Posterior Cornu 14'. Lower Sensory Root of Fift

Nerve

15. Fasciculus Solitarius

16. External Anterior Arcuat

Fibres

16'. Arcuate Nucleus

17. Lateral Nucleus


the dorsal grey cornu, which lies on the dorsal and lateral aspect: of the central canal. Like the nucleus gracilis it is elongated, anc increases in size as it ascends. It gives rise to the prominence 0: the funiculus cuneatus and to the cuneate tubercle, and the fibre: of the funiculus cuneatus, as they ascend, terminate at interval: around the cells of the nucleus.


Lateral to the nucleus cuneatus there is a small collection of grey matter which is known as the external or accessory cuneate nucleus. It is on a highe: level than the decussation of the pyramids, and it may be regarded as a detachec portion of the substantia gelatinosa.






THE NERVOUS SYSTEM


1461


The olivary nuclei are associated with the olive, and are three in imber—inferior, and two accessory (medial and dorsal).

The superior olivary nucleus is situated in the dorsal or tegmental part of 5 pons, and is not developmentally associated with those now dealt with.


The inferior olivary nucleus, which is the chief nucleus, is situated thin the olive. As seen in transverse sections through the olive, appears (Fig. 889) as a wavy lamina of grey matter, curved in such a inner as to form an incomplete capsule, which encloses white matter.


Nucl. Grac.


Nucl. Cun.

— Fasc. Solitar.

Sp. Nucleus of Trigeminal Sp. Root Fibres of Trigeminal Med. Longit. Fasc. (Post. Longit. Bundle) Nucl. Ambiguus Lateral Nucleus

Ant. Sp. Cerebellar Fasc. (Gowers)

Dorsal Acc. 01 .


ig. 889.—Section through the Lower Half of Inferior Olive (shows also the Medial and Dorsal Accessory Olives).


racile and cuneate nuclei are seen in position, but spinal tract of fifth is separated from surface by fibres passing to inferior peduncle; these are dorsal spinocerebellar and fibres from olive from opposite side; some fibres from olive pass between the nucleus and the nerve tract. Arrows show the direction of fibres on one side. The upper ones come from the dorsal nuclei and fi th nucleus, and run ventrallv to decussate. The lower fibies are running dorsally, and come mainly from opposite olive, and some from same side.


he open part of the capsule is called the hilum, and is diiected )wards the median line, but it stops short of either end of the nucleus.

great many nerve-fibres pass through the hilum, some inwards nd others outwards, and these form what is known as the olivary eduncle. The wavy lamina is traversed by nerve-fibres..

The medial accessory and dorsal accessory olivary nuclei are situated n the medial and dorsal aspects respectively of the inferior or chiet fivary nucleus, from which, however, they are distinct. Each consts of a band of grey matter, and the upper part of the medial xessory nucleus lies opposite the hilum of the chief nucleus.












A MANUAL OF ANATOMY


1462

Structure of Inferior Olivary Nucleus. —The wavy lamina consists of many small nerve-cells and nerve-fibres which traverse it. The axons of the nerve-cells leave the nucleus as nerve-fibres, and pass to the raphe of the bulb. Some of the nerve-fibres which traverse the wavy grey lamina terminate in connection with its cells, and other fibres pass through it (see Fig. 889).

There are at least two fibre tracts (in addition to those passing into the inferior peduncle) which connect the inferior olive with more distant parts of the nervous system, and are recognizable in sections; little is actually known about them otherwise. The smaller one (olivo-spinal or Helweg’s tract) lies on the surface of the lower medulla and cord immediately in front of the ventral spino-cerebellar fibres. The upper tract, much larger and longer, is the tractus pallido-rubro-olivaris, a name describing its apparent connections.

The structure of the two accessory olivary nuclei corresponds to that of the chief or inferior olivary nucleus.

Arcuate Nucleus. —This nucleus (seen in Fig. 889) consists of a lamina of grey matter which lies upon the ventral aspect of the pyramid of the bulb above the level of the decussation of the pyramids, and beneath the anterior superficial arcuate fibres as they arch outwards over the pyramid after emerging from the ventral median fissure. Superiorly it lies over the medial aspect of the pyramid close to the ventral median fissure. It contains small nerve-cells, in connection with which some of the anterior superficial arcuate fibres terminate, whilst others arise as axons of the cells, and many of them pass over the nucleus without entering it.

Fibres of various sorts, which may be termed in general circumolivary, may be found turning over the lower part of the olive. Some are superficial arcuate fibres, as just described, but others may come apparently from the pyramid, and others again, associated with the ponto-bulbar body, may be really of the nature of aberrant pontine fibres.

Nucleus Lateralis. —This is a special collection of nerve-cells in that portion of the formatio reticularis grisea which lies on the dorsolateral aspect of the olive. It is situated deeply between the olive and the substantia gelatinosa (see Fig. 889).

White Matter of the Medulla Oblongata. —The white matter is situated chiefly on the surface. Over the dorsal aspect of the upper or open part of the bulb, however, the grey matter comes to the surface, and covers the lower or bulbar half of the floor of the fourth ventricle. The white matter is disposed in tracts or strands which are chiefly longitudinal, but a few run transversely in an arched manner. The tracts are as follows:

1. Pyramidal tract (cerebro-spinal tract).

2. Dorsal spino-cerebellar tract (direct cerebellar tract).

3. Ventral spino-cerebellar tract (tract of Gowers).


/


THE NERVOUS SYSTEM


1463

4. Restiform body (inferior cerebellar peduncle).

5. Funiculus cuneatus.

6. Funiculus gracilis.

7. Medial or posterior longitudinal bundle.

8. Tecto-spinal tract.

g. Rubro-spinal tract.

10. Spino-tectal tract.

11. Superficial arcuate tract.

12. Deep arcuate tract.

13. Fillet (lemniscus).

14. Vestibulo-spinal tract.

15. Olivo-cerebellar tract.

The pyramid of either side and the decussation of the pyramids rave been already described. It may, however, be again stated Fat the path of their motor nerve-fibres is downwards into the spinal

ord. The pyramidal tract has descended from the pons.

Posterior (or Direct) Spino-cerebellar Tract. —This tract extends upwards from the lateral column of the spinal cord. It traverses

he lower part of the lateral area of the bulb nearly as high as the ower part of the olive, and immediately anterior to the tuberculum ^elatinosum, after which it passes backwards and upwards into the inferior peduncle, of which it forms a part (Fig. 883).

Anterior Spino-cerebellar Tract. —This tract, like the dorsal or lirect spino-cerebellar tract, extends upwards from the lateral column if the spinal cord. It is situated chiefly on the dorsal aspect of the ilive, but some of its fibres appear close to the outer side of that body. Whilst the dorsal spino-cerebellar tract passes into the restiform body, and so reaches the cerebellar hemisphere directly, the ventral spino-cerebellar tract is continued upwards into and beyond the pons before reaching the cerebellar hemisphere.

Restiform Body.— The restiform body, or inferior peduncle of the cerebellum, is situated on the dorsal aspect of the bulb in its upper ir open part, the funiculus gracilis and funiculus cuneatus occupying the dorsal aspect in its lower or closed part. It succeeds to the clava ind cuneate tubercle, in which these two funiculi respectively end, but it receives no nerve-fibres from the funiculi. It makes its first appearance in relation to the nucleus cuneatus, and above the cuneate tubercle it is a conspicuous massive bundle, which forms the lateral boundary of the lower or bulbar half of the floor of the fourth ventricle. Its course is upwards, outwards, and then suddenly backwards. It sinks into the corresponding hemisphere of the cerebellum.

This peduncle is composed of fibres which are derived from the

following sources:

1. The olivo-cerebellar fibres of the inferior olivary nucleus of the opposite

side

2. The posterior cerebellar tract of the lateral column of the spinal cord of

the same side.


4^4

A MANUAL OF ANATOMY


3. The anterior superficial arcuate fibres from the nucleus gracilis and

nucleus cuneatus of the opposite side.

4. The posterior arcuate fibres from the nucleus gracilis and nucleus

cuneatus of the same side.

5. Vestibular fibres from the vestibular nuclei of the vestibular division of

the auditory nerve.

The restiform body, from its composition, serves as an important means of connection between the cerebellar hemisphere superiorly and the medulla oblongata and spinal cord inferiorly.

Funiculus Cuneatus and Funiculus Gracilis. —These tracts are prolonged upwards from the posterior column of the spinal cord. As stated, each contains a grey nucleus, around the cells of which the corresponding sensory nerve-fibres terminate at intervals as they ascend. Towards the clava and cuneate tubercle the fibres become few and are spread over the clava and cuneate tubercle, finally ending in connection with the cells of the grey nuclei which give rise to these prominences (Fig. 885).

Posterior Longitudinal Bundle. —The fibres of this bundle ( fasciculus longitudinalis medialis ), when followed downwards into the anterior column of the spinal cord on the same side, represent the fibres of the ventral intersegmental tract. As these fibres are followed into the lower part of the bulb they form a bundle, which lies close to the median raphe and directly dorsal to the corresponding pyramid. This strand represents the longitudinal bundle in the lower part of the bulb. The deep arcuate fibres, to be presently described, pass obliquely through it to the median line, where they decussate with those of the opposite side. This decussation takes place in the interval between the right and left dorsal longitudinal bundles. Having now reached the other side, the deep arcuate fibres take an upward course, close to the median line, as the medial lemniscus. The dorsal longitudinal bundle and fillet are therefore now closely related to one another in the lower part of the bulb, both lying dorsal to the pyramid, the fillet lying close to the raphe.

In the upper part of the bulb the two tracts become distinct. The posterior longitudinal bundle is displaced dorsalwards during the formation of the fillet, and it comes into contact with the grey matter on the floor of the fourth ventricle, whilst the lemniscus lies on the dorsal aspect of the pyramid.

The posterior longitudinal bundle is prolonged into the ventral column of the spinal cord on the same side, where it is represented, as has been said, by the ventral intersegmental fibres.

A ventral or anterior longitudinal bundle (tecto-spinal tract) is described as lyi n g on the ventral aspect of the dorsal or posterior longitudinal bundle. This bundle, however, is not well defined. It descends into the anterior column of the spinal cord, and is accompanied by the ponto-spinal tract, the fibres of which spring from the cells of the formatio reticularis of the pons.

Arcuate Tracts. —These tracts form two goups—superficial and deep.


THE NERVOUS SYSTEM


1465


The superficial arcuate fibres are arranged in two sets—anterior md posterior.

The anterior superficial arcuate fibres arise from the nucleus gracilis and nucleus cuneatus of the opposite side, and a few arise from the arcuate nucleus of the same side. At the median line they decussate with those of the opposite side, and emerge at the ventral median fissure, where many of them arch over the medial and ventral aspects of the pyramid. Others pierce the pyramid, whilst some emerge at the ventro-lateral. sulcus between the pyramid and olive. The fibres now pass outwards and dorsalwards, some arching over the lower part of the olive, and finally enter the restiform body.

The posterior superficial arcuate fibres arise from the nucleus gracilis and nucleus cuneatus of the same side, and they enter the restiform body also of the same side.


Trigem. N.

Oblique Fasc.

Facial N.

Aud. N. Flocculus

Circumoliv. Fasc.


Basilar Groove


Pyramid


Fig. 890. —Front Aspect of Pons and Medulla, showing Oblique Fibres

of Pons and Arcuate Fibres on Medulla.


The deep arcuate fibres are disposed in two sets—lemniscal and olivo-cerebellar. The lemniscal deep arcuate fibres arise from the nucleus gracilis and nucleus cuneatus of the same side. They sweep forwards and inwards (Fig. 889) towards the raphe, passing obliquely through the dorsal longitudinal bundle. At the median line they decussate with those of the opposite side above the level of the decussation of the pyramids. Having reached the opposite side, the deep arcuate fibres change their course, and now pass upwards The ascending tract thus formed constitutes the medial lemniscus (or medial fillet).

The decussation which takes place between the deep arcuate fibres in the median line, immediately above the. decussation of the pyramids, is called the decussation of . the lemnisci (decussatio lemmscorum), or the superior sensory decussation, as distinguished from






1466


A MANUAL OF ANATOMY


the inferior sensory or spino-thalamic decussation, which takes place in the spinal cord.

The olivo-cerebellar deep arcuate fibres arise from the inferior olivary nucleus of one side. Emerging through the hilum, they pass across the median line to the opposite side. They then pass over or through the inferior olivary nucleus of that side, on the dorsal aspect of which they are collected into a distinct tract. This tract, arching backwards, applies itself to the restiform body on its deep aspect, and is thereby conducted to the cerebellar hemisphere. Its fibres terminate in the cortex of the vermis and cerebellar hemisphere. The olivo-cerebellar arcuate fibres constitute the olivo-cerebellar tract, which connects the inferior olivary nucleus of one side with the cerebellar hemisphere of the opposite side.

Lemniscus. —The lemniscus (or fillet), as seen in the bulb, is a wellmarked tract of fibres which lies on the dorsal aspect of the pyramid close to the raphe. As just stated, its fibres are derived from the lemniscal deep arcuate fibres of the opposite side. In the lower part of the bulb the fillet and posterior longitudinal bundle are closely related. In the upper part of the bulb, however, as already said, the posterior longitudinal bundle is displaced dorsalwards by the developing fillet, and the fillet, now distinct from the longitudinal bundle, lies on the ventral aspect of that bundle, and on the dorsal aspect of the pyramid. The ventral region of the bulb is thus traversed by four longitudinal tracts, all of which lie close to the median line. These tracts are related to each other in the following order from before backwards (ventro-dorsally):

Pyramid.

Fillet.

Tecto-spinal.

Posterior longitudinal bundle.

Olivo-cerebellar Tract.— This tract has already been described in connection with the olivo-cerebellar deep arcuate fibres.

Raphe of the Medulla Oblongata. —The raphe of the bulb occupies the median plane above the decussation of the pyramids, and is composed of fibres which, for the most part, cross obliquely from one side to the other. These fibres represent (1) the anterior superficial arcuate fibres, (2) the lemniscal deep arcuate fibres, and (3) the olivocerebellar deep arcuate fibres. A few fibres pass ventro-dorsally, and some are disposed longitudinally. The fibres are therefore arranged in an intersecting manner.

Central Canal of the Medulla Oblongata. —The central canal of the spinal cord is prolonged upwards through the lower or closed part of the bulb. As it ascends it is gradually displaced backwards, first by the decussation of the pyramids, and afterwards by the decussation of the lemnisci. It is surrounded by a thick layer of grey matter, which is derived from the basal portions of the ventral and dorsal grey horns of the spinal cord, Superiorly, at the level of the obex,


/


THE NERVOUS SYSTEM


1467


it opens into the lower part of the fourth ventricle in the angle between the two diverging clavse. The grey matter which surrounds the canal is now spread out, and forms a thick covering over the lower part of the ventricular floor, as has been said already.

Areas of Flechsig. —These areas involve the whole substance of the bulb, and are mapped out by the funiculi of the hypoglossal and vagus nerves. Seen in transverse section, these funiculi lie near each other as they arise from their nuclei in the grey matter of the lower part of the floor of the fourth ventricle. As the funiculi of the hypoglossal nerve pass forwards and those of the vagus nerve outwards they diverge from each other, and the substance of the bulb is thereby divided into three segments, which constitute the areas of Flechsig — ventral, lateral, and dorsal (see Fig. 891).


Fig. 891. —Plan to illustrate the Three Areas of Flechsig, showing the

Main Structures in Each of These.


The ventral area lies between the raphe of the bulb and the funiculi of the hypoglossal nerve. Throughout its thickness this area contains the following structures:

The pyramid and arcuate nucleus (Fig. 891, P).

The lemniscus, decussating (L).

The posterior longitudinal bundle (B).

The formatio reticularis alba.

The lateral area lies between the funiculi of the hypoglossal nerve and those of the vagus nerve. Throughout its thickness this area contains the following structures:

The olive and inferior olivary nucleus.

The nucleus lateralis (NL).

The nucleus ambiguus (to be afterwards described) (NA).

The formatio reticularis grisea.

The dorsal area is the region behind the funiculi of the vagus nerve. Throughout its thickness this area contains the following structures:

The inferior peduncle.

The upper part of the cuneate nucleus (C).

The descending root of the vestibular nerve ^ 'po be afterwards

The fasciculus solitarius (S) \ described.

The spinal root of the fifth cranial nerve (V) )

The substantia gelatinosa (G).





1468 A MANUAL OF ANATOMY

Course of Chief Nerve Funiculi of Spinal Cord through Medulla Oblongata. Spinal Cord. Medulla Oblongata.

Posterior Column.

Column of Goll (fasciculus gracilis).

Column of Burdach (fasciculus cuneatus).

Lateral Column.

(Crossed) lateral cerebro-spinal tract.

Anterior cerebro-spinal tract.

Dorsal (or direct) spino - cerebellar tract.

Ventral (or indirect) spino-cerebellar (tract of Gowers).

Prepyramidal or rubro-spinal tract.

Lateral intersegmental.

Anterior Column.

Anterior cerebro-spinal tract.

Tecto-spinal tract.

Ventral intersegmental.

Anterior marginal bundle (of Lowenthal).

Development of Medulla Oblongata.— The bulb is developed from the myelencephalon, which is the caudal division of the rhombencephalon.

2. The Pons.

The pons (Varolii) is situated above the medulla oblongata, and between the hemispheres of the cerebellum. With the exception of the inferior peduncles, all parts of the medulla oblongata are prolonged into it. The pons presents two surfaces (ventral and dorsal) and two borders (upper and lower). The ventral surface (Fig. 890) rests upon the upper part of the basilar groove of the occipital bone and the dorsum sellae of the sphenoid. It is convex from side to side, and from above downwards, and has a transversely striated appearance, due to the disposition of its superficial fibres. Along the median line it presents the basilar groove, which extends from the lower to the upper border, and lodges the basilar artery. On either side of this groove the ventral surface is rendered prominent by the prolongation upwards of the pyramids of the medulla oblongata, and the basilar groove is chiefly due to this circumstance. The sensory and motor roots of the fifth nerve, lying close together, appear on the lateral aspect of the ventral surface, the small motor root being the upper of the two. The portion external to these two nerveroots constitutes the middle peduncle of the cerebellum. It is composed of the transverse fibres of the pons, which pass backwards and laterally into the corresponding cerebellar hemisphere.


Funiculus gracilis and nucleus

gracilis.

Funiculus cuneatus and nucleus

cuneatus.


Inner three - quarters of opposite pyramid.

Outer one-quarter of pyramid of same side.

Lateral area below olive, and inferior peduncle.

Lateral area below olive, and formatio reticularis.


Outer one-quarter of pyramid of same side.

Posterior longitudinal bundle.


THE NERVOUS SYSTEM


1469


The dorsal surface is directed towards the cerebellum. It presents a triangular area which is covered with grey matter. This area is continuous with the dorsal surface of the upper or open part of the medulla oblongata, and it forms the upper or pontine part of the floor of the fourth ventricle. On either side it is bounded by the superior peduncle of the cerebellum as it passes upwards and inwards.

The upper border is slightly depressed at the centre, and on either side of the median depression it slopes outwards and downwards towards the middle peduncle of the cerebellum. The crura cerebri, right and left, sink into the pons at the upper border.


Internal Structure of the Pons. —dhe pons is composed of a large ventral and a small dorsal part.

Ventral Part.— This portion consists of (1) bundles of transverse fibres, (2) bundles of longitudinal fibres, and (3) a large amount ot


grey matter. , , ,, £ •

The bundles of transverse fibres intersect the bundles of longitudinal fibres, and on either side they are collected into e mi e peduncle of the cerebellum, which enters the corresponding cerebellar hemisphere. Some of the transverse fibres arise in, e cot ex of the cerebellum as the axons of the cells of Purkinje, an 1 terminate in the pons in arborizations round the cells of the nucleus pontis, mostly on the opposite side to that on which they arise Other transverse fibres arise in the pons as the axons of of the nucleus pontis on one side. They then cross to the other







I 47 °


A MANUAL OF ANATOMY


Corpora Mamillaria


side, and enter the cerebellar hemisphere of that side, where they terminate in arborizations in the cortex. The fibres, therefore, of which the middle peduncle of the cerebellum is composed may be regarded as being of two kinds—namely, efferent and afferent. The efferent fibres arise in the cerebellar cortex and terminate in the pons, whilst the afferent fibres arise in the pons and terminate in the cerebellar cortex.

The bundles of longitudinal fibres in each half of the ventral part of the pons are derived from the breaking up of the crusta or basis

pedunculi of the corresponding crus cerebri, which enters the pons at its upper border. Most of these bundles are collected together at the lower border of the pons, and form the pyramid of the medulla oblongata on the same side. Certain of the fibres of the basis pedunculi, however, terminate in the pons as follows: (i) some end in arborizations around the cells of the motor nucleus of the fifth cranial nerve, the nucleus of the sixth cranial nerve, and the nucleus of the seventh cranial or facial nerve ; and (2) others end in arborizations around the


Basis Pedunculi Cerebri --* 31 ; Locus Perforatus Posterior


Right Pyramidal Tract

Transverse Fibres of Pons

Middle Peduncle of Cerebellum


minL

Decussation of the Pyramids Ard :

■ f

Fig. 893.—Dissection of the Pons, showing the Course of the Pyramidal Tracts of the Medulla Oblongata (Hirschfeld and Leveille).

P., right pyramid; O.B., right olivary body.


cells of the nucleus pontis, all of the same side.


The fibres to cranial motor nuclei may run a more aberrant course, leaving the basis pedunculi in the mid-brain and running in the tegmentum of the pons to decussate and reach their objectives. Some also run a recurrent course, leaving the pyramid below the pons and turning upwards deeply.

The grey matter of the pons, which is large in amount, occupies the intervals between the intersecting transverse and longitudinal bundles, and contains small multipolar nerve-cells. It is known as the nucleus pontis, and is continuous with the arcuate nuclei of the medulla oblongata.

Corpus Trapezoides or Trapezium. —The trapezium is a fairly thick layer of transverse fibres on either side, which have no connection with the corresponding middle peduncle of the cerebellum. The fibres are situated in the lower part of the pons dorsal to the pyramidal bundles. Within the trapezium are large multipolar cells, which constitute the nucleus of the trapezium. The fibres of the trapezium arise chiefly as the axons of the cells of the ventral cochlear




THE NERVOUS SYSTEM


1471


mcleus, and also of the dorsal cochlear nucleus (or tuberculum icusticum), in which nuclei the fibres of the cochlear division of the mditory nerve terminate, Some of the fibres arise from the superior )livary nucleus; others are the axons of the cells of the nucleus of

he trapezium; whilst a third set (auditory strice) arise from the uberculum acusticum of the opposite side. Certain of the fibres


Fig 8q4—Vertical Transverse Section through the Upper Part of the Pons and Fourth Ventricle (from L. Testut’s ‘ Anatomie Humaine,’ after Stilling).


1. Fourth Ventricle

2. Superior Velum

3. Superior Root of Fifth Nerve

4. Nerve-cells which accompany this Root

5. Posterior Longitudinal Bundle

6. Formatio Reticularis

7. Lateral Fissure of Isthmus


8. Section of Superior Cerebellar Peduncle 9,9. Medial and Lateral Portions of the Lemniscus

10, 10. Transverse Fibres of the Pons

11, 11. Longitudinal Fibres of the Pons

12. Raph6

V. Fifth Nerve


the trapezium terminate in the superior olivary nucleus, but the laioritv cross the median plane, where they decussate with those E the opposite side. Having crossed to the opposite side, they ecome longitudinal, and form a well-marked ascending tract in the orsal part of the pons, called the lateral lemniscus, which lies on the

uter side of the main or medial fillet. . ,

Dorsal or Tegmental Part of the Pons.— This portion is divided ito two symmetrical halves by a median raphe, which is continuous ith that of the upper or open part of the medulla oblongata It insists of formatio reticularis, which is continued upwards from re formatio reticularis of the bulb. The formatio reticularis of the












1472


A MANUAL OF ANATOMY


dorsal part of the pons contains certain tracts of nerve-fibres and nuclei, with which important nerves are connected. These are so complicated that it is convenient to divide the dorsal part of the pons into two regions—lower and upper.

Lower Region. —This region corresponds to the level of the trapezium in the ventral part of the pons, and succeeds the upper end of the bulb. The inferior peduncle of the bulb lies for a short distance on the lateral aspect of this region, but soon passes backwards and sinks into the hemisphere of the cerebellum.

The tracts and nuclei of the formatio reticularis of the lower region, which will be described in this place, are as follows:

1. Spinal sensory root of the fifth cranial nerve.

2. Motor nucleus of the facial nerve.

3. Superior olivary nucleus.

4. Nucleus of the sixth cranial nerve.

5. Posterior longitudinal bundle.

6. Rubro-spinal tract.

7. Tecto-spinal tract.

8. Lemniscus.

The funiculi of the spinal or descending sensory root of the fifth cranial nerve appear ventro-medial to the mass of the inferior cere

Sixth Nucleus


Post. Long. Bundle Facial Nucleus

Corp. Trapez.


Basis Pontis


Fig. 895.—Nuclear Positions in Pons (Schematic).

bellar peduncle. Close to the inner side of this root, and accompanying it in its downward course, there is the inferior sensory nucleus of the fifth nerve, around the cells of which the fibres of the spinal or descending root of that nerve terminate at intervals. The inferior sensory nucleus is an upward prolongation of the substantia gelatinosa, and interiorly it extends to about the level of the second cervical spinal nerve (see Fig. 885).

The motor nucleus of the facial nerve is internal to the funiculi of the spinal root of the fifth nerve. It lies deeply in the lower region of the dorsal part of the pons on the dorsal aspect of the superior


Sensory Nucl. N.V. Motor Nucl. V.

Superior Olive Fifth Nerve











THE NERVOUS SYSTEM


1473


•livary nucleus. The motor fibres of the facial nerve arise as the xons of the cells of this nucleus (see Fig. 896).

The superior olivary nucleus is situated on the ventral aspect of he facial nucleus, and is close to the lateral part of the trapezium, he fibres of which arch round its ventro-lateral aspect. Some of hese fibres terminate in the superior olivary nucleus, whilst others .rise from its cells.

In connection with the superior olivary nucleus three other nuclei are lescribed: (1) an accessory superior olivary nucleus on the medial side of the )rincipal nucleus; (2) a lateral pre-olivary nucleus on the ventral aspect of the >rincipal nucleus; and (3) a medial pre-olivary nucleus on the ventral aspect of he nucleus of the trapezium.

The nucleus of the sixth cranial nerve lies immediately beneath he grey matter of the pontine part of the floor of the fourth ventricle, ind on the lateral side of the dorsal longitudinal bundle which separates he nucleus from the median raphe. It corresponds to that portion


supC- cerebellar peduncle.


facial colliculus _ _

nucleus of--., sixth H.

post- longitudbundie.

stalk of olive"'

median lemniscus


inferior! cerebellar middle j peduncle.

_\jai- vestibular

> x \ nucleus.

TV 7- 5 pinai root of V*- h n. facial nucleus


-facial nerve

^vestibular part ""'of 8th nerve.

. "Corpus trapezoides

""Supr- olivary nucleus

' oibducens nerve

pyramidal tract.

Fig. 896.—Diagrammatic Section through the Pons, to show Deep Origins of Sixth (Red) and Seventh (Black) Cranial Nerves.


f the facial colliculus which lies on the pontine part of the floor of he fourth ventricle directly above the auditory striae.

The medial or posterior longitudinal bundle lies close to the median iphe, and on the medial side of the nucleus of the sixth nerve. Like bat nucleus, it lies immediately beneath the grey matter of the pontine

art of the floor of the fourth ventricle. . , „

The lemniscus (medial, medial fillet) lies, as it <loes in the bulb, entral to the dorsal longitudinal bundle, but m the dorsal part of tl e ons the two strands are separated by a distinct interval occupied

93





474

A MANUAL OF ANATOMY


by the rubro- and tecto-spinal tracts. It will have been noticed tha J in the spinal cord the tecto-spinal tract lies ventral to the rubro-spinal but later on it will be seen that the tectum or quadrigeminal region i: dorsal to the red nucleus. It is therefore clear that somewhere ir their course they must reverse their relative positions, and the rubro spinal become ventral to the tecto-spinal. Where this happens i: not at present clear; indeed, the exact relations of these and man} other tracts, such as the vestibulo-spinal, spino-thalamic, and spino tectal in the upper part of their course, are still under investigation The lemniscus occupies a broad area in that portion of the lowei region of the dorsal part of the pons which is contiguous to the ventra part. The area extends outwards from the median raphe.

Upper Region of the Dorsal Part of the Pons. —This region lies abovt the level of the trapezium in the ventral part of the pons. The tracts and nuclei of this region, which will be described in this place, are as follows:

1. Superior peduncle of the cerebellum.

2. Nuclei of the fifth cranial nerve.

3. Medial or posterior longitudinal bundle.

4. Medial fillet or lemniscus.

5. Lateral fillet or lemniscus.

The superior peduncle of the cerebellum, after emerging from the corresponding cerebellar hemisphere, lies on the lateral aspect of this region, where it forms the lateral boundary of the upper or pontine pari of the floor of the fourth ventricle. Its dorsal aspect is connected with that of its fellow of the opposite side by the superior medullary velum l and ventrally it sinks into the upper region of the dorsal part of the pons.

The pontine nuclei of the fifth cranial nerve are motor and sensory. The motor and main sensory roots are pontine, and the sensory root is prolonged down as the spinal tract, and up as the mesencephalic root.

The motor nucleus is situated close to the superior peduncle of the cerebellum at the lower part of the lateral margin of the upper or pontine part of the fourth ventricle. It lies near the surface, and the axons of its cells form many of the fibres of the motor root of the nerve.

The main sensory nucleus is situated deeply on the outer side of the motor nucleus, and on the ventral aspect of the superior peduncle of the cerebellum. Some of the fibres of the sensory root ascend and terminate in arborizations around the cells of this nucleus.

The lower or spinal sensory nucleus succeeds to the main sensory nucleus, and is a continuation upwards of the substantia gelatinosa. It is elongated, and extends into the upper part of the spinal cord to about the level of the second cervical nerve. It lies on the medial side of the spinal or descending sensory root of the fifth nerve, and the fibres of that root (. spinal tract) terminate at intervals in arborizations around its cells.

The mesencephalic root arises from groups of small cells which are placed in the grey matter of the mid-brain, beside the aqueduct,


THE NERVOUS SYSTEM


1475

extending up as far as the canal of the lower end of the superior

olliculus. Fibres run down from this part, but their actual disposition n the fifth nerve is not yet settled.

There is some ground for supposing that this part of the nuclear arrangement of the fifth nerve is concerned with the reception of proprioceptive impulses from orbital muscles.

The posterior or medial longitudinal bundle has the same position in the upper region as it has in the lower region. It lies close to the median raphe, and immediately below the grey matter of the corresponding part of the floor of the fourth ventricle.

The main or medial lemniscus, like the main fillet in the lower region, lies in that portion of the upper region of the dorsal part of the pons which is near the ventral part, and it forms a layer of some breadth, extending outwards from the median raphe.

The lateral lemniscus is a strand of fibres which lies on the outer side of the medial fillet, and connects the cochlear nucleus with the opposite inferior corpus quadrigeminum. Associated with the lateral fillet, and lying between it and the medial fillet, there is a collection of nervecelis, called the nucleus of the lateral lemniscus.

Development of the Pons. —The pons is developed from the ventral and lateral walls of the metencephalon, which is one of the divisions of the rhombencephalon. The nuclear matter seems to be derived from the ponto-bulbar body, spreading over the surface of the neural tube.

3. The Cerebellum.

The cerebellum, or small brain, occupies the inferior occipital or cerebellar fossae of the occipital bone. It lies beneath the posterior parts of the hemispheres of the cerebrum, from which it is separated by a septum of the dura mater, called the tentorium cerebelli, and it is behind and above the medulla oblongata and pons. It is composed of white and grey matter, the white matter being situated in the interior, where it constitutes the medullary substance, and the grey matter being spread over the surface of the cortex. In appearance it is laminated or foliated, the laminae being separated from each other by parallel, slightly curved sulci. It is composed of two large lateral portions, called hemispheres, and a connecting median portion, termed the vermis, these parts being much more distinct below than above. When looked at from above it presents in the median line two notches, anterior and posterior. The anterior notch, which is wide, is known as the incisura semilunaris , and it contains the inferior pair of quadrigeminal bodies and the superior cerebellar peduncles. The posterior notch is narrow, and is occupied by the falx cerebelli. The most conspicuous sulcus of the cerebellum is the great horizontal fissure, which extends round the circumference, and passes for some distance into the interior. By means of this fissure the cerebellum is divided into two parts, upper and lower.

Relatively smaller in the new-born child, the cerebellum forms in the adult about an eighth of the whole mass of the brain.


I 47^


A MANUAL OF ANATOMY


The cerebellar surface is marked, as stated above, by the presence of numerous flattened or laminar gyri or folds, each fold being separatee from its neighbours by sulci of appreciable depth. Among these fissures are certain ones which are evident and deeper, and these car be taken to divide the surfaces into lobules or parts, which have some small descriptive value.


The presence of the foliated surface, and of certain striking appearances in different parts of the cerebellum, have led in the past to a wealth of terminology and description which, for the greater part, does not seem to be of much value or utility. Moreover, since these terms have in many instances come dowr from long past periods, they are archaic and fanciful. Thus it seems desirable to replace these with a short account of the cerebellar surface, broadly described, after which the older terms


Anterior Notch (Incisura Semilunaris)

Central Lobule

Anterior Crescentic Lobule

/

Primary Fissure

Posterior Crescentic Lobule


Postero-superior Fissure


f;A^SL Cor P° ,a Q ua drigemina


Postero-superior

Lobule


-Culmen Monticuli


.Preclival Fissure


Postclival Fissure


Postero-inferior Lobule


Clivus Monticuli


Posterior Notch Folium Cacuminis


Fig. 897.—The Cerebellum (Superior View).


and descriptions will be given in small print, for purposes of reference if required. Subsequently a short morphological consideration of the part can be added.

Upper Surface of the Cerebellum. —This surface presents in the median line the upper part of the vermis, known as the superior vermis (see Fig. 897). It extends from the incisura semilunaris to the posterior notch, and it forms a laminated elevation, which is higher in front than behind, the most prominent part being known as the monticulus cerebelli. On either side of the superior vermis the upper surface of each hemisphere inclines downwards to the circumference, and there is no distinct demarcation between it and the superior vermis.

This upper aspect is divided (Fig. 898) by two main fissures, primary and postclival, which are continued across the slight elevation of the superior vermis.


/



THE NERVOUS SYSTEM


x 4 77

The primary fissure (Fig. 898, PR) cuts across the vermis a little behind its highest point, the culmen. From this the fissure is continued with a slight forward curve on each side, to reach the horizontal fissure.

The fissure is termed ‘ primary ’ because it forms the posterior limit, at a fairly early stage, of the morphological entity, the anterior lobe.

The postlunate or postclival fissure (PC) is behind the primary fissure and below it; this is due to the descent of the vermis from the culmen, forming its ‘ declive * behind the primary fissure. The postclival fissure turns forward on each side with a bolder curve towards the horizontal fissure.


Fig. 898. —Upper Aspect of Cerebellum, with Main Subdivisions.

C, culmen; D, declive (or clivus); PR, primary fissure; PC, postlunate (or postclival) fissure; H, posterior end of horizontal fissure.

The anterior lobe lies above and in front of the primary fissure, including the parts of the vermis and lateral lobes as a continuous whole.

The posterior lunate lobe lies between the primary and postlunate fissures, and includes also the central vermis.

The posterior end of the horizontal sulcus (H) appears usually on this aspect of the cerebellum. It fails to reach the vermis, usually running into the postlunate sulcus.

The lobule which is seen on each side below the postlunate and above the horizontal fissure is frequently referred to as the superior crescentic lobule; it is also termed the superior (division of the) ansiform lobe.

The superior vermis is composed of five lobules, named, in order from before backwards, the lingula, central lobule, culmen monticuli, clivus monticuli, and folium cacuminis. The lingula is deeply placed, and consists of about four laminae or folia, which lie over the superior medullary



[478


A MANUAL OF ANATOMY


velum as it extends between the superior cerebellar peduncles. Its laminae may be continued on either side over the superior cerebellar peduncle, and, when this is so, the prolongation is known as the frenulum lingulae.

The central lobule is of small size, and lies at the bottom of the incisura semilunaris. It is separated from the lingula by the precentral fissure, and from the culmen monticuli by the postcentral fissure.

The culmen monticuli forms the summit of the superior vermis. It is composed of several laminae, and posteriorly is separated from the clivus by the preclival fissure.

The clivus monticuli represents the sloping part of the monticulus cerebelii. It is situated behind the culmen monticuli, and is composed of several laminae. Posteriorly it is separated from the folium cacuminis by the postclival fissure.

The folium cacuminis forms the posterior extremity of the superior vermis, and lies at the posterior notch, where it is placed above the great horizontal fissure.


i locculus


Biventral Lobule


Lobulus Gracilis 1 Great Horizontal

Fissure , I

t

Inferior Semilunar Lobule


Amygdala (Tonsil)


Pyramid Tuber Valvulae


-Pregracile Fissure — Mid-gracile Fissure —Post gracile Fissure -•-Small Horizontal Fissure


Fig. 899.— The Cerebellum (Inferior View).

1 he inferior semilunar lobule and the lobulus gracilis constitute the postero inferior lobule. Old terminology used.


The upper surface of each hemisphere is mapped out into lobules, which are continuous with the subdivisions of the superior vermis, with the exception of the lingula. These are called, in order from before backwards, the ala, anterior crescentic lobule, posterior crescentic lobule, and postero-superior lobule.

The ala is continuous with the central lobule, from which it is prolonged for a limited distance round the anterior part of the hemisphere in the region of the incisura semilunaris.

The anterior crescentic lobule is continuous with the culmen monticuli, and represents the anterior subdivision of the upper surface of the cerebellar hemisphere. It is limited posteriorly by a curved sulcus, called the anterosuperior fissure, which is continuous with the preclival fissure (fissura prima), and opens at the circumference into the great horizontal fissure. The right and left anterior crescentic lobules, together with the culmen monticuli, form the lobus culminis (see Fig. 897).

The posterior crescentic lobule is continuous with the clivus monticuli. It is limited in front by the antero-superior fissure, and behind by the postero-superior fissure, the latter being continuous with the postclival fissure, and opening at the circumference into the great horizontal fissure.








THE NERVOUS SYSTEM


1479


The right and left posterior crescentic lobules, together with the clivus monticuli, form the lobus clivi.

The postero-superior lobule (superior semilunar lobule) corresponds to, but is much more extensive than, the folium cacuminis. It is limited in front by the postero-superior fissure, and behind by the great horizontal fissure. The right and left postero-superior lobules, together with the folium cacuminis, form the lobus cacuminis.

Under Surface of the Cerebellum. —The under surface presents in

he median line a deep groove, called the vallecula, which is continuous Dehind with the posterior notch. Anteriorly it lodges the medulla Dblongata, and lying in the bottom of it there is the lower part of the /ermis, which is known as the inferior vermis. The vallecula separates the two cerebellar hemispheres from each other, and the inferior vermis s separated on either side from the corresponding hemisphere by a furrow called the sulcus valleculce. The under surfaces of the hemi


Fig. 900.—Lower Aspect of Cerebellum, showing the (Inferior) Ansiform Lobe and the Paramedian Lobule or Tonsil; also the Flocculus.


spheres are markedly convex, and are received into the inferior occipital or cerebellar fossae of the occipital bone.

The inferior aspect of the cerebellum (Fig. 900) presents on the lateral lobes, near the margins, the greater part of the horizontal fissure (H). The two additional fissures shown in the figure are of quite secondary importance; the upper one of the two was taken formerly as the lower boundary of the ‘ inferior crescentic lobule (ISL.), but it is not necessary now to subdivide the inferior surface in this way, but rather to term all this curved surface the posterior or posteio-inferior lobe, or the inferior (part of the) ansiform lobe. #

The paramedian lobule or tonsil, however, stands out as a striking formation on each side of the ‘ posterior notch, and is not included in the name given to the rest of the inferior surface. The base of the tonsil is received in a cup-shaped concavity on the inferior and medial surface of the ansiform lobe, from which it is separated by a deep retrotonsillar fissure. The loosely foliated flocculus (FLOCC) is





1480


A MANUAL OF ANATOMY


visible on each side, outside and in front of the tonsil. The flocculus has a white ' stalk/ which is continuous with the inferior medullary vellum, making with this a large part of the bed of the hollow which contains the tonsil. This velum is connected centrally with the

nodule/ the terminal piece of the inferior vermis. The inferior vermis is composed of four lobules, named, in order from behind forwards, tuber valvulse, pyramid, uvula, and nodule.

The tuber valvulae (tuber posticum) forms the posterior part of the inferior vermis, and is composed of several laminae. On either side it is prolonged into the corresponding hemisphere, and becomes continuous with the postero-inferior lobule. It is the only part of the inferior vermis which is prolonged into the cerebellar hemispheres.

The pyramid is situated in front of the tuber valvulae. It presents about four laminae, and is separated by deep sulci from the tuber valvulae behind and the uvula in front, whilst on either side it is separated from the cerebellar hemispheres by the sulcus valleculae. Laterally it is connected with the biventral lobule of the hemisphere by means of a faint ridge, but this lies low in the sulcus valleculae as it crosses.

The uvula is situated in front of the pyramid, and between the amygdalae or tonsils of the hemispheres. It is triangular, the base being directed backwards, and it consists of several laminae. It is separated on either side from the hemisphere by the sulcus valleculae. Laterally its narrow part is connected with the amygdala by a ridge of grey matter, but this lies low in the sulcus valleculae as it crosses. This ridge is notched at intervals, and is called the furrowed band.

The nodule forms the anterior part of the inferior vermis, and is composed of several laminae, which are largely concealed by the uvula. It is connected on either side with the flocculus by a thin semilunar band of white matter, which is the lateral portion of the inferior medullary velum.

The fissures of the inferior vermis are three: postpyr amidal, between the pyramid and the tuber valvulae; prepyramidal (fissura secunda), between the pyramid and the uvula; and postnodular, between the nodule and the uvula.

The under surface of each hemisphere is mapped out into four lobules, which are called, from behind forwards, the postero-inferior lobule, the biventral lobule, the amygdala, and the flocculus.

The postero-inferior lobule is situated at the back part of the under surface of the hemisphere. It is divided into four curved parts by three curved Assures. The anterior two parts are known as the lobulus gracilis, and the posterior two as the inferior semilunar lobule. The right and left postero-inferior lobules, together with the tuber valvulae, form the lobus tuberis.

The biventral lobule is composed of curved laminae, and is somewhat triangular. The pointed end is directed backwards and inwards, and it is connected with the pyramid by a faint ridge, which lies low in the sulcus valleculae. The base is directed forwards towards the flocculus. The lobulus gracilis lies external to it, and the amygdala is on its inner side. The biventral lobule is divided by a sulcus into two portions, outer and inner; hence the name ‘ biventral.’ The right and left biventral lobules, together with the pyramid, form the lobus pyramidis.

The amygdala (tonsil) forms a conspicuous prominence between the uvula and the biventral lobule. It is situated in a depression of the vallecula, which is known as the nidus avis (‘ bird’s nest ’), and its long axis is almost sagittal. It is connected with the narrow part of the uvula by the furrowed band in the sulcus valleculae. The right and left amygdalae, together with the uvula, form the lobus uvulae.

The flocculus (subpeduncular lobule) is a small irregular lobule which


THE NERVOUS SYSTEM


1481

is situated between the front of the biventral lobule and the middle peduncle of the cerebellum. Internally it is connected with the nodule by the lateral portion of the inferior medullary velum. The right and left flocculi, together with the nodule, form the lobus noduli.

In the foetus a structure, known as the paraflocculus, lies behind and to the outer side of the flocculus, and occupies a depression in the petrous bone. In the lower monkeys it persists throughout life, but in man it atrophies after birth.

The fissures of the under surface of each hemisphere are: (1) a continuation of the postnodular sulcus, between the biventral lobule and the flocculus; (2) a continuation of the prepyramidal sulcus, between the am yfl ( lala an< d the biventral lobule; (3) the pregracile, or anterior arcuate sulcus, between the biventral lobule and the lobulus gracilis; (4) the midgracile, or middle arcuate sulcus, within the lobulus gracilis; (5) the postgracile, or posterior arcuate sulcus, between the lobulus gracilis and the inferior semilunar lobule; and (6) the small horizontal sulcus within the inferior semilunar lobule.

Cerebellar Morphology.—Extensive examination of the types and varieties of the cerebellum found in different classes of animals has gradually established the fundamental parts of this organ. It has )een shown to consist essentially of three lobes—anterior, middle, and bosterior—of which the anterior is the most primitive, the middle and posterior appearing in higher forms; in birds both these are present, md in mammals reach a more extensive development, while in man md the higher mammals the middle lobe reaches its most expanded

orm, varying much in the different orders of mammals below these, [t may be added that the vermis is to be looked on as a more primitive md older part of the organ than the lateral lobes, in which the paired donations exhibit much variety.

The anterior lobe is represented in man by that portion of the cerebellum lying above and in front of the primary fissure; this has received ts name from this relationship, and various names have been given

o the part thus marked off, known now as the anterior lobe. The obe includes the vermis in this part, as far back as behind the culmen, vhere the fissure cuts through it.

The middle lobe is a simple lens-shaped formation (Fig. 901 a), as

een on the surface in lower vertebrates. In the mammals, however, t is found to present a simple transverse bar immediately behind the brimary fissure, but behind this it shows medial and lateral parts, of vhich the lateral portions exhibit (Fig. 901 a) two main divisions—an ipper or anterior one, curved on itself, and hence termed the ansiform obule \ and a lower one (continuous with the ansiform lobule) placed beside the median formations, and hence named the paramedian obule.

The posterior lobe is also composed of a median part (posterior nedian lobule) and two lateral portions; these consist on each side of 1 flocculus and paraflocculus, as illustrated in the figure.

The human cerebellum possesses a relatively simple anterior lobe, is pointed out above. There is also a recognizable ‘ transverse bar,’ narking the upper portion of the middle lobe, in the so-called ‘ lobulus Implex ' (posterior lunate lobe or posterior crescentic), which includes


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A MANUAL OF ANATOMY


the central declive. It is behind this that the middle lobe is particularly concerned in forming the greater part of the human lateral lobe from the ansiform lobule on each side, while the posterior lobe remains centrally, but degenerates in part in its lateral portions.


Fig. 901A.—Illustrations of Cerebellar Structural Morphology, based

on Figures by Ingvar.

1, higher reptile; 2, bird; 3, mammal; A, M, P, anterior, middle, and posterior lobes; ANS, PM, ansiform and paramedian lobules; F, PF, flocculus and paraflocculus.

The schematic drawings in Fig. 901B may make this transformation clearer. In the first scheme the recognition of the fundamental parts, as already described, is evident and straightforward; the anterior lobe (A) is separated by the primary fissure from the ‘ lobulus simplex,' marked by the upper M; the lower M indicates the lower portion of the


Fig. 901 b.— To illustrate Hypothetical Stages in the Evolution of

Form of Human Cerebellum.

(References as in previous figure.)


middle lobe, with its two lateral pieces consisting of ansiform (ANS) and paramedian (PM) lobules. The overgrowth of the ansiform lobule accounts for the greater part of the lateral lobe (behind the lobulus simplex), as shown in the second figure, while the paramedian lobule remains as the ‘ tonsil.' The great ansiform enlargement is naturally






THE NERVOUS SYSTEM


1483

in a doisal and lateral direction mainly, so that the paramedian lobule is overlapped, and finds itself on the anterior aspect of the lower and median part of the enlargement.

The posterior lobe, stippled, is seen (as in the primitive forms, Fig. 901A) to have a central and two lateral pieces; these show floccular (F) and parafloccular (PF) enlargements. In the human foetus these ire represented, but the paraflocculus is lost, and the flocculus remains done in the adult condition, connected still with the median part of the posterior lobe.

When estimating the comparative values of the parts of the cerebellum, as above, it must be remembered that the growing thickness of the organ affects not only the lateral, but also the median parts; thus, the vermis is very thick in the middle lobe and fades rapidly in the bosterior lobe, so that this last is turned down and comes to look iownwards and forwards. 1 his is associated with the overgrowth bf the ansiform lobule, whence the paramedian lobule is visible from below and in front, and not from behind, as in the scheme; it is covered behind by the increasing growth of the ansiform lobule extending medially behind it and the buried posterior derivatives.

To sum up shortly: the lateral lobes of the cerebellum are, for their posterior, lateral, and greater part, overgrowths of the ansiform portion of the middle lobe, the tonsil being paramedian. Above this is another part (lobulus simplex, upper crescentic) of the middle lobe, separated by the primary fissure from the simple anterior lobe, the oldest lobe of the organ. The vermis is represented in both anterior and middle lobes as the central portion, thickened very much in these parts, but getting rapidly smaller (and hence reversed, as it were) in the posterior lobe. The flocculus is the remnant of the lateral portion of the posterior lobe.

The vermis in front of the primary fissure is the central part of the interior lobe, behind this fissure, down to and including the declive; t belongs to the upper portion (lobulus simplex) of the middle lobe,

he lower part of which includes the ‘ tuber vermis ' and ends at the prepyramidal fissure/ The central part of the posterior lobe includes

he pyramid, uvula, and nodule, and it is of interest to note that these barts are particularly connected (Holmes and Stewart) with the medial iccessory olive, the oldest part of the inferior olivary structures; the arge inferior olive of higher mammals has appeared with the lateral growth of the cerebellum in them, and in less direct connection with

he development of the cerebral cortex. Peduncles of the Cerebellum.—The peduncles are three in number Dn either side—superior, middle, and inferior—and they are composed bf fibres which enter or leave the central white medullary substance.

The superior peduncles (brachia conjunctiva) are largely composed bf efferent fibres, and are at first concealed from view by the upper br anterior portions of the hemispheres. After they leave the hemi

i 4 8 4


A MANUAL OF ANATOMY


spheres they pass upwards on the lateral aspects of the dorsal surface of the pons in a converging manner towards the quadrigeminal bodies (or colliculi of mid-brain). They form the lateral boundaries of the upper part of the floor of the fourth ventricle, and by their convergence they project slightly over that part of the ventricle, so as to take part in its roof. The superior medullary velum extends between the two peduncles, and closes the interval between them. On reaching the inferior pair of quadrigeminal bodies the two peduncles pass beneath them and enter the mesencephalon, where their course will be subsequently described (see p. 1553). Most of the fibres of each superior peduncle are derived from the corresponding nucleus dentatus, but a few come from the grey matter of the cerebellar cortex. In addition to these there are the fibres of the ventral (or indirect) cerebellar tract (of Gowers).

The middle peduncles are of large size, and are formed by the transverse fibres of the pons, these being gathered together on either side into a large bundle, which passes backwards and laterally into the white central medullary substance of the corresponding hemisphere. The fibres of each middle peduncle are both afferent and efferent. The afferent fibres arise in the pons from the cells of the nucleus pontis of the opposite side, and terminate in arborizations around the cells of the cerebellar cortex. The efferent fibres arise from the cells of the cerebellar cortex of the same side, and terminate in arborizations around the cells of the nucleus pontis, mostly on the opposite side.

The inferior peduncles are principally composed of afferent fibres, which are derived chiefly from the dorsal (or direct) cerebellar tract and the olivo-cerebellar tract of either side. The fibres of the dorsal cerebellar tract terminate in the cortex of the superior vermis on both sides of the median line. The fibres of the olivo-cerebellar tract , which are derived from the inferior olivary nucleus of the medulla oblongata on the opposite side, terminate in the cortex of the vermis and cerebellar hemisphere. The superficial arcuate fibres, which form part of the inferior peduncle, are connected with the cortex of the vermis and cerebellar hemisphere. There are also fibres connecting the vermis with the vestibular nerve, thus forming the direct sensory cerebellar tract (see p. 1623).

It should be noted that the inferior peduncle comes up from below to a position between the other two, the middle peduncle being external and the superior internal. Having reached this position, the inferior peduncle suddenly bends backwards and passes into the cerebellum.

White and Grey Matter of the Cerebellum.—In the hemispheres and vermis the white matter is situated in the centre as the medulla, and the grey matter is disposed superficially as the cortex. The white matter in the interior of the vermis is occasionally termed the corpus trapezoides. When sagittal sections of a hemisphere are made, the mass of white matter in the centre is seen to send offshoots into the lobules. From the sides of these offshoots secondary processes are given off, and these in turn furnish tertiary processes, the white


THE NERVOUS SYSTEM


1485

latter in all cases being covered by grey matter. When the section 5 made across the direction of the laminae or folia the appearance •resented is like the trunk and branches of a tree; hence the name rbor vitce cerebelli is applied to it (see Fig. 902).

Nuclei.—The corpus dentatum (Fig. 902) is a collection of grey latter which is situated within the white matter of each hemisphere, nd is very like the inferior olivary nucleus in the olivary body of the aedulla oblongata. It is composed if a wavy grey lamina, disposed n the form of a capsule, which ncloses white matter. The capsule >resents an opening or hilum at its ipper and inner part, and through his a large number of the fibres >f the superior cerebellar peduncle merge from the interior.

There are three other nuclei on

ach side as follows (1) the nucleus imboliformis, close to the inner

ide of the hilum of the corpus lentatum; (2) the nucleus globosus, nternal to the preceding ; and 3) the nucleus fastigii, or rooflucleus, situated in the vermis, dose to the median line, and contiguous to its fellow of the opposite dde.

Commissural and Association Fibres.—The commissural fibres pass

horn the white matter of one hemisphere to that of the opposite hemisphere. They traverse the vermis in two sets, superior and inferior. Die association fibres are confined to each side, and they connect idjacent laminae, passing across the bottom of the fissures which separate them.

Medullary Vela.—These are thin laminae or curtains of white matter, and are two in number, superior and inferior.

The superior medullary velum (or valve of Vieussens) is continuous with the white matter of the vermis. It extends between the converging superior cerebellar peduncles, bridging over the interval between them, and becoming continuous with their inner margins. Superiorly it extends to the inferior pair of quadrigeminal bodies, and inferiorly it passes into the corpus trapezoides or white matter of the vermis. It forms a large portion of the roof of the upper part of the fourth ventricle, and its dorsal surface supports the lingula of the superior vermis. From the upper part of the superior medullary velum a band of white fibres, called the frenulum veil, passes to the lower part of the median longitudinal groove which separates the lateral pairs of quadrigeminal bodies. Immediately below the inferior pair of quadrigeminal bodies the fourth pair of cranial nerves emerge from the superior medullary velum on either side of the frenulum veli. The


Fig. 902.—Anteroposterior Section through Lateral Lobe, showing Dentate Nucleus.


I486


A MANUAL OF ANATOMY


tract of Gowers (ventral spino-cerebellar tract), after having traversed the formatio reticularis of the medulla oblongata and the dorsal part of the pons, passes into the superior medullary velum, and then descends in the superior cerebellar peduncle to the cerebellum.

The inferior or posterior medullary velum is a thin lamina of white matter which consists of three parts—median and two lateral, right and left.

The median part supports dorsally the nodule of the inferior vermis. It is a prolongation of the white matter of the vermis, and lies on the upper or ventral aspect of the nodule, to which it is adherent. As it leaves the white matter it is contiguous to the superior medullary velum, but the two laminae take different directions. As they diverge they make the cerebellar recess or apex of the roof of the fourth ventricle.


Layer of Purkinje's Cells


Fia Mater


_ Molecular Layer


Granular Layer


White Medullary Substance


Fig. 903. —Structure of a Lamina ■the Cerebellum (magnified).


of


The superior medullary velum passes upwards between the dorsal parts of the superior peduncles of the cerebellum. The median part of the inferior medullary velum passes ventralwards and then downwards. It is succeeded in a downward direction by the ependymal epithelium and pia mater (tela chorioidea inferior), which form a large portion of the lower part of the roof of the fourth ventricle. The middle part of the inferior medullary velum forms the upper portion of


the lower part of the roof of the fourth ventricle.

Each lateral part of the inferior medullary velum extends laterally to the corresponding flocculus in the form of a semilunar band. The ventral surface of this band is directed towards the fourth ventricle, and the dorsal surface is related to the amygdala or tonsil. One border of the band is free and concave, whilst the other is continuous with the white matter of the corresponding cerebellar hemisphere.

The inferior medullary velum forms a part of the lobns noduli, the other parts being the nodule and the two flocculi.


Minute Structure of the Cerebellar Laminae. —Each lamina or folium of the cerebellum consists of (1) a central part or core of white matter, which is an offshoot from the white medullary substance; and (2) an external part or cortex of grey matter.

Grey Cortex. —The grey cortex is composed of two layers, an outer molecular layer and an inner granular layer. Between these two layers there is a stratum of characteristic large cells, called the cells or corpuscles of Purkinje.

The cells of Purkinje are pyriform or flask-shaped, and are situated, as just stated, between the molecular and granular layers. The narrow or superficial end of each cell projects into the molecular layer, and the broad or deep end rests








THE NERVOUS SYSTEM


1487


1 the granular layer. From the broad or deep end of each cell a single axon isses off, which enters the granular layer, where it soon becomes medullated, id then forms a nerve-fibre of the white medullary substance. The axon of urkinje’s cell gives off a few collateral recurrent branches, some of which end 1 the granular layer, whilst others enter the molecular layer.

From the narrow or superficial end of each cell one or two dendrons are given ff. These divide and subdivide at frequent intervals in the molecular layer like the antlers of a deer.' The dendritic processes so formed are arranged 1 an arborescent manner, and are distinct from those of adjacent cells. They ermeate the molecular layer as far as the surface.

The molecular layer consists of a few nerve-cells and many nerve-fibres.

The nerve-cells are situated partly in the inner or deep portion of the lolecular layer, and partly in its outer portion.

The inner cells are known as the basket-cells, and they lie in the vicinity of tie cells of Purkinje. Each basket-cell has several dendritic processes which imify in all directions. In addition to these processes there is an axon, which prings from the side of the cell and takes a transverse course. It gives off a umber of collaterals which pass towards the bodies of the cells of Purkinje. 'hese collaterals terminate by ramifying very freely around the cells of Purkinje s well as around the axons of these cells for a short distance. The minute srminal ramifications form a close basket-work, which encloses the ceil of hirkinje and its axon for a short distance.

The outer cells of the molecular layer are small, and each has several denritic processes and an axon. Each axon springs from the side of the cell, nd, taking a transverse course, it ends in numerous ramifications.

The fibres of the molecular layer are derived from the following sources: 1) The dendritic processes, and the recurrent collaterals of the axons of the ells of Purkinje; (2) the dendritic processes and axons of the outer cells; (3) the .endritic processes of the inner or basket cells; (4) the axons of the granule-cells >f the granular layer; (5) the fibres of Bergmann, which represent the processes I glia-cells in the granular layer; (6) the dendrons of the cells of Golgi; and 7) some fibres from the white medullary substance of the lamina.

The granular layer consists of (1) nerve- and glia-cells, and (2) fibres.

The nerve-cells are of two kinds—namely, granule-cells and cells of Golgi.

The granule-cells are small and very numerous. They are closely packed ogether, and impart to this layer a granular appearance. Each has several [endrons and one axon. The dendrons soon ramify, and the dendritic processes erminate in minute clusters within the granular layer, which are closely related o the granule-cells. The axon of each granule-cell passes into the molecular ayer, where it ramifies, its branches diverging and being closely related to the [endritic processes of the cells of Purkinje.

The cells of Golgi lie near the cells of Purkinje, and are larger than the granuleells. They are stellate, and each has several dendrons and an axon. The lendrons enter the molecular layer, in which they ramify. The axon ramifies r ery freely in the granular layer. The cells of Golgi may be regarded as associaion cells.

The glia-cells are situated close to the cells of Purkinje, and lie between the mter granule-cells. The superficial processes enter the molecular layer, and onstitute the fibres of Bergmann, which pass as far as the pia-matral covering if the lamina. Their deep processes pass between the granule-cells of the

ranular layer, and some of them enter the white medullary substance. The fibres of the granular layer are derived from the following sources: 1) The axons of the cells of Purkinje; (2) the moss-fibres of Cajal; (3) the denIritic processes of the granule-cells; (4) the ramifications of the axons of the

ells of Golgi; (5) some of the deep processes of the glia-cells; and (6) some fibres rom the white medullary substance.

White Matter.—The white matter of a cerebellai lamina is an >ffshoot of the principal white medullary substance, and composed


A MANUAL OF ANATOMY


1488

of nerve-fibres. (1) Some of these are the axons of the cells of Purkinje, and these enter the white matter. (2) Others pass through the granular layer into the molecular layer, where they divide into branches which are closely related to the more deeply placed dendritic processes of the cells of Purkinje. (3) A third set terminate in the granular layer, where they divide into branches which present moss-like swellings, furnished with short delicate filaments. These fibres are known as the moss-fibres of Cajal.

Development of the Cerebellum. —The cerebellum is developed from the dorsal laminae of the metencephalon, where this forms the front limb of the pontine flexure. In its growth it extends into the roof-plate.


4. The Fourth Ventricle.


The fourth ventricle (ventriculus quartus) is situated behind (1) the upper or open half of the medulla oblongata or bulb, and (2) the pons. It has two walls—ventral and dorsal.

Ventral or Anterior Wall.—This wall is usually referred to as the floor. It is formed by (1) the dorsal surface of the upper or open

half of the bulb, and


Fig. 904.— Diagram to show the Composition of Floor and Roof of Fourth Ventricle.


SMV, IMV, upper and lower medullary vela.


(2) the dorsal surface of the pons between the converging superior peduncles of the cerebellum. In shape it is rhomboidal, the bulbar and pontine parts being triangular and having their bases applied to each other. The floor is sometimes spoken of as the fossa rhomboidea (see Fig. 892).

The lower end is tapering, and lies between the clavae of the funiculi graciles. In this situation the cavity of the ventricle is continuous with the central canal of the spinal cord after that canal has


traversed the lower or closed half of the bulb. The upper end is somewhat tapering, and lies between the converging superior peduncles of the cerebellum. In this situation the cavity of the ventricle is continued into the aqueduct (of Sylvius), which traverses the mesencephalon and opens superiorly into the third ventricle.

The floor is widest across its centre, which is on a level with the upper ends of the ' restiform bodies ’ of the bulb. The cavity of the ventricle is here prolonged on either side round the outer aspect




THE NERVOUS SYSTEM 1489

[ the corresponding restiform body towards the olive. This probation is known as the lateral recess.

An opening in the lateral part of this recess, involving the wering pia mater and opening into the subarachnoid space, is known 5 the lateral aperture of the fourth ventricle.

The floor is covered by a thick layer of grey matter, which is mtinuous with the central grey matter of the lower or closed half [ the bulb. This grey matter is covered by ependyma, the epithelial ills being continuous with those which line the central canal of the unal cord.

The floor is traversed in the median line by a slight longitudinal roove, which divides it into two symmetrical longitudinal halves.


Fig. 905.—Dorsal View of Mid- and Hind-Brains with Thalamus

(Cerebellum Removed) .

iach half is crossed at its widest part by bundles of white fibres, ailed auditory stricB. They wind round the upper part of the restiorm body, and pass transversely across the corresponding half of he floor as far as the median longitudinal groove, into which they ink. They belong to the cochlear division of the auditory nerve, in onnection with which they will be described.

The floor is divided into two parts, lower and upper, by the striae

if either side.

Lower or Bulbar Part.— The bulbar part is formed by the dorsal urface of the upper or open part of the bulb. It is bounded on either ide by fi) the clava of the funiculus gracilis, (2) the cuneate tubercle if the funiculus cuneatus, and (3) the restiform body, in this order

94






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A MANUAL OF ANATOMY


from below upwards. The lower end constitutes, as stated, a tapering point which carries a small ridge of grey matter, the obex, at its extremity. The bulbar part is traversed in the median line by a longitudinal groove, already referred to, and this groove subdivides it into twc symmetrical halves.

Immediately below the auditory striae on either side, and not far from the median longitudinal groove, there is a small triangular depression, known as the fovea inferior. Its apex extends to the striae and the lateral angles of its base are prolonged downwards as two grooves—inner and outer. The inner groove passes in a somewhat curved manner towards the point of the lower end, and the outer groove passes downwards and outwards towards the lateral boundary.


Between these two diverging grooves there is a triangular area, called the vagal triangle. Its apex is at the fovea inferior, and its base is directed downwards and outwards. It has a dark colour, and from this circumstance it is known as the ala cinerea. Deep to it there is the dorsal nucleus of the vagus and glosso-pharyngeal nerves (Fig 905).

A second triangular area, called the hypoglossal triangle, is situated between the median longitudinal groove and the medial of the two grooves prolonged from the angles of the base of the fovea inferior. Its base is directed upwards towards the striae, and its apex downwards towards the lower point. The area is slightly elevated, and is associated with the lower part of the eminentia medialis. Subjacent to this area is the upper part of the nucleus of the hypoglossal nerve.










THE NERVOUS SYSTEM 1491

A third triangular area, called the vestibular area or triangle, is ituated between the lateral boundary of the floor and the outer of he two grooves prolonged from the angles of the base of the fovea iferior. Its base, like that of the hypoglossal triangle, is directed pwards, and is continued into an eminence, over which the auditory triae pass. This eminence is known as the vestibular area or tubercle, ubjacent to the vestibular area and tubercle there is the dorsal or hief terminal nucleus of the vestibular division of the eighth nerve.

Upper or Pontine Part.—The pontine part of the floor is formed y the dorsal surface of the pons between the converging superior •eduncles of the cerebellum, which constitute its lateral boundaries, ts upper somewhat tapering end adjoins the lower end of the aqueduct, ike the bulbar part, it is traversed in the median line by a longiudinal groove, already referred to, which subdivides it into two ymmetrical halves.

Above the auditory striae, and in line with the fovea inferior, here is a slight depression, called the fovea superior, the two foveae >eing separated from each other by the vestibular tubercle. Between he fovea superior and the median longitudinal groove there is a ^ell-marked prominence, called the eminentia medialis. Deep to his eminence immediately above the striae acusticae, is the ibducent nucleus, or nucleus of the sixth cranial nerve, and the minentia medialis is really formed by fibres of the seventh nerve curvound the dorsal surface of the sixth nucleus just deep to the floor )f the ventricle. For this reason an alternative name for the eminence s the colliculus facialis. The eminence is continued downwards nto the trigonum hypoglossi, and superiorly it extends towards the ower end of the aqueduct of the mid-brain. Extending upwards rom the fovea superior towards the region of the lower end of the iqueduct there is a slight depression, known as the locus cseruleus, vhich has a dark grey or somewhat blue colour. This colour is due

o a subjacent group of deeply pigmented nerve-cells, known as the substantia ferruginea. This group may belong to the chief motor lucleus of the fifth cranial nerve, or it may be a terminal nucleus for some of the sensory fibres of that nerve.

Dorsal or Posterior Wall of Fourth Ventricle.— this wall is usually eferred to as the roof, and it is divisible into two parts—upper and ower.

The upper part is formed chiefly by the superior or anterior medulary velum, which extends between the inner margins of the dorsal ispects of the superior peduncles of the cerebellum. It is also formed

o a certain extent by these peduncles as they converge and slightly overhang the angular space between them. I he lower part of the roof s formed, from above downwards, by (1) the inferior or posterior nedullary velum, and (2) the ependymal epithelium of the ventiicle, covered by pia mater. The inferior medullary velum is separated from

he superior velum by the recess (Fig. 9 ° 4 )> within which the cerebellum ictually forms a part of the roof; the inferior velum terminates in a free


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A MANUAL OF ANATOMY


margin. Beyond this free margin there is the ependymal epitheliun of the ventricle, covered, as stated, by pia mater. This portion o pia mater is called the tela chorioidea inferior.

The epithelial part of the roof presents superficially threi laminae of white nervous matter—namely, the obex and thi ligulae. The obex is a thin triangular lamina which is situatec at the lower point of the ventricle, being attached laterally t( the diverging clavae. The ligulce are right and left. Each is c narrow band, which is continuous interiorly with the obex It is attached inferiorly to the clava and the cuneate tubercle It then passes transversely outwards over the dorsal aspect of the restiform body. The transverse part of the ligula forms the lowei boundary of the lateral recess of the ventricle.


Fig. 907. —The Fourth Ventricle and Right Hemisphere of the Cerebellum (in Section) (Hirschfeld and Leveille).


In the lower part of the roof of the fourth ventricle, below the lower limit of the inferior medullary velum, there is a perforation through the pia mater and ependyma, known as the foramen of Magendie. This, situated in the mid-line, forms an opening between the fourth ventricle and the subarachnoid space, and with the lateral recesses allows the cerebro-spinal fluid to pass from the interior to the surface of the brain. Its modern name is median aperture of fourth ventricle.

Choroid Plexuses of Fourth Ventricle.—These are two in number,

right and left. Each is a longitudinal inflexion of the pia mater which forms the tela chorioidea inferior, and it invaginates the ependymal epithelium of the lower part of the roof of the ventricle, by which it is covered on its ventricular surface. Each choroid plexus consists of two parts—longitudinal and transverse—and the two plexuses are disposed thus: ] |". The longitudinal parts lie on either






THE NERVOUS SYSTEM


1493


ide of the median line, and extend upwards from the region of the oramen of Magendie. Each transverse part extends outwards into he corresponding lateral recess of the ventricle

Development of Fourth Ventricle.—The lower or bulbar part is developed from the myelencephalon, and the upper or pontine part is developed from the metencephalon, these being the two divisions of the rhombencephalon. The cavity is the cavity of the neural tube, dilated and made diamond-shaped as a result of the formation of the pontine flexure (p. 57).


TELENCEPHALON.

Cerebral Hemispheres.

The cerebral hemispheres are right and left. Each is semi-ovoid, md presents two extremities and three surfaces.

The extremities are anterior and posterior. The anterior is thick md round, and its most projecting part is called the frontal pole, rhe posterior extremity is narrow and pointed, and its most projecting Dart is called the occipital pole. The surfaces are lateral, medial, and inferior. The medial surface is convex, in adaptation to the concavity Df the cranial vault. The lateral surface is flat and vertical, and it forms the lateral boundary of the great longitudinal fissure. For the most part it is in contact with the falx cerebri. The inferior surface is irregular, being adapted to the corresponding lateral divisions of the anterior and middle fossae of the interior of the base of the skull and the upper surface of the tentorium cerebelli. It is crossed transversely by a deep cleft, representing the stem of the lateral fissure.. The portion in front of this fissure is known as the orbital area, and is concave, in adaptation to the convexity of the orbital plate of the frontal bone, upon which it rests. The extensive portion behind the stem of the lateral fissure is known as the tentorial area, and is prominent and arched. Its anterior portion is received into the lateral division of the middle cranial fossa, and its posterior portion rests upon the tentorium cerebelli.

The borders of each hemisphere are four—supero-medial, mferolateral, superciliary, and internal occipital. The supero-medial border separates the lateral from the medial surface. The infero-lateral border separates the lateral surface from the tentorial area of the inferior surface. The superciliary border separates the front part of the lateral surface from the orbital area of the inferior surface. The internal occipital border separates the medial surface from the tentorial area of the inferior surface, and it extends from the occipital pole to the splenium of the corpus callosum.

The exterior of each hemisphere is broken up into tortuous eminences, called gyri or convolutions, and these are separated from each other by clefts, called sulci or fissures. The exterior is composed of grey matter,


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A MANUAL OF ANATOMY


which is spoken of as the cerebral cortex, and the interior is occupied by white matter, which forms the medullary centre. The breaking up of the hemispheres into gyri, with the intervening sulci, greatly increases the amount of cerebral cortex, and to a proportionate extent of pia mater.

It is sometimes the practice to distinguish between fissures and sulci of the brain, though many regard it as a refinement. If it is necessary, the fissures may be defined as clefts which either pass from one surface of the brain to another or, if they do not do that, cause an elevation in the wall of the lateral


biG. 908.— Views of Brain, not showing Convolutions. A, from above; B, from left; C, from below; D, from behind.


ventricle. The difficulty is that, with this definition, the same depression is sometimes a fissure and sometimes a sulcus.

Subject to this explanation, the following clefts would rank as fissures and them claims will be dealt with as they are described: (1) Lateral, (2) central, .( 4 ) hippocampal, (5) calcarine, (6) collateral, (7) parieto-occipital. ^Mwessions which do not fulfil these requirements merely rank as sulci.

the fissures thus defined are deeper and more constant in arrangement than the sulci. 0

Each hemisphere presents six principal clefts, called interlobar, and by means of these it is divided into six lobes.





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Interlobular Clefts.

1. Lateral fissure [Sylvian). 4. Cingulate sulcus.

2. Central fissure [Rolando). 5. Collateral fissure.

3. Parieto-occipital fissure. 6. Circular or limiting sulcus.

Interlobular Fissures.—The lateral fissure (O.T., fissure of Sylvius), which is the first fissure to appear in the course of development, begins on the inferior surface of the hemisphere at the anterior perforated substance in a depression, called the vallecula cerebri (or Sylvii). From this point it passes horizontally outwards to the external surface of the hemisphere, where it divides into three diverging branches. It is a deep cleft, which is overhung posteriorly by the front part of the temporal lobe, and it separates the orbital surface of the frontal from



Fig. 909.—Lateral View of Left Hemisphere.

In this specimen the horizontal (AH) and ascending (AV) anterior rami arise separately from lateral fissure. The pars triangularis lies between them.

the temporal lobe. The posterior border of the small wing of the sphenoid bone faces the fissure, which lodges the middle cerebral artery. The limbs into which the fissure divides are anterior horizontal, ascending, and posterior horizontal. The anterior horizontal limb passes forwards into the frontal lobe, its length being about f inch. The ascending limb passes upwards and slightly forwards into the frontal lobe for about 1 inch, but its length is variable. The posterior horizontal limb is the longest and most conspicuous. It passes backwards on the external surface of the hemisphere for at least 2 inches, having poi tions of the frontal and parietal lobes above it, and the temporal lobe below it. Finally, it turns upwards into the parietal lobe for a very short

distance

The central fissure (see Fig. 909), also known as the central sulcus and fissure of Rolando , begins at the supero-medial border of the hemi

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A MANUAL OF ANATOMY


sphere a little behind its mid-point, and ends above the centre of the posterior horizontal limb of the lateral fissure. It does not usually open into this limb, but may do so. Superiorly the fissure in most cases intersects the supero-medial border to reach the medial surface of the hemisphere, upon which it passes backwards for a very short distance. The direction of the fissure is irregularly downwards and forwards over the external surface of the hemisphere, and it separates the frontal from the parietal lobe. It describes two bends. The upper genu has its concavity directed forwards, and is situated about the junction of the upper and middle thirds of the fissure. The lower genu has its concavity directed backwards, and is situated on a more anterior plane than the upper genu. Below the lower genu the direction of the fissure is almost vertical, with a slight inclination backwards. The fissure is sometimes interrupted.


Central Sulcus


Medial Frontal Gyrus


Cingulate Gyrus

Callos. Sulcus •


--- Paracentral Lobule


Cingulate

Sulcus

Suprasplenial Fissure Parieto-occ.

Fissure Cuneus


Calcarine

Fissure


Uncus


Rhinal Sulcus


, Lingual Gyrus

Collateral Fissure

\\ Occipito-Temp. Gyrus ' Hippocampal Gyrus Inf. Temp. Gyrus


Fig. 910.—Medial Aspect of Right Hemisphere. Approximate position of lateral ventricle marked in blue.


The parieto-occipital fissure is situated about 2 inches behind the upper end of the central fissure, and separates the parietal from the occipital lobe. It is composed of two limbs, external and internal, which are continuous with each other at the supero-medial border of the hemisphere, where they form a right hngle. The external limb is situated on the lateral surface of the hemisphere, upon which it passes transversely outwards for about £ inch, when it is arrested by the convolution which connects the parietal and occipital lobes. The internal limb appears as a deep, almost vertical cleft on the medial surface of the hemisphere, which opens into the calcarine fissure a short distance behind the splenium of the corpus callosum (see Figs. 910 and 917).

The cingulate sulcus is situated on the medial surface of the hemisphere. It commences below the rostrum of the corpus callosum,


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near the anterior perforated area, and, bending round the genu, it passes backwards above the corpus callosum, from which it is separated by the cingulate gyrus. At a point a little behind the centre of the internal surface of the hemisphere it turns upwards, and terminates at the supero-medial border a short distance behind the upper end of the central fissure. The cingulate fissure lies between the frontal and limbic lobes, the medial frontal gyrus being above it and the cingulate gyrus below it (see Fig. 910).

The collateral fissure is situated on the inferior or tentorial surface of the hemisphere. It starts near the occipital pole, and extends forwards towards the temporal pole. Posteriorly it has the calcarine fissure above, and in line with it, and anteriorly the hippocampal gyrus holds this position on its medial side. It separates the temporal lobe from the hippocampal portion of the limbic lobe. The middle portion of the collateral fissure gives rise to the eminentia collateralis in the floor of the lateral ventricle.

The circular or limiting sulcus is situated deeply in the anterior part of the posterior horizontal limb of the lateral fissure. It almost surrounds the convolutions which constitute the insula, and is composed of three parts-—superior, inferior, and anterior. The superior part separates the insula from the frontal and parietal lobes, the inferior part separates it from the temporal lobe, and the anterior part separates it from the frontal lobe. The circular fissure is deficient in the region of the apex of the insula (see Fig. 920).

Lobes of the Cerebral Hemisphere—Frontal Lobe.—This is of large size. On the external surface of the hemisphere it is bounded behind by the central fissure and below by the posterior horizontal limb of the lateral fissure. On the inferior surface it is bounded behind by the stem of this fissure. On the internal surface it is bounded by the cingulate fissure. The frontal lobe has three surfaces—lateral, inferior, and medial.

Lateral Surface.—This surface presents three principal sulci—precentral, superior frontal, and inferior frontal.

The precentral sulcus is more or less parallel to the central fissure, the ascending frontal or precentral gyrus intervening between the two. It may be a single cleft, but it more frequently consists of two parts, superior and inferior. The superior part is usually joined above by the superior frontal sulcus. The inferior part passes superiorly into the middle frontal gyrus for a short distance in a forward and upward direction (see Fig. 909).

The superior and inferior frontal sulci extend forwards from the precentral sulcus.

The gyri of the external surface are as follows: precentral or ascendingfrontal, superior frontal, middle frontal, and inferior frontal (see Fig. 909) The ascending frontal or precentral gyrus (Fig. 909, A) is bounded behind by the central sulcus, and in front by the superior and inferior parts of the precentral sulcus. It extends from the supero-medial border of the hemisphere to a little behind the Sylvian point, which


A MANUAL OF ANATOMY


1498


corresponds to the place where the stem of the lateral fissure appears on the external surface of the hemisphere, and divides into its three branches. Below the lower end of the central fissure it is, as a rule, connected with the ascending parietal or postcentral gyrus by an annectant gyrus.

The superior or first , middle or second, and inferior or third frontal gyri

(C, D, E) are arranged in tiers, which are disposed antero-posteriorly, but the first and second often are subdivided, so as to make five tiers in

all. They are separated from the ascending frontal or precentral gyrus by the superior and inferior parts of the precentral sulcus.

The superior frontal gyrus is narrow, and lies between the supero-medial border of the hemisphere and the superior frontal sulcus. It is continuous with the medial frontal gyrus on the medial surface of the hemisphere, and is partially broken up into two parts, upper and lower.

The middle frontal gyrus, which is broad, is usually connected with the ascending frontal or precentral gyrus by an annectant gyrus. It is broken up anteriorly into two parts, upper and lower, by an anteroposterior secondary sulcus; and it is cut into behind by the upper portion of the inferior part of the precentral sulcus.

The inferior frontal gyrus lies below the inferior frontal sulcus, and in front of the lower part of the precentral sulcus. The anterior horizontal and the ascending limbs of the lateral fissure enter it and subdivide it into three parts—namely, pars orbitalis, pars triangularis, and pars basilaris, or, better still, orbital, frontal, and fronto-parietal opercala. The orbital operculum lies below the anterior horizontal limb of the lateral fissure; the frontal operculum is situated between the anterior horizontal and the ascending limbs of the fissure; and the fronto-parietal operculum is placed between the ascending limb of the fissure and lower part of the precentral sulcus. The inferior frontal gyrus is connected posteriorly with the lower end of the ascending frontal or precentral gyrus by an annectant gyrus.

Inferior or Orbital Surface of the Frontal Lobe.—This surface presents two sulci, olfactory and orbital (see Fig. 914).

The olfactory sulcus is parallel to the medial border, from which it is separated by the gyrus rectus. It lodges the olfactory tract and olfactory bulb. The orbital sulcus is of very variable form, but, as a rule, bears some resemblance to the letter )-(. It has, therefore, three limbs


Fig. 911.—The Left Cerebral Hemisphere (Superior Surface).

Red=frontal lobe. Orange=parietal lobe. Blue=occipital lobe.







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—medial, lateral, and transverse. The medial limb is separated from the olfactory sulcus by the medial orbital gyrus. The lateral limb is curved, and has external to it the orbital part of the inferior frontal gyrus. The transverse limb passes in a more or less curved manner between the other limbs.

The gyri of the orbital surface are: gyrus rectus, medial orbital gyrus, anterior orbital gyrus, lateral orbital gyrus, and posterior orbital gyrus.

The gyrus rectus lies between the olfactory sulcus and the medial border. The medial orbital gyrus is placed between the olfactory sulcus and the inner limb of the orbital sulcus. The lateral orbital gyrus is external to the other limb of the orbital sulcus. The anterior orbital gyrus is situated in front of the transverse limb of the orbital sulcus. The posterior orbital gyrus lies behind the transverse limb of the orbital sulcus.


Fig. 912.—The Right Cerebral and Cerebellar Hemispheres (Lateral

Surface).

Red=frontal lobe. Blue=occipital lobe.

Orange—parietal lobe. Green=temporal lobe.

Purple=cerebellar hemisphere.

Medial Surface of the Frontal Lobe.— The medial surface presents only one convolution, the medial frontal or marginal gyrus, which is situated between the supero-medial border of the hemisphere and the cingulate sulcus. It is continuous with the superior frontal gyrus, and anteriorly is broken up by one or two sulci. Its posterior part is almost completely detached, and forms the paracentral lobule, so named because it contains the upper end of the central fissure (Fig. 910).

Parietal Lobe.— This lobe lies between the large frontal and small occipital lobes, and above the temporal lobe. It is bounded anteriorly by the central fissure, which separates it from the frontal lobe. Posteriorly it is bounded by (1) the external parieto-occipital fissure, and (2) a line drawn across the external surface of the hemisphere from the extremity of this fissure towards the pre-occipital notch. on the inferolateral border of the hemisphere, from i-| to 2 inches in fiont of the occipital pole.


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A MANUAL OF ANATOMY


The parietal lobe has two surfaces—lateral and medial.

Lateral Surface.—This surface presents the following sulci: the intraparietal sulcus, composed of four parts; and the terminal portions of [a) the posterior limb of the lateral fissure, ( b ) the first temporal or parallel sulcus, and (c) the second temporal sulcus.

The inferior and superior postcentral sulci may be distinct, or continuous with each other. They lie behind the central fissure, with which they are parallel, and from which they are separated by the ascending parietal or postcentral gyrus (Fig. 909, B).


Cerebral Hemisphere


OccipitaL

Poie


_Frontal

Pole


Pons

—Medulla Oblongata


Fig. 913.— The Encephalon (Right Lateral View) (Hirschfeld

and Leveille).


1. Central Fissure

2. Posterior Horizontal Limb of Lateral Fissure

3. Ascending Limb of Fissure

4. Anterior Horizontal Limb of Fissure 5,5. Intraparietal Sulcus

6. Ramus Horizontalis


7. Ramus Occipitalis

8. Transverse Occipital Sulcus

9. Lateral Occipital Sulcus

10. External Occipito-parietal Fissure

11. Superior Temporal, or Parallel, Sulcus

12. Inferior Temporal Sulcus


The intraparietal sulcus is often in two parts, horizontal and occipital (see Fig. 913).

The ramus horizontalis passes backwards and slightly upwards from the upper end of the inferior postcentral sulcus. It has the superior parietal lobule above it, and the inferior parietal lobule below it.

The ramus occipitalis is usually continuous with the last branch, and passes back into the occipital lobe as the lower boundary of the arcus parieto-occipitalis.






THE NERVOUS SYSTEM


1501


The terminal portions of (a) the posterior horizontal limb of the lateral fissure, (b) the first temporal or parallel sulcus, and (c) the second temporal sulcus, are confined to the lower part of the external surface of the parietal lobe, where they lie in the order named from before backwards.

The gyri of the lateral surface are as follows: ascending parietal; superior parietal; and inferior parietal, with its supramarginal, angular, and postparietal gyri.

The ascending parietal or postcentral gyrus is situated immediately behind the central fissure, which separates it from the ascending frontal or precentral gyrus in front of that fissure. Posteriorly it is limited by the superior and inferior postcentral sulci. It extends from the supero-medial border of the hemisphere to the posterior horizontal limb of the lateral fissure, and it lies parallel to the ascending frontal or precentral gyrus, with which it is connected below the central fissure.

These two gyri, from their relation to the central fissure, are often spoken of by neurologists as the ‘ central gyri,’ though the name, if not clearly understood, is apt to lead to confusion with the gyri of the central lobe or insula.

The superior parietal lobule is situated between the ramus horizontalis and the supero-medial border of the hemisphere, where it is continuous with the quadrate lobule, or precuneus, of the internal surface. Anteriorly it is limited by the superior postcentral sulcus, round the upper end of which it is continuous with the postcentral gyrus. Posteriorly it is bounded by the external part of the parieto-occipital fissure, round the extremity of which it is connected with the occipital lobe by the arcus parieto


Fig. 914. —The Inferior Surface of the Left Cerebral Hemisphere, showing the Gyri and Sulci.


occipitalis.

The inferior parietal lobule is situated behind the inferior postcentral sulcus, and below the ramus horizontalis and ramus occipitalis. It is broken up into several gyri, three of which the supramarginal, angular, and postparietal—lie in this order from before backwaids. The supramarginal gyyus arches over the ascending extremity of the posterior limb of the lateral fissure. The angular gyrus arches over the







1502


A MANUAL OF ANATOMY


ascending extremity of the first temporal or parallel sulcus, and is continuous with the second temporal gyrus. The postparietal gyrus arches round the ascending extremity of the second temporal sulcus, and is continuous with the third temporal gyrus. These three subdivisions of the inferior lobule are sometimes described simpfy as anterior, middle, and posterior parts.

Medial Surface of the Parietal Lobe.—The medial surface is of quadrilateral outline, and constitutes the quadrate lobule or precuneus. It is bounded in front by the upturned posterior extremity of the cingulate sulcus, behind by the internal parieto-occipital fissure, and below by the suprasplenial sulcus and a portion of the gyrus cinguli (Fig. 916).


Corpus Callosum


Pineal Body Splenium


Anterior Pillar of Fornix Septum Lucidum


Genu passing into Rostrum


Corpora Quadri- g gemina


Anterior Commissure Optic Thai, and Connexus Thalamus Nerve Pituitary Body Tuber Cinereum Corpus Mamillare


Cerebellum


Fourth Ventricl


Third Nerve

1 1 Pons

i Crus Cerebri Aqueduct

Medulla Oblongata


fG. 915 .—The Medial Surface of the Left Cerebral Hemisphere

(Hirschfeld and Leveille).


Occipital Lobe.—This lobe lies behind the parietal and temporal lobes, and forms the posterior part of the cerebral hemisphere.

Laterally the lobe is bounded in front by the external parietooccipital fissure, and a line connecting this fissure with the pre-occipital notch on the infero-lateral border of the hemisphere. Medially it is bounded in front by the internal parieto-occipital fissure, which separates it from the quadrate lobule, or precuneus, of the parietal lobe. Inferiorly it is continuous with the temporal and hippocampal regions, but the separation may be indicated by a line connecting the preoccipital notch with the portion of the hippocampal formations which lie below the splenium of the corpus callosum, this portion being known as the ‘ isthmus.’



THE NERVOUS SYSTEM


1503


The occipital lobe is pyramidal, having an apex and three surfaces— lateral, medial, and inferior.

The apex forms the occipital pole of the cerebral hemisphere.

Lateral Surface (see Fig. 913).—This surface presents two sulci, transverse occipital and lateral occipital. The transverse occipital sulcus is formed by the bifurcation of the posterior end of the ramus occipitalis of the intraparietal sulcus, and it crosses the upper part of the occipital lobe obliquely. Its upper limb lies a little behind the external part of the parieto-occipital fissure, from which it is separated by a portion of the arcus parieto-occipitalis, and its lower limb is behind the postparietal gyrus. The lateral occipital sulcus is situated on the external surface of the occipital lobe, and extends almost horizontally from behind forwards. It divides the external surface of the lobe into two parts, upper and lower, which are connected with the


Gyrus Dentatus

Fig. 916.—The Medial Surface of the Right Cerebral Hemisphere

(Hirschfeld and Leveille).

parietal and temporal lobes by annectant gyri. These sulci and gyri are very variable in appearance.

Medial Surface.—On the medial surface is the calcarine fissure.

This is a deep cleft which starts on the internal aspect of the occipital pole in a bifurcated manner. It takes a curved course forwards, passing at first upwards and then downwards, and ends by reaching the hippocampal gyrus beneath the splenium of the corpus callosum. It is joined at a point anterior to its centre by the internal parietooccipital fissure, and between the two fissures is the cuneus. The calcarine fissure is composed of two parts: precalcarine, representing the portion in front of the internal part of the parieto-occipital fissure; and postcalcarine, representing the portion behind that fissure. The precalcarine fissure gives rise to the calcar avis, on the inner wall of the posterior cornu of the lateral ventricle (Fig. 917) The gyri of the internal surface are two in number—namely, the

cuneus and the gyrus lingualis.





1504


A MANUAL OF ANATOMY


The cuneus is triangular, and is wedged in between the posterior calcarine fissure and the internal parieto-occipital fissure. The gyrus lingualis (infracalcarine gyrus) is situated between the calcarine fissure above and the posterior part of the collateral fissure below. Anteriorly it becomes narrow, and joins the hippocampal gyrus. The lower portion of this gyrus is visible on the inferior surface of the lobe.

Inferior Surface.—The inferior or tentorial surface presents the posterior part of the occipito-temporal gyrus, medial to which is the posterior part of the collateral fissure, and internal to this again there is the lower portion of the gyrus lingualis (see Fig. 918).

Temporal Lobe.—The temporal lobe (see Fig. 913) is prominent, and of large size. It is situated below the posterior horizontal limb of the lateral fissure, and behind the stem of that fissure. Superiorly it is bounded by the horizontal portion of the posterior limb of the


Fig. 917.—Medial Aspect of Parieto-occipital Region of Left Hemisphere, to show Internal Parietal Occipital Fissure, Anterior and Posterior Calcarine.

Visual area coloured.

fissure, and a line prolonging this limb backwards to meet the anterior boundary of the occipital lobe. Anteriorly it is bounded by the stem of the fissure, which separates it from the orbital area of the frontal lobe. Posteriorly it is continuous with the occipital lobe, but the separation may be indicated by the following lines: externally by a line connecting the extremity of the external parieto-occipital fissure with the pre-occipital notch, and below and medially by a line connecting the pre-occipital notch with the splenium of the corpus callosum. Its medial surface above is separated from the hippocampal gyrus by the collateral fissure. The temporal lobe is somewhat pyramidal, the rounded apex being directed forwards. The apical part forms the temporal pole, and underlies the stem of the lateral fissure. The uncus of the hippocampal gyrus lies on its inner side, but on a more posterior level, and separated from it by the temporal sulcus.


THE NERVOUS SYSTEM


1505


The lobe presents three surfaces—superior, lateral, and inferior.

The superior or opercular surface is concealed within the lateral fissure, and is directed towards the insula.

The lateral surface has two horizontal sulci and three convolutions, the latter being disposed one above the other.

The sulci are called first and second temporal. The first temporal sulcus is parallel to the posterior limb of the lateral fissure, from which

ircumstance it is called the parallel sulcus. Starting near the temporal pole, it turns upwards posteriorly into the parietal lobe, where the angular ?yrus arches over it. The second temporal sulcus is parallel to the first, below which it lies, and it is usually broken up into two or more parts by annectant ^yri. Posteriorly it turns upwards into the parietal lobe, where the postparietal ^yrus curves round it.

The first temporal gyrus is situated uetween the posterior limb of the lateral ussure and the parallel sulcus. Posteriorly it is continuous with the infraparietal lobule. The second temporal ?yrus lies between the parallel and second temporal sulci. The third temporal gyrus lies below the second temporal sulcus, and posteriorly is con:inuous with the lower part of the ixternal surface of the occipital lobe.

On the inferior or tentorial surface )f the temporal lobe is the occipito:emporal sulcus and the occipito:emporal gyrus. The occipito-temporal iulcus extends from before backwards, ying near the infero-lateral margin of he hemisphere, and lateral to the colateral fissure (see Fig. 918). It is lsually broken up into parts by anlectant convolutions. The occipitotemporal gyrus is situated between the )ccipito-temporal sulcus and the collateral fissure, and extends from he occipital pole to the temporal pole. Lateral to the occipitoemporal sulcus there is the narrow inferior or tentorial surface of the hird temporal gyrus.

Insula (Island of Reil) (see Fig. 920).—This lobe is situated deeply vithin the lateral fissure, and is concealed from view by the opercular [yri, to be presently described. It is triangular, the apex being lirected downwards towards the vallecula cerebri and anterior per 95


Fig. 918.—The Inferior Surface of the Left Cerebral Hemisphere, showing the Gyri and Sulci.






1506


A MANUAL OF ANATOMY


forated area. The circular or limiting sulcus being here absent, the grey matter of the apex is continuous with that of the perforated area, this point being called the limen insulce. Elsewhere the island is surrounded by the circular or limiting sulcus, which has been already described. The insula presents several sulci, which diverge as the}/ pass from the apical region to the base, and these map it out into gyri. One of these sulci is known as the sulcus centralis insulae. It extends from the apex to the base in an upward and backward direction almost in line with the central fissure, and it divides the insula into two lobules, precentral and postcentral.

The precentral lobule is composed of three or four short gyri, called the gyri breves , which converge as they descend from the base, but they do not reach the apex or pole of the precentral lobule. The post


Fig. 919.—The Insula exposed by Removal of Opercula.

C, sulcus centralis insulae.


central lobule is formed by the gyrus longus, which is usually broken up into two gyri towards the base of the insula.

The direct internal or medial relation of the insula is the claustrum, internal to which there are, in succession, the external capsule, the nucleus lentiformis, the internal capsule, and the nucleus caudatus.

Opercula Insulae.—The parts of the cerebral hemisphere which bound the three limbs of the lateral fissure and overhang the insula are called; the opercula insulae. They are four in number—fronto-parietal, temporal, frontal, and orbital—and have been mentioned already (p. 1498).

Limbic Lobe.—This name was given in former times to a part of the brain, on its medial aspect, which included what is now known as the rhinencephalon, and also the cingulate gyrus.



THE NERVOUS SYSTEM


1507


The interrelations of the various parts will be considered more appropriately under the heading of rhinencephalon, but the preliminary description of these parts can be taken here, including that of the cingulate gyrus.

The gyrus cinguli arches round the corpus callosum. It begins at the anterior perforated substance below the rostrum of the corpus callosum, and it ends below the splenium of that body. Between these two points it pursues a semicircular course, passing forwards beneath the rostrum, upwards in front of the genu, backwards above the body of the corpus callosum, and finally, curving round the splenium, it is continued into the hippocampal gyrus through the isthmus. It is bounded superiorly by the cingulate sulcus, which separates it from the medial frontal gyrus (Fig. 921) and paracentral lobule, and posterior


Central Fissure


Fig. 920.— The Left Insula (Poirier, from Eberstaller). 1, 2, 3, gyri breves; 4, 5, gyri longi; X, limen insulae.


to the latter it is partially separated from the precuneus by the suprasplenial sulcus. The gyrus is separated from the corpus callosum by

the callosal sulcus.

The hippocampal gyrus, below the splenium of the corpus callosum, is joined above to the callosal gyrus by the isthmus , and behind and below it is continuous with the lingual gyrus (Fig. 921). As it passes forwards it has the hippocampal fissure above it, and the anterior part of the collateral fissure below it. Anteriorly , near the apex of the temporal pole and close behind the anterior perforated substance, it forms an enlargement, known as the caput gyri hippocampi, which is separated from the temporal pole by a slight fissure, called the incisura temporalis. From the caput a hook-like process, the uncus, passes backwards for a short distance above the anterior part of the hippocampal or dentate fissure. The caput represents the largely-developed



A MANUAL OF ANATOMY


1508

lobus pyriformis of many mammals, and it constitutes an olfactory centre of the cerebral cortex. Along with the uncus it forms part of the rhinencephalon or rhinopallium, a large part of the hippocampal gyrus belonging to the neopallium.

The incisura temporalis, which separates the caput gyri hippocampi from the temporal pole, represents the ecto-rhinal fissure , defining the well-developed rhinencephalon in some animals.

The cingulum, a narrow, tape-like band of white matter, is associated with the cingulate gyrus, and, according to Cajal, its fibres arise as the axons of cells of the gyrus, to the under surface of which the cingulum adheres. On entering the cingulum some fibres pass forwards and others backwards, whilst a few are


Central Sulcus


Rhinal Sulcus (Inc. temp.)


'Lingual Gyrus Collateral Fissure ' O’cipito-Temp. Gyrus

Hippocampal Gyrus Inf. Temp. Gyrus


Medial Frontal Gyrus


Cingulate

Gyrus


Callos. Sulcus

■= Paracentral Lobule


- Cingulate Sulcus

Suprasplenial Fissure Parieto-occ.

Fissure Cuneus

Calcarine Fissure


Fig. 921. Medial Aspect of Hemisphere, with Approximate Position of Lateral Ventricle represented in Colour.


described as branching into forward and backward branches. The anterior branches pass as far as the caput of the corpus striatum, where they are described as blending with the fibres which enter the internal capsule. Some may even pass to the cortex of the prefrontal region. The posterior branches turn round the splenium, and then lie upon the subiculum, or upper part of the hippocampal gyrus, as far forwards as the caput and uncus. The posterior fibres are described as ending in the cortex of (1) the subiculum hippocampi, and (2) occipital lobe. The cingulum belongs to the class of long association fibres.

Hippocampal or Dentate Fissure. —This fissure commences behind the splenium of the corpus callosum, where it is continuous with the callosal sulcus. It is directed forwards, lying between the gyrus denta



THE NERVOUS SYSTEM


1509


tus above and the hippocampal gyrus below, and it terminates within the uncus of the hippocampal gyrus.

The hippocampal fissure is a complete fissure. It appears in the course of the fifth week, and is parallel to the temporal portion of the choroidal fissure, below which it lies. The portion of the vesicular wall between these two fissures is the gyrus dentatus, and the portion below the hippocampal fissure forms the hippocampal gyrus.



Fig. 922.—To show the Arrangement of Structures below the Level of the Splenium.

F, fimbria and posterior pillar of fornix; D, dentate gyrus; CALC, beginning of calcarine fissure; COLL, collateral fissure; HG, hippocampal gyrus. The * band of Giacomini, continuous with the dentate gyrus, is shown at G crossing the base of the uncus. C, choroidal fissure ; Fiss, hippocampal fissure.


Fig. 923.—Outline of Section across Hippocampal Region.

Shows how the upper part of the region is bent on itself to make a prominence, the hippocampus, which projects in the ventricle (LV), while the thick lower part makes the hippocampal gyrus (H). The concavity of the upper bent part is provided by the hippocampal fissure (HF); the dentate gyrus (DG) is only a surface prominence on the part. The fimbria (F) is receiving fibres from the white covering of the hippocampus, known as the alveus (ALV). S, the tail of caudate nucleus.


The hippocampal fissure is associated with an internal elevation— namely, the hippocampus —on the wall of the descending cornu of the

lateral ventricle (Fig. 923). , ,

Gyrus Dentatus Fascia Dentata).— The gyrus dentatus is situated

above the hippocampal gyrus, and below the fimbria It is separated

from the hippocampal gyrus by the hippocampal fissure, and from

the fimbria by a slight groove, called the fimbno-dentate sulcus - The

dentate gyrus is narrow, and its free margin is indented or n ,

hence the name dentatus. It begins behind the splenium of the corpus

callosum, and is directed forwards above the hippocampal gyrus




i5io


A MANUAL OF ANATOMY


the curve of the uncus. Here it describes a bend, after which it emerges from the curve of the uncus, and, crossing the recurved part, is lost on its lateral aspect. This portion, the tail of the dentate gyrus , is often called the band of Giacomini.

Posteriorly it is continuous round the splenium with the rudimentary gyrus supracallosus, or indusium griseum, which contains the medial and lateral longitudinal strice of one-half of the upper surface of the corpus callosum.

Fimbria.—The fimbria is the prolongation of the posterior pillar of the fornix. It is situated above the gyrus dentatus, from which it is separated by the fimbrio-dentate sulcus. Posteriorly it turns upwards round the posterior extremity of the thalamus, and so becomes continuous with the posterior pillar of the fornix. Anteriorly it enters the uncus. It receives fibres along its length from the dentate gyrus and from the layer of white fibres ( alveus ) covering the ventricular surface of the hippocampus.

Development of the Cerebral Hemispheres. —Each hemisphere is developed from the wall of the cerebral vesicle, and is a hollow protrusion from the upper and lateral part of the telencephalon, the anterior subdivision of the prosencephalon. The anterior wall of that portion of the telencephalon which lies between the two cerebral vesicles is called the lamina terminalis.

The hemispheres grow out of proportion to the other parts of the encephalon in a forward, upward, and backward direction. Their backward growth is so great that they completely cover the other parts of the encephalon by the seventh month of intra-uterine life.

The sulci and gyri of the hemispheres first appear about the fifth month of intra-uterine life.

Development of the Insula and Lateral Fissure. —The insula, or island of

Reil, appears as the floor of a depression, called the lateral fossa, on the lateral aspect of the cerebral vesicle. The wall of this fossa becomes developed into the opercula insulce, and as these grow they cover the insula, and give rise to the limbs of the fissure. The insula is the superficial surface of the mass of the corpus striatum, which does not increase in surface area so quickly as the thin walls of the pallium round it, whence it is overlapped by these walls, which form the opercula.

Olfactory Lobe.

The olfactory formations, taken as a whole, are rudimentary in man. Although they are developments of the cerebral vesicles (with the exception of the olfactory nerves) they can be divided on each side for descriptive purposes into [a) external, lying apparently on the surface of the hemisphere-; and ( b ) internal, forming part of the hemisphere, on its medial aspect.

(a) The external formations comprise the olfactory bulb and tract, with the dispositions of the ‘ roots ' or * olfactory striae 5 and of the formations in their immediate neighbourhood.

The olfactory bulb is the enlarged anterior extremity of the olfactory tract. It is oval, and its upper surface is in contact with the orbital surface of the frontal lobe, whilst its lower surface rests upon one half of the cribriform plate of the ethmoid bone. The lower surface receives the olfactory nerves, which arise from the olfactory cells of the olfactory


THE NERVOUS SYSTEM


5*1

nucous membrane, and pass through the foramina of the cribriform date.

The olfactory tract is a white band which extends backwards from

he olfactory bulb, both of them occupying the olfactory sulcus on the medial part of the orbital surface of the frontal lobe. Posteriorly it divides into two roots, medial and lateral, which diverge and enclose between them the trigonum olfactorium.

The medial root passes medially and upwards in a curved manner to reach the subcallosal region. Some of its fibres pass into this area, and others enter the anterior extremity of the callosal gyrus.

The lateral root passes backwards and laterally over the outer part of the anterior perforated area, and enters the anterior part of the hippocampal gyrus.

The trigonum olfactorium is the area of grey matter which lies between the diverging medial and lateral roots of the olfactory tract. It is sometimes described as the middle or grey root of the olfactory tract.

The anterior perforated substance lies behind and between the diverging roots of the olfactory tract, and is limited behind and medially by the diagonal band, frequently not very well defined, which lies between it and the optic tract. At its anterior and medial end, where the olfactory roots are beginning to diverge, there may be a slight prominence, the olfactory tubercle. The perforations are made by central branches of the anterior and middle cerebral arteries.

The olfactory tubercle is, when present at all, a very small elevation. It represents the remnant of a large rounded mass which is found in macrosmatic brains, receiving an intermediate tract from the olfactory bulb.

The grey matter of the anterior perforated substance is continuous superiorly with the grey matter of the lentiform and caudate nuclei.

Development. —The olfactory lobe is developed from the antero-inferior part of the cerebral vesicle; an area is marked off by a groove, which deepens, and the area, growing, thus becomes a protrusion. This protrusion becomes solid, and gives rise to the olfactory tract and olfactory bulb.

Development of the Olfactory Apparatus.

This is developed in two parts—the olfactory lobe, and the olfactory epithelium. The olfactory lobe is intracranial, and is an outgrowth from the anterior part of the ventral aspect or floor of the telencephalon, which is the anterior


Fig. 924. —Plan of Structures Round Right Anterior Perforated Substance.

M, L, medial and lateral olfactory roots; D, diagonal band; T, olfactory tubercle. Bulb and tract are seen at B and TR.


1512


A MANUAL OF ANATOMY


subdivision of the prosencephalon or fore-brain. It constitutes the olfactorylobe (rhinencephalon) of the brain, and it becomes transformed into several parts, which will presently be stated, its terminal portion being the olfactory bulb, which rests upon one-half of the cribriform plate of the ethmoid bone.

The olfactory epithelium is intranasal, and occupies the upper part of the nasal fossa of either side. It represents a neuro-epithelium, which is derived from an invagination of the surface ectoderm. The axons of its sensory cells constitute the olfactory nerve-filaments, which pass upwards through the foramina of the cribriform plate, and enter the under surface of the olfactory bulb.

Olfactory Bulb. —The olfactory bulb appears as part of a hollow protrusion, slowly lengthening, of the anterior cerebral vesicle on its ventral aspect, and near its anterior part. The cavity of this protrusion, which is continuous with the lateral ventricle, soon undergoes obliteration, and the protrusion becomes solid. Its terminal extremity undergoes enlargement, and the entire protrusion becomes differentiated into the following parts: (i) the olfactory bulb; (2) the olfactory tract; (3) the inner or medial, and outer or lateral, olfactory roots; (4) the trigonum olfactorium. Of these parts, the olfactory bulb is the enlarged terminal extremity of the original protrusion, and rests upon one-half of the cribriform plate of the ethmoid bone, through the foramina of which half it receives the olfactory filaments, which are the axons of the sensory cells of the olfactory epithelium of the upper part of the nasal fossa.

Olfactory Epithelium. —The first indications of the olfactory organ are the two olfactory or nasal areas. They consist of thickened ectoderm, and are placed on the ventral aspect of the anterior cerebral vesicle on either side of the medial nasal process of the fronto-nasal process, and on the cephalic side of the orifice of the stomodaeum. Each olfactory area soon becomes depressed, and lies in the olfactory or nasal pit. The formation of the olfactory pits has been described on pp. 83 et seq.

The olfactory epithelium is deeply placed in the upper part of the nasal pit, in the roof of which the cribriform plate of the ethmoid bone will develop. The ectodermic cells of the upper part of the nasal pit constitute a neuroepithelium, and each cell is prolonged into a slender process, which is an axiscylinder process, or axon. These axons form the olfactory nerve-filaments, which are non-medullated, and they are connected with the olfactory area of the brain from an early stage. Within the olfactory bulb they break up into arborizations, which intermingle with the arborizations of the mitral cells of the bulb.

For the development of the organ of Jacobson and further details about nasal fossae, see pp. 1360 et seq.

(b) Internal Formations—Rhinencephalon.—The rhinencephalon is that part of the cerebral hemisphere which receives and relays olfactory impulses which have been transmitted to it through the olfactory roots. It is feebly developed in man. It includes (Fig. 925) the formations which make a ring round the passage into the cerebral vesicle, a ring which is closed in front by the olfactory roots. They are the uncus and caput hippocampi, the dentate gyrus, fimbria and fornix, and probably a large part of the hippocampal gyrus; the hippocampus (in the ventricle), and the continuity (fasciola cinerea or splenial gyrus) between the hippocampal and dentate formations and the indusium griseum, is carried over the front of the corpus callosum to join the subcallosal region. The medial olfactory root reaches the subcallosal region, the lateral root reaches the uncus, and the diagonal band stretches also between these two parts. All the.se structures are thus included in the rhinencephalon, and to them can be added the septum lucidum and the anterior commissure.


THE NERVOUS SYSTEM


1513


Many of these formations have been described (p. 1507) already; others will be described in their proper place, and the developmental aspect of the part will also be considered.

Morphologically considered, the cerebral hemisphere is composed of three parts—namely, the stem, rhinopallium, and neopallium. The stem or stalk is formed by the corpus striatum; the rhinopallium consists of the parts which compose the rhinencephalon; and the neopallium represents the remainder of the hemisphere.

Corpus Callosum.—The corpus callosum is the great neopallial commissure, and connects the two cerebral hemispheres. It is situated at the bottom of the great longitudinal fissure, and extends nearer to the front than back of the hemispheres. It is arched and thicker in front and behind than at the centre, its greatest thickness being posteriorly , where more fibres cross in it than elsewhere, on account of there being more of the hemisphere behind it than in front of it.


F IG . 925.— Plan of the Structures constituting the Rhinencephalon.


The superior surface is related to the falx cerebri, but is in contact nth it only posteriorly. It is covered by a thin layer th ’

nd presents a transversely striated appearance, ind:icatv iirection of its fibres. In the median line there s a slight antero losterior furrow or raphd, and on either side of this there 1 . s l % T ongitudinal band, called the stria longitudinalis medialis. ,Tf „s the nd left striae longitudinales mediales are sometimes spoken of as the

Lateral to each medial stmi and si uated under over of the callosal gyrus, there is another band, composed g V

natter called the stria longitudinalis lateralis. . ,

The strifof each side are lying in the grey layer alreadymentioned,

.nd may be traced postenorly round ^ ' s dentatus.

mown as fasciola cmerea, into the .f^rrespona b gy , it

interiorly each medial stria, along with the grey matter ^ ies, passes round the genu and backwards on t

















1514


A MANUAL OF ANATOMY


rostrum under the name of the geniculate gyrus. This enters the subcallosal gyrus, and finally passes to the temporal pole.

The medial and lateral longitudinal striae of each side, together with the thin layer of grey matter, represent a rudimentary convolution of the rhinencephalon called the supracallosal gyrus. The grey matter in the human brain is termed

the indusium griseum.

The posterior extremity of the corpus callosum is called the splenium, and is rolled upon itself, so that its lower part is directed forwards and lies over the mesencephalon and pineal body.


Great Longitudinal Fissure

Forceps Minor • Ganu of Corpus Callosum


Median Raph£

Stria

_ Longitudinalis

ijffljjjjfo Mesialis

Stria

. Longitudinalis Lateralis


Tapeturri F orceps


Splenium of Corpus Callosum


Great Longitudinal Fissure

Fig. 926. —The Corpus Callosum (Superior View) (Hirschfeld

and Leveill£).


Anteriorly the corpus callosum is bent upon itself, and passes at ! first downwards and then backwards. The bent portion is called the genu, and the portion which passes backwards the rostrum. The rostrum ends by joining the lamina terminalis in the mid-line, and on either side it passes into the so-called peduncles of the corpus callosum , otherwise known as the subcallosal gyri. Each subcallosal gyrus, with the contained stria longitudinalis medialis, passes downwards on the internal surface of the cerebral hemisphere to become continuous with the anterior perforated substance, lying in front of the lateral portion of











































THE NERVOUS SYSTEM


515 he lamina terminalis. The gyrus then passes backwards and outwards ilong the posterior margin of the perforated area, forming now the liagonal band, and so reaches the temporal pole.

The inferior surface of the corpus callosum is divisible into a medial md two lateral portions. The medial portion is connected posteriorly with the fornix, and over the remainder of its extent with the septum Aicidum. Each lateral portion enters into the roof of the body and interior horn of the corresponding lateral ventricle (see Fig. 930).

Destination of the Callosal Fibres.—The transverse fibres of the corpus callosum, on entering the white medullary substance of each cerebral hemisphere, traverse it in a radiating manner as they pass


Corpus Callosum

Pineal Botly _ -'v^'f

Splenium v


Anterior Pillar of Fornix Septum Luciuum


Genu passing into Rostrum

■UU -V l X

X A 4


Corpora

Quadri gemina


Cerebellum


Fourth Ventricle


Crus Cerebri / j \ Aqueduct

Medulla Oblongata


Fig. 927. —The Medial Surface of the Left Cerebral Hemisphere

(Hirschfeld and Leveille;).


to the cerebral cortex. They constitute the radiatio corporis callcsi, and intersect in their course the fibres which pass between the internal capsule and the cerebral cortex, which form the corona radiata. The fibres from the central portion or body and upper part of the splenium of the corpus callosum constitute the tapetum. This forms the roof of the body of the lateral ventricle, the chief part of the roof, and the outer wall of the commencement of the middle or descending horn, and the roof and outer wall of the posterior horn. Most of the fibres of the tapetum ultimately pass into the temporal and occipital lobes. The fibres from the region of the genu curve forwards into the front part of the frontal lobe, and form the roof of the anterior horn of the lateral ventricle. They constitute the forceps minor . The fibres from the





A MANUAL OF ANATOMY


1516

lower part of the splenium curve backwards into the occipital lobe, and give rise to an eminence on the inner wall of the posterior horn of the lateral ventricle. They constitute the forceps major.

Development. —The corpus callosum is developed fro m the lamina terminalis, but extends beyond this. It is the commissure of the neopallium.

Fornix.—This is an arched lamina of white longitudinal fibres which lies beneath the corpus callosum, with which it is connected posteriorly, but from which it is separated anteriorly by the septum lucidum. It is composed of two lateral halves, which are united together in the median line to form the body of the fornix; but in front and behind they are separated from each other, and form the anterior and posterior pillars. The fornix is thus composed of a body, two anterior pillars, and two posterior pillars (Fig. 928).


%

Gyrus Dentatus

Fig. 928.—The Medial Surface of the Right Cerebral Hemisphere

(Hirschfeld and Leveille).

The body is triangular, being narrow in front, where it is continuous with the anterior pillars, and broad behind, where it is prolonged into the posterior pillars. The superior surface of the body is connected posteriorly with the corpus callosum, and anteriorly with septum lucidum. Each lateral border is well defined, and projects slightly into the lateral ventricle. The inferior surface rests directly upon the tela chorioidea, beneath which, in the median line, is the third ventricle, and on either side the upper surface of the thalamus.

The anterior pillars are two round bundles, which are continuous with the anterior part of the body, and are slightly separated from each other. They pass downwards in front of the interventricular foramina, traversing the grey matter on the sides of the third ventricle. On reaching the base of the brain each pillar becomes twisted in the form of a loop, and forms the white portion of the corresponding corpus mamillare. The fibres of the anterior pillar terminate in the grey nucleus of the corpus, and from this nucleus a bundle of fibres,




THE NERVOUS SYSTEM


1517


called the mamillo-thalamic tract, or bundle of Vicq d’Azyr, passes upwards and backwards into the thalamus.

The anterior pillars lie behind the anterior commissure, but give off a few precommissural fibres which, passing down in front of the


Fig.


>29> _Part of Corpus Callosum cut away to expose Fornix and

Right Ventricle.

Inferior and posterior horns also opened from above.


Choroid Plexus Corp. Call. Fornix


Tail of Caudate Nucleus Hippocampus


Collateral Trigone Bulb

Calcar Avis


commissure, reach the anterior perforated substance and subcallosal

^ The posterior pillars are prolongations of the posterior part the body on either side. They are flattened bands, which at firs







A MANUAL OF ANATOMY


1518

adhere to the under surface of the corpus callosum. Subsequently, however, each curves laterally and downwards round the posterior extremity of the thalamus, and enters the descending horn of the lateral ventricle. Here the posterior pillar comes into contact with the hippocampus, upon the surface of which some of its fibres become spread out, forming the alveus. The rest of the fibres are prolonged as a narrow band of white matter, called the fimbria, or tcmia hippocampi , along the concave border of the hippocampus, to which it is attached, as far as the uncus (see Fig. 929)* As the two posterior pillars diverge from each other they enclose between them a small triangular space on the under surface of the corpus callosum posteriorly. This space is crossed by transverse fibres, and is known as the lyra, from its supposed resemblance to a lyre. The transverse fibres form a commissure between the two hippocampi, and the lyra is therefore known as the hippocampal commissure. Each lateral half of the fornix establishes a communication between the hippocampus, in which the majority of its fibres originate, and the thalamus of’the same side by means of the anterior pillar, the corpus mamillare, and the bundle of Vicq d’Azyr (mamillo-thalamic tract).

Development.— The fornix is developed from the lamina terminalis.

Anterior Commissure.—This is a round bundle of white fibres which crosses the middle line immediately in front of the anterior pillars of the fornix. Anteriorly its central portion is connected with the lamina terminalis (Fig. 930), and posteriorly the central portion appears between the anterior pillars of the fornix, where it forms part of the anterior boundary of the third ventricle, and is covered by the ventricular ependyma. On either side the commissure enters the cerebral hemisphere, and divides into two parts, olfactory and temporal. The olfactory portion is of small size, and enters the coi responding olfactory tract. Some of its fibres serve to connect the olfactory bulb of one side with that of the other side. The other fibres connect the olfactory bulb of one side with the temporal lobe of the opposite side. The temporal portion is of large

size, and its fibres disappear in the white matter of the temporal lobe.

The anterior commissure, therefore, serves to connect the olfactory bulbs and the temporal lobes.

Septum Lucidum.— This is a thin vertical partition which is situated between the anterior horns of the lateral ventricles, as well as between the front parts of the bodies of these ventricles. It is triangular, being broad in front and narrow behind. Posteriorly it is attached above to the under surface of the corpus callosum, and below to the upper surface of the body of the fornix. Anteriorly it occupies the concavity behind the genu of the corpus callosum, being attached above to the corpus callosum and below to the rostrum of that body. It is seen in section in the first figure in Fig. 930. The septum lucidum is composed of two delicate laminae. The lateral


THE NERVOUS SYSTEM


1519


surface of each lamina looks into the corresponding lateral ventricle, and is covered by the ventricular ependyma (epithelium). The medial


Corp. Call. r


Caud. N. and Int. Capsule


Rostrum i*


Ext .Caps, and Claustrum

Fornix *


Ant. Comm.


Fig. 930.

The upper section is through the anterior horn, cutting the body and rostrum of corpus callosum. The lower section is through the lamina terminalis and anterior commissure, and has cut tangential slips from the anterior pillars of fornix.


surface faces that of its fellow, a narrow lymph space, formerly called the fifth ventricle, but now the cavity of the septum lucidum, intervening between the two. Each lamina consists of white matter, which





1520


A MANUAL OF ANATOMY


is covered by grey matter on the surface looking towards the fifth ventricle.

The two laminas are formed from portions of the medial wall of the two cerebral hemispheres, which have become detached in the course of the development of the corpus callosum and fornix.

Cavity of the septum lucidum, formerly known as the fifth ventricle, is the narrow cleft-like interval between the two laminae of the septum lucidum. It is a closed space, and has therefore no communication with the other ventricles. It is destitute of any ependymal lining, and contains a very little fluid.


Caudate Nucleus-.—


Internal Cerebral Vein


Vein of Corpus Striatu Choroid Vein ^


Choroid Plexus J entering Descending Horn of Lateral Ventricle


Septum Lucidum

Space in Septum Fornix


Tela Chorioidea


Vena Magna Cerebri


' Lyra

Anterior Pillar of Fornix

Fig. 931. —The Tela Chorioidea and Internal Cerebral Veins.


As regards development, it differs from the true ventricles in being originally a part of the great longitudinal fissure.

Tela Chorioidea.—This is also known as the tela chorioidea superior, in contradistinction to the tela chorioidea inferior, which is the pia mater forming the roof of the lower part of the fourth ventricle. It lies immediately beneath the fornix, and rests upon the ependymal roof of the third ventricle, and also upon the adjacent portions of the thalami (Fig. 932). It consists of two layers of pia mater, and is triangular, the apex being situated behind the anterior pillars of the fornix at the interventricular foramina and the base lying beneath the splenium of the corpus callosum. In the latter situation the two layers of the tela become continuous with the pia mater, which has entered through the transverse fissure, situated between the splenium


/













THE NERVOUS SYSTEM


1521


>f the corpus callosum and the corpora quadrigemina. On either ide the tela chorioidea projects beyond the lateral border of the ornix, and appears as a vascular fringe in the lateral ventricle, where t is covered by the ventricular ependyma. This fringe is known as he choroid plexus of the lateral ventricle. Posteriorly it is prolonged nto the descending cornu. Anteriorly it approaches its fellow of the >pposite side, and the two unite in the median line behind the interventricular foramina. From this junction two other choroid plexuses extend backwards on the inferior surface of the velum interposi;um, one on either side of the median line. They form the choroid plexuses of the third ventricle, and lie superficial to the ependymal •oof of the cavity. The choroid dexuses are composed of a lighly vascular villous arrangenent of the pi a mater, and are

he structures which secrete the cerebro-spinal fluid.

The principal veins in connection with the velum interpositum are the two choroid veins and the two internal

erebral veins. The choroid vein of each side is situated in the choroid plexus of the lateral ventricle. It passes forwards md inwards to a point behind the corresponding foramen, where it joins the vein of the corpus striatum, which lies between the thalamus and the corpus striatum. In this manner the

internal cerebral vein of one side is formed.

The internal cerebral veins (or veins of Galen) are right and left. Each is formed by the union between the choroid vein, the vein of the corpus striatum, and the vein of the septum lucidum, behind the corresponding interventricular foramen. The two veins pass backwards within the tela chorioidea, one on either side of the median line. At first they are near each other; then they diverge; but subsequently they come together again and unite to form one vessel, called the vena magna cerebri, which opens into the anterior extremity of the straight sinus. Each vein receives numerous tiibutaries from the corresponding choroid plexus of the third ventricle, the thalamus, corpus callosum, corpora quadrigemina, and pineal body. Near its termination it is joined by the large basilar vein, which is formed at the anterior perforated area by the union of the anterior cerebral vein with the deep middle cerebral vein. The vena magna receives tributaries from the upper surface of the cerebellum and from the occipital lobes of the cerebral hemispheres.


Fig. 932.—Schematic Section to show Disposition of Tela Chorioidea, (T.C.).

LV, lateral ventricle; F, F, fornix; CN, caudate nucleus; IC, internal capsule.


96




I 5 22


A MANUAL OF ANATOMY


Lateral Ventricles.

The lateral ventricles are cavities in the right and left cerebral hemispheres. They are of irregular shape, and each is about twothirds of the length of the corresponding hemisphere. They are lined with ependyma (epithelium), and contain cerebro-spinal fluid, Each ventricle communicates with the third ventricle by the interventricular foramen, which is situated between the anterior pillar


Genu of

Corpus Callosum


Caudate Nucleus


Interventricular

Foramen


Stria Semicircularis


Thalamus


Choroid Plexus of Central Part of Late al Ventricle


Choroid Plexus of Inferior Horn


Calcar Avis


Bulb of Posterior Horn (due to Fibres of the Forceps Major)


Anterior Horn of Lateral Ventricle


Cavity of Septum Lucidum


Posterior Pillai of Fornix Hippocampus


Great Cerebral Vein


Posterior Horn of Lateral Ventricle


Cerebellum

Fig. 933.— The Lateral Ventricles of the Cerebrum (after

Rirschfeld and Leveille).


of the fornix and the front part of the thalamus. The lateral ventricle of either side consists of a body or central part and three horns— anterior, middle or descending, and posterior.

The central part extends from the foramen to the level of the splenium of the corpus callosum. The anterior horn is situated in front of the foramen, and curves forwards and laterally into the frontal lobe. The inferior horn enters the temporal lobe, and describes a remarkable curve as it sweeps round the posterior extremity of the thalamus. Its direction is backwards, laterally downwards ,


/


























THE NERVOUS SYSTEM


1523


mvards, and finally medially to a point about 1 inch from the temporal ole. The posterior horn curves backwards and laterally, and then ackwards and medially into the occipital lobe.

The central part of the lateral ventricle has a roof, a medial wall, nd a floor. The roof is formed by the corpus callosum (tapetum). 'he medial wall is formed by the posterior part of the septum lucidum, nd, behind this, by the attachment of the body of the fornix to tie under surface of the corpus callosum. Laterally the cavity is mited by the meeting of the roof and floor. The floor presents he following structures, in order from within outwards: (1) the harp lateral border of the fornix; (2) the choroid plexus of the


Third Ventricle


- Anterior Horn


Descending Horn


Suprapineal Recess


Fourth Ventricle

Lateral Recess of Fourth Ventricle

Impression for Bulb of Posterior Horn

Posterior Horn


Fig. 934. —Drawing of a Metal Cast of the Ventricles of the Brain of an Adult (Superior View) (Retzius).


ateral ventricle; (3) a portion of the upper surface of the thalamus, covered by ependyma of ventricle; (4) an oblique groove, extending orwards and inwards between the thalamus and caudate nucleus, n which there are (a) a white band, called the stria semicircularis, ind (b) the vein of the corpus striatum; and (5) the narrow part of

he nucleus caudatus of the corpus striatum. The anterior horn is compressed from side to side; its roof is formed iy the forceps minor of the corpus callosum; its lateral wall by the lead of the caudate nucleus, round which the cavity is moulding tself; its inner wall by the septum lucidum; and its floor by the neeting of the outer and inner walls.








^524


A MANUAL OF ANATOMY


The posterior horn has its roof and lateral wall formed by th tapetum of the corpus callosum. The medial wall presents tw elongated curved eminences, upper and lower. The upper eminenc is made by the fibres of the forceps major as they sweep backward from the lower part of the splenium of the corpus callosum to th occipital lobe. It is called the bulb of the posterior horn. The lowe eminence is called the calcar avis, and is invaginated by the precalcarin fissure on the medial surface of the cerebral hemisphere.

The inferior or descending horn is situated in the temporal lobe The roof is formed chiefly by the tapetum of the corpus callosum


Trigonum Ventriculi-^


Bulb of Posterior Horn


Posterior Horn


Inferior Horn Hippocampus


Pes Hippocampi Uncus

Gyrus Dentatus Hippocampal Gyrus

- Fimbria

Posterior Pillar of Fornix Splenium


Calcar Avis


Fig. 935. —The Inferior and Posterior Horns of the Left Lateral Ventricle (after Hirschfeld and Leveili.e).


The inferior cornu has been laid open throughout its entire extent.


and at its anterior end presents the amygdaloid tubercle, which is produced by a collection of grey matter, called the amygdaloid nucleus. The narrow part or tail of the nucleus caudatus and the taenia semicircularis are prolonged into the roof, and extend in it as far as the amygdaloid nucleus, The floor of the descending horn presents the following structures: (1) the hippocampus; (2) the fimbria; (3) the trigonum collaterals; and (4) the choroid plexus of the descending horn. The hippocampus is a prominent curved elevation which traverses the entire length of the descending cornu, accurately adapting itself to its curves. It enlarges as it descends, and beneath the amygdaloid









THE NERVOUS SYSTEM


i 5 2 5


ubercle it terminates in a swelling, which is notched on the surface, 'his swelling is called the pes hippocampi.

The hippocampus is invaginated by the dentate or hippocampal ssure on the medial surface of the cerebral hemisphere (see Fig. 923).

The fimbria is the continuation of the posterior pillar of the fornix, t lies along the inner concave border of the hippocampus, to which t is attached, and it is composed of white fibres, some of which form he layer on the surface of the hippocampus, called the alveus.

The trigonum collaterals is an elevation which is situated in the ,ngle between the descending and posterior horns, where there is a mall triangular space, called the trigonum ventriculi. It extends backyards into the posterior horn, and for a variable distance into the lescending cornu.


Fig. qs6._Plan of Relations to Choroidal Fissure and to Each Other

of Structures found in Lateral Ventricle.


H, hippocampus;


B, bulb of posterior horn; C.A., calcar avis; C.T., collateral

trigone.


The trigonum collaterals is produced by the central portion of the 'ollateral fissure on the tentorial surface of the cerebial hemisp eie.

The choroid plexus of the descending horn rests upon the surface jf the hippocampus, and is continuous with that of the body of the ateral ventricle It is covered by the ependyma of the medial wall jf the descending cornu, which it mvagmates. When the choroid Dlexus is removed its ependymal covering comes away along with it, md the choroidal fissure then becomes apparent.

The choroidal fissure is situated between the fimbria and the roof 3 f the descending cornu, and, curving round the back part of the

halamus, it is traceable as far forwards as the mterventncular foramen if the same side. In the other direction it extends to the lower ex tremity of the inferior cornu.




1526


A MANUAL OF ANATOMY


It is produced by an infolding or invagination of the epithelia medial wall of the cerebral vesicle of one side over the choroid plexus of the descending horn of the lateral ventricle. On either side it is continuous with the lateral and lower part of the transverse fissure.

When the choroid plexus is withdrawn from the descending horr of the lateral ventricle the epithelial or ependymal covering of the plexus comes away with it, or is broken down. Under these circumstances the descending horn opens freely upon the exterior.

Development. —The lateral ventricles represent the cavities of the primitive cerebral vesicles. The choroidal fissure is developed as an invagination of the medial wall of the cerebral vesicle; and the choroid plexus is developed from a growth of mesoblast into the choroidal fissure.


Basal Ganglia of the Cerebral Hemispheres.

The basal ganglia of each cerebral hemisphere are the nucleus caudatus and nucleus lentiformis of the corpus striatum, the claustrum, and the amygdaloid nucleus.

The corpus striatum is a large ovoid mass, which is situated in front, and on the outer side of the thalamus. It is composed of two collections of grey matter, one of which is intraventricular and the other extraventricular. The intraventricular portion is called the nucleus caudatus. The extraventricular portion is embedded in the white matter of the cerebral hemisphere, and is termed the nucleus lentiformis. Between these two nuclei there is a part of the thick tract of white fibres which constitutes the internal capsule ; and on the outer side of the nucleus lentiformis there is the thin lamina of white matter, called the external capsule. When a coronal section is made through the corpus striatum on a level with the anterior part of the nucleus lentiformis (see Fig. 939), the white matter of the front part of the internal capsule is seen to be intersected by striae of grey matter which pass between the nucleus caudatus and nucleus lentiformis. From the striped appearance thus produced the body has received the name of corpus striatum.

The nucleus caudatus is pyriform. The large round end is directed forwards, and projects into the anterior horn of the lateral ventricle. The narrow portion is directed laterally and backwards in the floor of the central part of the lateral ventricle, where it lies lateral to the thalamus, from which it is separated by the stria semicircularis. Its tapering tail is continued into the roof of the descending horn of the lateral ventricle, and is prolonged in the roof as far as the amygdaloid nucleus, in which it terminates. The nucleus caudatus is composed of grey matter, and its cells are of the multipolar variety.

The nucleus lentiformis is embedded in the white matter of the cerebral hemisphere, and lies on the outer side of the nucleus caudatus and thalamus, from both of which it is separated by the internal capsule. It is of more limited extent than the nucleus caudatus, and receives its name from the fact that in certain sections it has the


THE NERVOUS SYSTEM


1527


Lppearance of a biconvex lens, the broadest part being on a level vith the front of the thalamus. Anteriorly it is closely related to he front part of the nucleus caudatus, being continuous with it ineriorly, and connected with it superiorly by striae of grey matter which ntersect the white matter of the front part of the internal capsule.


Great Longitudinal Fissure


Genu of Corpus Callosum


Anterior Horn of Latera Ventricle

Caudate Nucleus

Cavity of the Septum

Anterior Pillars of Fornix Ant. Tub. of Thalamus Stria Semicircularis . Connexus Thalami

~ Tasnia Thalami Third Ventricle

-Posterior Commissure

-Vn \

^ Y\“V~ Habenular Commissure

.xV A.


Thalamic Groove for Margin of Fornix


Trigorium Habenulae

Lower Quadrigeminal Body


' Pulvinar of Thalamus 'y v ’Pineal Body

\ Superior Peduncle of Cerebellum

Upper Quadrigeminal Body

ig. 937 ._The Third Ventricle, Portions of the Lateral Ventricles, Pineal Body, and Corpora Quadrigemina (Superior View) (Henle).

The‘corpus callosum, fornix, and tela chorioidea have been removed.

Vhen either a horizontal or a coronal section is made through the centre he nucleus has a triangular outline, the base being directed towards the nsular surface; it is, therefore, clear that the nucleus is rea y a PJ 1 ^ 1 j ying on its side with the base outwards, m contact with the external

apsule, while above, behind, and in front the wa Is are surrou





































1528


A MANUAL OF ANATOMY


by the internal capsule. Below lies the anterior commissure and the temporal lobe. In such a section the nucleus is seen to be traversed vertically by two white bands, called the medullary lamina, which divide it into three zones. The outer zone, which has a dark reddish colour, is the largest, and is called the putamen. The inner two zones, which are somewhat yellowish, are together known as the globus pallidus. The putamen and globus pallidus, which consist of grey matter, are traversed by white fibres.

The grey matter of the nucleus caudatus and nucleus lentiformis comes to the surface at the base of the brain in the region of the anterior


Entrance to Descending Horn Hippocampus

of Lateral Ventricle

Fig. 938. Horizontal Section of the Brain through the Genu AND SPLENIUM OF THE CORPUS CALLOSUM (DALTON).

perforated substance, where it is continuous with the grey matter of the cerebral cortex.

The internal capsule is the thick tract of white matter which lies between the nucleus lentiformis externally, and the nucleus caudatus, stria semicircularis, and thalamus internally. As seen in horizontal section it describes the bend opposite the front part of the thalamus (see Fig. 938). This bend is called the genu, and its convexity is directed inwards. The part of the internal capsule in front of the genu is called the anterior limb. It forms about one-third of the entire capsule, and its direction is forwards and outwards. The part behind the genu is called the posterior limb. It forms about



















THE NERVOUS SYSTEM


1529


two-thirds of the entire capsule, and its direction is backwards and outwards.

The anterior limb of the internal capsule is situated between the front part of the nucleus lentiformis and the nucleus caudatus. Anteriorly it is intersected by the striae of grey matter which pass between the two nuclei.

The fibres which compose the anterior limb are partly corticipetal and partly corticifugal. The corticipetal fibres are as follows: (1) thalamo-frontal fibres, which pass from the thalamus to the cortex of the frontal lobe; (2) thalamo-striate fibres, which pass from the thalamus to the corpus striatum (thalamo-caudate and thalamo

Great Longitudinal Fissure c ~

, Genu of Corpus Callosum

Cavity of Septum Lucidum \ -—■" - 1 ■— L


Anterior Horn of Lateral Ventricle


Left Anterior Pillar of Fornix


Connexus- _ Thalami


Third Ventricle


Caudate Nucleus


Thalamus

Internal Capsule


- External Capsule


Claustrum


—=3 Grey Matter of Insula


White Matter of Insula


Optic Tract (cut)


Infundibulum


Lentiform Nucleus


Fig. 939. —Coronal Section of the Frontal Portions of the Cerebral Hemispheres passing through the Anterior Horns of the Lateral Ventricles (Posterior View).

1, putamen of lentiform nucleus; 2, 3, globus pallidus of lentiform nucleus.


lenticular fibres); and (3) strio-frontal fibres, which pass from the corpus striatum to the cortex of the frontal lobe.

The chief corticifugal fibres constitute the fronto-pontine tract. The fibres of this tract arise in the cortex of the prefrontal region. They traverse the anterior limb of the internal capsule, and then descend in the inner part of the basis pedunculi of the crus cerebri to the pons, within which they terminate in connection with the cells of

the nucleus pontis. .

Other corticifugal fibres constitute fronto-thalamic , fronto-striate,

and strio-thalamic tracts. .

The posterior limb of the internal capsule is situated between the back part of the nucleus lentiformis and the thalamus, and is pro













1530


A MANUAL OF ANATOMY


longed backwards for a little beyond the posterior limit of the nucleus lentiformis. It is therefore conveniently divided into two parts—lenticular, representing the anterior two-thirds; and postlenticular, representing the posterior third.

The lenticular part of the posterior limb, like the anterior limb, is composed of centripetal and centrifugal fibres. The corticipetal fibres arise in the thalamus, and their destination is the cerebral cortex. The corticifugal fibres represent the pyramidal or motor fibres , and they


Corpus Callosum


- Ant. Horn and Head

of Caudate Nerve •••• Internal Capsule

External Capsule ---• Claus trum ■ -* Putamen

-• Globus Pallidus ... Thalamus


... Internal Capsule ..... Caudate Nucleus’ Tail


-Posterior Horn


Optic Radiation


Fig. 940.—Horizontal Section through Right Hemisphere, showing Disposition of Corpus Striatum, etc.


occupy the anterior portion of the lenticular part of the posterior limb of the internal capsule. These fibres descend from the central region of the cerebral cortex. Some of them pass to the nucleus of the facial nerve; others pass to the nucleus of the hypoglossal nerve; but the majority of them are destined for the motor cells in the anterior grey column of the spinal cord. The fibres which pass to the facial nucleus lie close to the genu, and those which pass to the hypoglossal nucleus lie close behind the facial fibres. The fibres of the pyramidal tract



THE NERVOUS SYSTEM


1531


FP


CAUD. HA. '


occupy the central portion of the crusta of the crus cerebri in their downward course.

The postlenticular part of the posterior limb contains the following sets of fibres: (1) the fibres of the optic radiation on their way from and to the thalamus, lateral geniculate body, and superior quadrigeminal body; (2) the fibres of the auditory radiation, passing between the auditory region of the temporal lobe and the medial geniculate body; and (3) the fibres of the temporo-pontine tract (cortico-protuberantial fibres), which pass from the cortex of the temporal lobe through the outer part of the basis pedunculi to the pons, where they terminate in the nucleus pontis. The internal capsule is continuous inferiorly with the crusta or basis of the crus cerebri. Superiorly its fibres diverge in a radiating manner on their way to the cerebral cortex, forming the corona radiata, the fibres of which are intersected by those of the radiatio corporis callosi.

The external capsule is a thin lamina of white matter which is situated on the outer side of the nucleus lentiformis, where it lies between that nucleus and the claustrum. In front of and behind the nucleus lentiformis it is continuous with the internal capsule. The external capsule is, as stated, only loosely connected with the putamen of the nucleus lentiformis. The fibres of which it is composed are probably derived from the anterior white commissure and the thalamus.


A UD .


Fig. 941.— Plan of Internal Capsule in Horizontal Section, to show Positions of Main Fibretracts.

FP, fronto-pontine; T, thalamo-cortical; CR, motor for head and neck; A, for arm; L, for leg; S, sensory from thalamus; OPT, AUD, optic and auditory fibres.


Connections of the Corpus Striatum. —(1) The nucleus caudatus and nucleus lentiformis are partly continuous with each other, and partly connected by stria of grey matter. (2) The corpus striatum is connected with the thalamus by strio-thalamic and thalamo-striate fibres. (3) The nucleus caudatus is said to be connected with the substantia nigra by a tract of fibres known as the stratum intermedium. (4) The nucleus lentiformis is connected with the thalamus by the ansa lenticularis. (5) The corpus striatum is connected with

the cerebral cortex by cortico-striate fibres. . Development. _The corpus striatum is developed as a thickening of the floor

and outer wall of the cerebral vesicle.

t

The claustrum is a thin lamina of grey matter which is situated on the outer surface of the external capsule. It lies embedded in the white matter which occupies the region between the lentiform nucleus






















1532


A MANUAL OF ANATOMY


and the insula. Superiorly it is narrow and tapering, but interiorly it expands and reaches the surface at the base of the brain in the region of the anterior perforated substance. Its outer surface presents elevations and intervening depressions, which correspond to the sulci and gyri of the insula.

The claustrum is regarded as an isolated portion of the grey matter of the insula.

The amygdaloid nucleus is an oval collection of grey matter which is situated in the anterior part of the temporal lobe, where it lies in the roof of the extremity of the inferior horn of the lateral ventricle. The putamen of the nucleus lentiformis lies above it; anteriorly it is continuous with the cerebral cortex; posteriorly it receives the narrow


Subthal. N. and. Caudate Tail


Dentate Gyrus


Hippocampal

Gyrus


Fig. 942. —Transverse Section across Cerebrum.

part or tail of the nucleus caudatus; and the stria semicircularis arises from it.

The stria semicircularis is a narrow white band of fibres arising from the amygdaloid nucleus. It passes backwards in the roof of the descending horn of the lateral ventricle, and then sweeps upwards and forwards into the central part of the lateral ventricle, lying between the nucleus caudatus and the thalamus. Anteriorly in the region of the interventricular foramen it dips downwards towards the anterior pillar of the fornix and the anterior white commissure. In this situation its fibres are variously disposed. Some pass into the anterior pillar of the fornix; others pass in front of the anterior commissure, and enter the grey matter between the head of the nucleus caudatus and the septum lucidum; whilst a few are regarded as entering the nucleus caudatus.


THE NERVOUS SYSTEM


1533


Relation of Structures in the Region of the Corpus Striatum.—

When a coronal section has been made the relation of structures, from within outwards, is as follows (see Fig. 942):


1. Nucleus caudatus.

2. Internal capsule.

3. Nucleus lentiformis.


4. External capsule.

5. Claustrum.

6. Insular cortex.


THIRD VENTRICLE.

The third ventricle is the cleft-like interval which is situated in the median line between the two thalami. It extends from the pineal body posteriorly to the anterior pillars of the fornix in front, is very narrow from side to side, and is deeper in front than behind. The cavity presents a roof, a floor, two lateral walls, an anterior boundary, and a posterior boundary.

The roof is formed by a delicate layer of epithelium which extends across between the upper margins of the lateral walls, and is continuous with the ependymal lining of the ventricle. Lying on this epithelial roof, and intimately connected with it, is the tela chorioidea, from the under surface of which the two choroid plexuses of the ventricle project downwards, one on either side of the middle line, each invaginating the epithelium of the roof. The epithelium of the roof is so intimately connected with the tela that, when the latter is removed, the epithelium comes away with it, and the cavity of the ventricle is exposed. Above the tela chorioidea is the ‘ body ’ of the fornix, and above this again is the ‘ body ' of the corpus callosum.

Summary of the Roof.— To expose the ventricle from above, the following structures must be removed, in the order named: (1) the body of the corpus callosum; (2) the body of the fornix; and (3) the tela chorioidea, along with the

epithelium of the roof.

The floor, which is sloped downwards and forwards (see Fig. 944 ). is formed by the structures which lie within the interpeduncular space at the base of the brain, from behind forwards: the locus perforatus posterior, the corpora mamillaria, and the tuber cmereum, with the upper end of the infundibulum. The tegmenta of the crura cerebri enter to a certain extent into the floor posteriorly, and the optic commissure lies across it anteriorly. Above the optic commissure the floor presents a depression, called the optic r«c«ss, and behind i there is another depression or diverticulum, called th e infundibular recess. The latter forms the upper part of the infundibulum, which

leads to the posterior lobe of the hypophysis

The lateral wall is slightly convex and is formed for the m part by the inner surface of the thalamus, which has covering of grey matter. Towards its centre it presents a furrow, which leads from the interventricular foramen m a backward direction

towards the upper opening of the aqueduct T'^^ralwall Ihere ^s hypothalamic sulcus. At the upper part of the lateral wall there


1534


A MANUAL OF ANATOMY


a delicate band of white fibres, called the stria thalami, which runs back toward the root of the pineal body, and passes to the anterior pillar of the fornix. Connecting the two lateral walls (the thalami), in front of the centre of the ventricle, there is a fragile band of grey matter, formerly called the middle or soft commissure, but now usually known as the massa intermedia or connexus thalami , since it is not really a commissure. At the anterior part of the lateral wall the corresponding anterior pillar of the fornix passes downwards and backwards.

The anterior boundary is formed inferiorly by the lamina terminalis, which extends upwards from the optic commissure to the rostrum of the corpus callosum, and superiorly by the anterior pillars of the fornix and the central portion of the anterior commissure.


Fig. 943. —Drawing of a Metal Cast of the Ventricles of the Brain of

an Adult (Right Lateral View) (Retzius).

The posterior boundary is formed by the pineal body and the posterior commissure, and under cover of the latter is the upper opening of the aqueduct. The posterior boundary presents two recesses, pineal and suprapineal. The pineal recess passes backwards for a very short distance above the posterior commissure into the stalk of the pineal body, separating the stalk into two portions, dorsal and ventral. The suprapineal recess is connected with the back part of the epithelial roof of the ventricle, and passes backwards over the pineal body. The third ventricle has thus four diverticula—namely, the optic recess, the infundibular recess (both of which recesses are associated with the floor), the pineal recess, and the suprapineal recess. The cavity communicates with the fourth ventricle by means of the aqueduct of the mid-brain, and with the two lateral ventricles by means of the interventricular foramina.






THE NERVOUS SYSTEM


1535


Interventricular Foramina (Foramina of Monro).—These two openings lead one on each side from the third ventricle into the lateral ventricles. Each foramen is situated between the anterior pillar of the fornix in front and the anterior tubercle of the thalamus behind. From this point the foramen of each side leads medially and slightly downwards, and opens into the third ventricle at the anterior and upper part of the corresponding lateral wall. By means of the foramina the lateral ventricles communicate with the third ventricle, and through that ventricle with each other. The choroid plexuses of the lateral ventricles also become continuous with each other and with those of the third ventricle just above the roofs of these foramina.

Development. —The posterior and greater part of the third ventricle is the cavity of the thalamencephalon or diencephalon; and the anterior part in the region of the foramina represents the cavity of the telencephalon.


Fig. 944. —Left Wall of Third Ventricle.


The foramen of each side represents the original wide communication between the cavity of the cerebral vesicle and the cavity of the telencephalon.

Thalami.—The thalami (O.T. optic thalami) are two large ovoid masses of grey matter which lie obliquely, with their long axes directed backwards and outwards, for the most part on the sides of the third ventricle. Their anterior extremities are near each other, but their posterior extremities stand apart, the superior corpora quadrigemina being situated between them. Over their anterior two-thirds they are separated from each other by the third ventricle.

Each thalamus presents four surfaces—superior, inferior, lateral, and medial; and two extremities—anterior and posterior.

The superior surface is limited laterally by an oblique groove, which separates it from the nucleus caudatus, and contains the stria semicircularis, and anteriorly the vein of the corpus striatum. Medially it is bounded, from before backwards, by (1) the stria thalami, (2) the trigonum habenulae, and (3) the corpora quadrigemina. It is divided into two areas, lateral and medial, by a groove which is





A MANUAL OF ANATOMY


1536

directed backwards and laterally from near the anterior extremity to the lateral end of the posterior extremity. This groove corresponds to the lateral margin of the body of the fornix. The lateral area enters descriptively into the body of the lateral ventricle, but is covered by the ependyma of that ventricle. The medial area is excluded from


Great Longitudinal Fissure


F ig . 945 .—The Third Ventricle, Portions of the Lateral Ventricles, Pineal Body, and Corpora Quadrigemina (Superior View) (Henle).

The corpus callosum, fornix, and tela chorioidea have been removed.

the lateral ventricle, and is covered by portions of the tela chorioidea and body of the fornix (see Fig. 933). The superior surface is covered by a thin layer of white fibres called the stratum zonale, these fibres being derived from the optic tract and optic radiation.

The inferior surface lies posteriorly upon the upward prolongation





































THE NERVOUS SYSTEM


1537


of the tegmental fibres of the crus cerebri, which constitutes the subthalamic tegmental region, but anteriorly it rests upon the corpus mamillare and a portion of the tuber cinereum.

The lateral surface is directly related to the posterior limb of the internal capsule, which separates it from the nucleus lentiformis (see Fig. 940). Many fibres emerge from this surface, and enter the internal capsule on their way to the cerebral cortex, whilst others from the cerebral cortex enter the thalamus through this surface. These fibres constitute the thalamic radiation. On its surface the fibres form a well-marked reticular layer of white matter, which is called the external medullary lamina.


A, surface covered by ependyma of lateral ventricle; B, groove caused by fornix.


The medial surface faces its fellow of the opposite side, with which it is connected by means of the connexus thalami. It forms the lateral wall of the third ventricle, and superiorly is limited by the stria thalami. It is covered by a thick layer of grey matter, which is continuous with that around the aqueduct of the mid-brain.

The anterior extremity is marked by a prominence, called the anterior tubercle , which enters into the body of the lateral ventiicle, and forms the posterior boundary of the corresponding interventricular

foramen. . - ,

The posterior extremity presents at its inner end a well-marked

prominence, called the 'posterior tubercle or pulvinar . It lies over the








1538 A MANUAL OF ANATOMY

brachia of the corpora quadrigemina, which it almost conceals. Below and external to the pulvinar there is an oval swelling, called the corpus geniculatum externum. Medial to this body is the brachium of the upper corpus quadrigeminum, and inferior to this is the corpus geniculatum internum (see Fig. 946).

Metathalamus or Corpora Geniculata.—The corpora geniculata are external or lateral and internal or medial. They are associated with the posterior extremity of the thalamus, and the medial also with that portion of the mesencephalon which constitutes the corpora quadrigemina (see Fig. 946).

The corpus geniculatum laterale is an oval eminence situated on the posterior extremity of the thalamus below and lateral to the pulvinar. Internally it is connected with the upper quadrigeminal body by the superior brachium.

It consists of grey and white curved lamellae, which alternate with each other. The fibres of the white lamellae belong to the outer or visual root of the optic tract. The axons of the cells of the grey matter enter the optic radiation as corticipetal fibres. The lateral or outer geniculate body is associated with sight.

The corpus geniculatum mediale is a small oval eminence which is situated below the pulvinar, and on the lateral aspect of that portion of the mesencephalon which constitutes the corpora quadrigemina. The inferior brachium, which is beneath it, connects it with the lower quadrigeminal body.

The medial geniculate body contains many nerve-cells, the axons of which become corticipetal fibres, their destination being the cortex of the temporal region of the brain. By means of the lower quadrigeminal body and the inferior brachium this geniculate body receives fibres from the lateral or acoustic fillet , which terminate in arborizations around its cells. The axons of these cells become corticipetal fibres, the destination of which is the cortex of the temporal region of the brain. The medial geniculate body is associated with hearing.

Development. —The corpora geniculata appear as elevations on the lateral wall of the thalamencephalon or diencephalon.

Structure of the Thalamus. —The thalamus is composed chiefly of grey matter. Its superior surface is covered with a layer of white matter, known as the stratum zonale, and its lateral surface is covered with a reticular layer of white matter, called the external medullary lamina. The medial surface has a thick coating of grey matter, which is continuous with the grey matter around the aqueduct.

The grey matter of the interior of the thalamus is traversed by a plate of white matter, called the internal medullary lamina, which divides it into tw r o nuclear areas—lateral and medial. The lateral nuclear area lies between the internal and external medullary laminae, and extends backwards as far as the pulvinar. The medial nuclear area lies between the internal medullary lamina and the thick layer of grey matter wdiich coats the medial surface of the thalamus. It extends backwards as far only as the habenular region, and anteriorly it is separated from the anterior tubercle by a lamina of white matter. The region of the anterior tubercle therefore constitutes a third or anterior nuclear area of grey matter. The grey nuclear areas are consequently three in number —lateral, medial, anddnterior.


THE NERVOUS SYSTEM


1539

Lateral Nuclear Area. —1 his area includes the pulvinar, the geniculate bodies, md the radiate nucleus. The pulvinar and geniculate bodies have just been described. Ihe radiate nucleus is associated with the fibres of the thalamic radiation, referred to later.

Anterior Nuclear Area. —This area includes the anterior tubercle, and is the chief sensory nucleus. It receives corticifugal fibres, and its cells furnish corticipetal fibres. It also receives many of the fibres of the lateral lemniscus as well as those of the superior cerebellar peduncle, and the fibres of the bundle of Vicq d’Azyr, the mamillo-thalamic tract.

Medial Nuclear Area. —This area contains the ganglion habenulae, to be presently described.

Connections of the Thalamus. —(1) Viewing the thalamus as an aggregation of ‘ cell-stations ' in the course of the centripetal fibres of the tegmentum of the crus cerebri, the tegmental fibres probably all terminate in the thalamic cells. (2) Through the lateral geniculate the thalamus is connected with the optic tract and optic radiation. (3) The cells of the anterior nucleus receive the fibres of the mamillo-thalamic tract, which are connected through the corpus mamillare with the fibres of the anterior pillar of the fornix. (4) Thalamic Radiation. —This is composed of thalamo-cortical fibres which arise within the thalamus as the axons of the thalamic cells. They issue from its lateral and inferior surfaces, and pass to all parts of the cerebral cortex. They are conveniently arranged in four groups or stalks —frontal, parietal, occipital, and inferior or ventral, (a) The fibres of the frontal stalk, having emerged from the front part of the external surface, traverse the lateral part of the anterior limb of the internal capsule, and most of them pass to the cortex of the frontal lobe. Some of these fibres are thalamo-caudate and thalamolenticular as regards their destination. ( b ) The parietal stalk, having issued from the thalamus, passes for the most part through the internal capsule, but also to a certain extent through the external capsule, to the cortex of the parietal lobe, and the central region of the frontal lobe, (c) The occipital stalk issues from the pulvinar, and, having traversed the postlenticular portion of the posterior limb of the internal capsule, it passes backwards and outwards lateral to the posterior horn of the lateral ventricle, and so reaches the cortex of the occipital lobe, (d) The inferior or ventral stalk emerges from the front part of the inferior surface of the thalamus, and its fibres arise as the axons of the cells of the lateral and medial nuclei. The most superficial of these fibres constitute a band, called the ansa lenticularis, which enters the nucleus lentiformis, where it terminates. The remaining fibres pass outwards beneath the nucleus to the cortex of the temporal lobe and insula.

Besides the thalamo-cortical fibres there are cortico-thalamic fibres, which pass from the various parts of the cerebral cortex into the thalamus, where they terminate in arborizations around the thalamic cells.

Development. —The thalamus is developed as a thickening of the dorsal lamina of the thalamencephalon.

Subthalamic Tegmental Region.—This region represents the upward prolongation of the tegmental fibres of the crus cerebri beneath the posterior portion of the thalamus. The parts to be noted are the upward prolongations of the red nucleus and substantia nigra of the tegmentum of the crus; the medial lemniscus; the fibres of the superior peduncle of the cerebellum; and the corpus subthalamicum (or nucleus of Luys). The red nucleus and the substantia nigra gradually disappear, and are no longer visible at the level of the corpus mamillare. The medial lemniscus lies on the superficial and lateral aspects of the red nucleus. The fibres of the superior peduncle of the cerebellum partly terminate in connection with the cells of the red nucleus, but many of them surround it in the form of a capsule. Beyond the red


154°


A MANUAL OF ANATOMY


nucleus the medial fillet, fibres of the superior cerebellar peduncle, and fibres which issue from the red nucleus enter the inferior surface of the thalamus, and terminate in connection with the thalamic cells. Some of these fibres may pass through the thalamus into the internal capsule, and thence to the cortex of the central (Rolandic) region of the cerebral hemisphere. The corpus or nucleus subthalamicum (or nucleus of Luys) is a small lenticular mass of grey matter, surrounded by white fibres, which lies above the substantia nigra.

Epithalamus.—The epithalamus includes the following parts:

1. Pineal body. 3. Trigonum habenulae.

2. Stria thalami. 4. Posterior commissure.


Subthal. N. and Red N.


bm. 947.— Section showing the Intermediate Subthalamic Area, where the Red Nucleus is appearing and the Subthalamic Nucleus has not yet Disappeared.


Pineal Body, or Epiphysis Cerebri.—The pineal body resembles a small pine-cone. It is situated on the dorsal or superior surface of the mesencephalon, and occupies the depression between the upper quadrigeminal bodies. It is of small size, dark red in colour, and somewhat conical in shape. Superiorly it is intimately related to the pia mater as that membrane passes through the transverse cerebral fissure to form the tela chorioidea, and the splenium of the corpus callosum lies above it with the intervention of the pia mater. Inferiorly it is in contact with the depression between the upper quadrigeminal bodies. Its apex,, which is directed downwards and backwards, is free. Its base is directed upwards and forwards, and contains the pineal recess, which is continuous anteriorly with the cavity of the third ventricle. The portion of the base which lies below this recess is



THE NERVOUS SYSTEM


I 54 I


onnected with the posterior commissure, which separates it from the jpper opening of the cerebral aqueduct. The portion above the recess

ontains the habenular commissure. Structure of the Pineal Body. —The pineal body is free from nervous constituents. It consists of a number of follicles lined with epithelial cells, and containing a variable amount of calcareous matter, called acervulus cerebri or brain-sand, which is composed of calcium phosphate, calcium carbonate, magnesium phosphate, and ammonium phosphate.

Development. —The pineal body is developed as a diverticulum of the posterior part of the dorsal aspect of the thalamencephalon or diencephalon. This diverticulum for the most part becomes solid, but a portion of its cavity persists as the pineal recess of the third ventricle.

The pineal body is usually regarded as the representative of one of the stalks of the two median eyes of some of the higher arthropods, such as the king crab, among the Jnvertebrata, and is important in suggesting the possible line of evolution of the Vertebrata. In many of the reptiles the pineal eye as well as the eye-stalk is present, though it is never functional.

Striae Thalami or Habenulae.—Each stria is a narrow strip of white longitudinal fibres lying along the upper part of the medial surface of the corresponding thalamus. It constitutes the habenula. Anteriorly most of its fibres are derived from the olfactory lobe, more particularly the olfactory bulb and anterior perforated substance. Some, however, may be derived from the anterior pillar of the fornix, and through the fornix from the cells of the hippocampus. Posteriorly the fibres are disposed in two ways: (i) The lateral fibres enter the ganglion habenulae, and terminate in connection with its cells. (2) The medial fibres curve inwards towards the base of the pineal body, in which they cross to the opposite side, lying above the pineal recess. As they cross the median line they decussate with the medial fibres of the opposite stria medullaris, and they terminate in the ganglion habenulae of the side to which they have crossed. Their decussation is known as the habenular commissure.

Trigonum Habenulae.—This is a small triangular area (Fig. 948) which is bounded posteriorly by the upper quadrigeminal body, internally by the posterior part of the stria thalami, and laterally by the adjacent part of the thalamus. It contains an important group of multipolar nerve-cells, known as the ganglion habenulae. This ganglion belongs to the medial area of the thalamus. It receives some of the fibres of the stria, which come from the olfactory lobe, and, it may be, from the anterior pillar of the fornix. The axons of the ganglionic cells issue from the ventral surface of the ganglion and form a bundle, called the fasciculus retroflexus. This bundle passes downwards and forwards in the tegmentum of the crus cerebri, lying on the medial side of the red nucleus. Its fibres terminate in connection with the cells of the ganglion interpedunculare , which is situated in the lower part of the posterior perforated substance directly above the pons.

The ganglia habenularum are connected with each other by fibres which constitute the habenular commissure or commissure of the habenular


A MANUAL OF ANATOMY


542

ganglia. These fibres cross in the dorsal part of the base of the pineal body, and are on a higher plane than the posterior commissure.

The striae thalami, or habenulae, and the ganglia habenularum are associated with the rhinencephalon or olfactory brain.


C. Call.

Thalamus Str. Haben.


Pineal

Splenium and Great Vein


Cerebellum


Fig. 948. —Thalamus partly exposed by Removal of Portions of Corpus Callosum and Fornix, with Tela Chorioidea.

Shows trigonum and stria habenulae.

Posterior Commissure.—This is a band of white fibres which is situated at the back part of the third ventricle. It lies in the posterior wall of the ventricle directly above the upper opening of the aqueduct and underneath the base of the pineal body. Its fibres are regarded



THE NERVOUS SYSTEM


1543


as arising from a nucleus in the grey matter of the lateral wall of the third ventricle near the upper opening of the aqueduct. Some of the fibres of either side, after crossing, may descend in the tegmentum of the crus cerebri as part of the medial longitudinal bundle of that side, and so reach the medulla oblongata.

Hypothalamus.—The hypothalamus consists of two parts—mammillary and optic. The pars mamillaris hypothalami represents the


Fig. 949. —Diagram to show Position and Relations of Structures in

Tegmental Subthalamus.

Supposed to be viewed from the medial aspect. R, red nucleus. The subthalamic nucleus is shown antero-lateral to this. Dotted line shows course of fasciculus retroflexus from habenula to interpeduncular ganglion. Course of anterior pillar of fornix is indicated, also mamillo-thalamic tract (bundle of Vicq d’Azyr) passing up medial to front part of subthalamic nucleus. Substantia nigra is seen near pontine level, but passes upwards and laterally out of the section higher up.


two corpora mamillaria. The pars optica hypothalami includes the following structures:


1. Tuber cinereum.

2. Infundibulum.

3. Posterior or cerebral lobe of the hypophysis.

4. Optic chiasma.

5. Lamina terminalis.


The corpora mamillaria are two small, white, pea-like bodies, which lie side by side directly in front of the posterior perforated area Each is composed of white matter externally, and of a grey nucleus internally. The white matter is derived from the corresponding anterior pillar of the fornix, the fibres of which terminate in connection with the cells of the grey nucleus. This grey nucleus contains many cells the axons of which give rise to two fasciculi—namely, the mamillo-thalamic tract (or bundle of Vicq d’Azyr). and the peduncle of the corpus mamillare. The mamillo-thalamic tract, which is


l 5 44


A MANUAL OF ANATOMY


apparently a continuation of the anterior pillar of the fornix, enters the thalamus, and its fibres terminate in connection with the cells of the anterior nucleus. The peduncle of the corpus mamillare passes downwards and backwards in the grey matter of the floor of the third ventricle to the tegmental region of the mesencephalon, but the mode of termination of its fibres is not known.

Development. — The corpora mamillaria are developed from the ventral aspect of the thalamencephalon or diencephalon. Up to the third month of intra-uterine life they are represented by a single corpus mamillare, but after that period this divides into two corpora.

The tuber cinereum is an elevated area of grey matter which lies in front of the corpora mamillaria and behind the optic commissure, the anterior portion of each optic tract being on either side. It is continuous anteriorly with the lamina terminalis, and on either side with the grey matter of the anterior perforated substance.

In the lateral part of the tuber cinereum, in the vicinity of the optic tract, there is a collection of nerve-cells, which is variously spoken of as the basal ganglion of Meynert or the supra-optic nucleus of Cajal, and which is connected

with the fibres of the commissure of Gudden.

Behind the tuber cinereum, and in front of the corpora mamillaria, there is a small prominence, medially placed, called the eminentia saccularis of Retzius, who regards it as the homologue of the saccus vasculosus of some lower vertebrates— e.g., fishes.

Ihe infundibulum is a funnel-shaped stalk which extends downwards from the anterior part of the inferior surface of the tuber cinereum to the posterior lobe of the hypophysis, or pituitary body. Its upper part is hollow, and contains the infundibular recess or diverticulum of the cavity of the third ventricle. The infundibulum is the peduncle of the posterior lobe of the hypophysis.

Hypophysis (Pituitary Body).—As this structure is seldom removed in the course of dissection with the brain, it has already been described on p. 1171 with the pituitary fossa, in which it lies. It may be well, however, to repeat in this place the fact that the anterior lobe is a derivative of the ectodermal lining of the primitive mouth; that the posterior lobe, which is connected to the infundibulum, is a downgrowth from the brain (hypophysis cerebri); and that, between the two, lies the pars intermedia, which is only the posterior wall of the ectodermal pouch. The name (pituitary) was derived from the old belief that the gland secreted the pituita or mucus of the nose.

Lamina Terminalis.— This is a thin plate of grey matter which extends between the upper surface of the optic commissure and the rostrum of the corpus callosum near the genu. On either side it is connected with the grey matter of the anterior perforated substance. It forms the lower part of the anterior wall of the third ventricle.

Development. The lamina terminalis represents the terminal part of the ventral wall of the embryonic neural tube.


THE NERVOUS SYSTEM


1545


Optic Nerve, Optic Chiasma, Optic Tract, and Optic Radiation.

The optic nerves, or nerves of sight, in the cranial cavity are onnected together at the optic commissure or chiasma , where some >f the fibres decussate. From the back part of the commissure each lerve, under the name of the optic tract, passes backwards round he crus cerebri to its cerebral connections.

The optic chiasma rests upon the tuberculum sellae and above the iptic groove of the sphenoid bone. It lies in front of the tuber cinereum ,nd infundibulum, and its superior surface is connected with the lamina erminalis, and is intimately related to the anterior part of the floor >f the third ventricle. On either side of the commissure is the anterior >erforated substance. Most of the fibres of the commissure proceed rom each retina in the corresponding optic nerve, being afferent or

entripetal; but at the back part of the commissure there are the ibres of the medial roots of the optic tracts, which have no connection vith either retina. The decussation of fibres in the commissure is >nly partial. The fibres which arise in the nasal or medial half of he retina cross and enter the optic tract of the opposite side. The ibres which arise in the temporal or lateral half of the retina take no >art in the decussation, but pass directly backwards into the optic tract )f the same side (see Fig. 950).

Occupying the back part of the commissure there are, as stated, iome fibres which have no connection with either retina.. These ibres constitute the commissure of Gudden. I hey lie behind the lecussating fibres, and represent the fibres of the medial root of the )ptic tract of each side. They form the innermost fibres of each >ptic tract, and connect one medial geniculate body with its fellow )f the opposite side.

Summary. —The fibres which arise in the nasal half of one retina cross in fie optic commissure, and enter the optic tract of the opposite side. The fibres \fiich arise in the temporal half of one retina pass directly backwards into the )ptic tract of the same side. The fibres of the inner ropt of each optic tract

ross in the back part of the commissure, and form the commissure of Gudden, he fibres of which have no connection with the optic nerves, but connect the •wo medial geniculate bodies, right and left. The optic commissure therefore insists of the following groups of fibres: (1) The crossed fibres, which arise n the nasal portion of each retina; (2) the uncrossed fibres, which arise m the temporal portion of each retina, and occupy the outer part of the commissure; ind (3) the fibres of the commissure of Gudden, which occupy the back part of fie commissure.

The optic tract of each side is a flattened white band which passes backwards from the optic chiasma. It curves round the crus cerebri, ind in the region of the posterior extremity of the thalamus it divides into two roots, lateral and medial. Ihe lateral or visual foot is the larger of the two. It is chiefly composed of afferent fibres, which pass from the retina to the brain; but it also contains efferent ibres, which pass from the brain to the retina.. The efferent fibres ire derived from (1) the temporal half of the retma of the same side,


A MANUAL OF ANATOMY


1546

and (2) the nasal half of the retina of the opposite side, the latter having crossed in the optic chiasma. The fibres of the lateral root terminate in the lateral geniculate body and the upper quadrigeminal body, reaching the last-named body through the superior brachium. They form arborizations around the cells of these bodies which constitute the terminal nuclei or lower visual centres of the


Yellow, uncrossed fibres; red, crossed fibres ; blue, Gudden’s commissure. Interrupted lines, connections with occipital pole—that is, within the hemisphere ; mainly afferent, but some efferent fibres here.

outer or visual root. These lower visual centres are connected with the higher or cortical visual centre by the strand of fibres forming the optic radiation, the higher visual centre being situated in the cortex of the cuneate and lingual gyri of the medial surface of the occipital lobe. The medial or commissural root of the optic tract passes beneath the medial geniculate body, which represents the nucleus of most of its fibres. As stated, these fibres have no connection with the optic










THE NERVOUS SYSTEM


1547


nerve. Having traversed the inner part of the optic tract, they cross in the back part of the optic commissure behind the decussating fibres, and are continuous with the corresponding fibres of the opposite side. These are the fibres which constitute the commissure of Gudden.

Optic Radiation. —The strand of fibres which forms the optic (or thalamo-occipital) radiation of either side establishes a connection between the lower visual centres (lateral geniculate body and upper quadrigeminal body) and the higher or cortical visual centre, which is situated (1) on the medial surface of the occipital lobe close to the calcarine fissure in the region of the cuneus and lingual gyrus, and (2) on the adjacent part of the postero-lateral surface of the occipital lobe. The strand passes through the post-lenticular part of the internal capsule, and then backwards in the medullary substance of the occipital lobe, lying on the lateral side of the posterior horn of the lateral ventricle. Thereafter the fibres pass in a radiating manner to the higher or cortical visual centre.

This visual area in the neighbourhood of the calcarine fissure is distinguishable to the naked eye in a section of a fresh brain by the white band of Gennari which traverses it.

The optic radiation consists of afferent or corticipetal and efferent or corticifugal fibres. The corticipetal fibres for the most part arise as the axons of the nerve-cells within the lateral geniculate body, which are terminal nuclei of the retinal nerve-fibres, and they end in the higher or cortical visual centre. Some corticipetal fibres arise in the higher or cortical visual centre of the opposite side and cioss in the splenium of the corpus callosum. These fibres are of a commissural character. The corticifugal fibres arise as the axons of the pyramidal cells of the cortex of the visual area of the occipital lobe, and they terminate in the pulvinar, geniculate, and upper quadrigeminal

body.


The lower visual centres are connected with the nuclei of origin of the nerves which supply the ocular muscles, probably through the medial longitudinal

bundle.


Mesencephalon.

The mesencephalon is composed of the corpora quadngemma, which form its upper or dorsal portion; the crura cerebri, which form its lower or ventral portion; and the aqueduct, which passes through it from the fourth ventricle below to the third ventricle above.

Corpora Quadrigemina. —These are four rounded eminences, which, as just stated, form the dorsal portion of the mesencephalon. They are covered by the splenium of the corpus callosum, and are arranged m pairs, upper and lower, the upper pair being larger than the lower pair but not quite so prominent. The four eminences are separated from each other by two grooves, longitudinal and transverse, which are arranged in a cruciform manner. The longitudinal groove extends upwards as far as the posterior commissure and it separates ^Udd e and lower quadrigeminal bodies of one side from those of the other side.


1548


A MANUAL OF ANATOMY


Its upper part lodges the pineal body, and from its lower part a band of white fibres, called the frenulum veli , passes downwards to the superior medullary velum, which lies below the lower pair of eminences. The transverse groove separates the upper pair of quadrigeminal bodies from the lower pair. Laterally each eminence is connected with a white band, called the brachium, the two brachia being separated by a continuation of the transverse groove.

The superior brachium extends outwards and forwards from the upper quadrigeminal body to the lateral geniculate body and the lateral root of the optic tract. It passes between the pulyinar of the thalamus and the medial geniculate body.


Connexus Thalami

Pineal Peduncle ; Tllird Vtntricle


Thalamus


..Upper Quadrigeminal Body W m( i!i'i(.( //).' VjV - Lower Quadi igeminal Body


Superior Fovea Auditory Striae

Inferior Fovea_


Cuneate Tubercle -

Basis Pedunculi


Frenulum Veli Sup. Med. Velum ~-»»Supei ior Peduncle of Cerebellum

Mid. ped. (cut)


"" Auditory Striae Vestibular Trigone

I dim J'-' Trigonum Hypoglossi

•^Trieonum Vagi

( Spinal Tract of Fifth Nerve

tf# \ciava ppT-- Funiculus Gracilis

Funiculus Cuneatus

Fig. 951. —The Floor of the Fourth Ventricle and Adjacent Parts. I he pineal body has been removed to show the upper quadrigeminal bodies.


The superior brachium is associated with the visual apparatus. The inferior brachium, though connected with the medial geniculate body, with which body the inner or commissural root of the optic tract (commissure of Gudden) is also connected, is associated with the acoustic apparatus.

The superior brachium contains two sets of fibres—namely, retinal fibres, derived from the lateral root of the optic tract; and occipital fibres, from the cortex of the occipital lobe of the cerebrum.

The inferior brachium passes upwards from the lower quadrigeminal body to the under aspect of the medial geniculate body, which is a small oval mass on the lateral aspect of the mesencephalon, under cover of the pulvinar of the thalamus. Though the inner root of the optic tract is connected with this geniculate body, the inferior brachium







THE NERVOUS SYSTEM


1549


passes clear of it, and most of its fibres are traceable to the thalamus through the tegmentum.

Structure of Corpora Quadrigemina. —The lower quadrigeminal body {colliculus

inferior ) is composed of the following parts:

1. A central nucleus of grey matter.

2. A dorsal layer of white matter.

3. A ventral layer of white matter.

The central grey nucleus consists of many multipolar cells and nerve-fibres. The axons of the cells pass partly to the dorsal and partly to the ventral layers


Pulvinar

Superior Quad. Body

Inferior Quad. Body • Fourth Nerve Lateral Fillet - ~ — Basis Pedunculi Superior Cerebellar Peduncle—

Inferior Cerebellar Peduncle. _

Middle Cerebellar Peduncle Restiform Body Eighth Nerve ^

Olive


Med. Geniculate Body

-1 Lat. Geniculate Body

.Optic Tract __ . Corpus Mam.


Tuber Cinereum

- Third Nerve


Fifth Nerve


- - Seventh Nerve

" * Sixth Nerve


Fig. 952.— Side View of the Mesencephalon.


of white matter. The nerve-fibres are derived from the lateral or acoustic lemniscus , and terminate in arborizations around the cells of the central nucleus.

The dorsal white layer derives its fibres from the lateral lemniscus and from the axons of the cells of the central grey nucleus. The fibres pass into the inferior brachium, by which they are conducted to the medial geniculate body.

The ventral white layer also derives its fibres from the lateral lemniscus and from the axons of the cells of the central grey nucleus. This layer separates the central nucleus from the subjacent grey matter of the aqueduct. Some of the fibres cross the median plane, and decussate with corresponding fibres of the opposite side superficial to the roof of the aqueduct. Others enter the tegmentum of the crus of the same side and also of the opposite side, in which their course is downwards in the lateral lemniscus.












L55°


A MANUAL OF ANATOMY


The lower quadrigeminal body ( colliculus inferior), which receives its fibres from the lateral or acoustic lemniscus, is associated with the acoustic apparatus The upper quadrigeminal body ( colliculus superior) is composed of the follow, ing layers :

1. Stratum zonale. 3. Stratum opticum.

2. Stratum cinereum. 4. Stratum lemnisci.

The stratum zonale is the most superficial layer, and probably consists of retinal fibres which are derived from the outer root of the optic tract. Many of these fibres pass into the stratum cinereum and terminate in connection with its cells. Others cross the median plane and decussate with corresponding fibres from the opposite side superficial to the roof of the aqueduct.

The stratum cinereum, or second layer, lies beneath the stratum zonale, and consists of a crescentic layer of grey matter containing many nerve-cells. It represents the grey nucleus of the upper quadrigeminal body, and the axons of its cells pass to the more deeply seated strata.


Infundibulum Tuber Cinereum _

Mamillary Body Mesial Root of Optic Tract Lateral Root of Optic Tract /\

Lateral Geniculate Body b.

Locus Perforatus Posterior


Sixth Nerve Motor Root, Facial Nerve Sensory Root Auditory Nerve " Glosso-pharyngeal Nerve —3 Vagus Nerve

Superficial Arcuate Fibres Accessory Ner y e

First and Second Cervical Nerv


Optic Chiasma

3-)ptie Nerve


.Optic Tract


Third Nerve


_Fourth Nerve

-Motor Root of Fifth Nerve

-Sensory Root of Fifth Nerve


Middle Peduncle of Cerebellum


Restiform Body —.Hypoglossal Nerve ■ --Anterior Median Fissure

.Decussation of the Pyramids


Fig. 953.—The Medulla Oblongata, Pons, and Interpeduncular Region. C.C., crus cerebri; P., pyramid; O.B., olivary body.


The stratum opticum is the third layer, and consists of grey matter which contains numerous nerve-cells and nerve-fibres. The fibres are conducted to this stratum by the superior brachium, and are of two kinds: (1) Many are retinal fibres, and are derived from the outer root of the optic tract. (2) Others are corticifugal fibres, which come from the higher visual centre in the cortex of the occipital lobe, and form part of the optic radiation. The fibres pass into the stratum cinereum, and terminate in arborizations around its cells. The axons of the cells of the stratum opticum pass into the stratum lemnisci.

The stratum lemnisci is the deepest layer. Like the stratum opticum, it consists of , grey matter, which contains numerous nerve-cells and nerve-fibres. The fibres are derived from the following sources: (1) Many are derived from the medial or main lemniscus; and (2) some are the axons of cells belonging to the stratum opticum and stratum lemnisci. The lemniscal fibres terminate in the stratum lemnisci. The fibres formed by the axons of the cells of the stratum opticum and stratum lemnisci cross the median plane, below the aqueduct, and decussate with the corresponding fibres of the opposite side. This


/



THE NERVOUS SYSTEM


I55i


decussation is known as the fountain decussation (of Meynert). The fibres, after crossing, form the tectospinal tract of that side, and this bundle or tract descends through the pons and medulla oblongata into the corresponding ventral or anterior column of the spinal cord.

The upper quadrigeminal body, by means of the superior brachium, is one of the lower visual centres, the other being the external geniculate body.

Development of Corpora Quadrigemina. —The corpora quadrigemina are developed from the dorsal wall or roof of the mesencephalon. They are at first in the form of elongated paired swellings, later divided transversely.


Dorsal Tegmental Decussation (or Commissure of Meynert). —This commissure or decussation consists of fibres which issue from each upper quadrigeminal body, and cross partly to the opposite posterior longitudinal bundle, but mostly form tecto-spinal tracts, in which they descend towards the pons.

Crura Cerebri. —The crura or pedunculi cerebri are two large strands which are situated above the pons. They lie at first near each other, being separated by the interpeduncular fossa, but afterwards diverge as they pass upwards and laterally to the cerebral hemisphere. The medial surface of each crus bounds the interpeduncular region, and has a furrow, the oculo-motor sulcus, through which the roots of the third nerve emerge near the pons.

The lateral surface looks towards the temporal lobe of the brain, which to a large extent overlaps the crus, and this surface also has a furrow, the sulcus lateralis. The slender fourth cranial nerve lies upon this surface. Close to the cerebral hemisphere the ventral and lateral aspects of the crus are embraced by the optic tract of the corresponding side.

Each crus is composed of two parts—ventral and dorsal. The ventral part is the basis (or crusta), and the dorsal the tegmentum. The separation between these is indicated superficially by the sulcus lateralis and the oculo-motor sulcus. Within the crus the two parts are separated by a mass of dark grey matter, called the substantia


Aqueduct


Corpora Quadrigemina Tectum

Tegmentum


Lateral Sulcus


Subst. Nigra

-- Basis Pedunculi


Oculo-Motor Sulcus


Fig. 954. —Topography of the Crus Cerebri (after Poirier).


uigia* # • i • 1 i

The basis pedunculi (crusta) is continuous superiorly with the internal capsule of the corpus striatum, and inferiorly its fibres enter the ventral part of the pons.

Structure of the Basis.— The crusta, or basis, as seen m transverse section, presents a crescentic outline, the concavity of the crescent being occupied by the convexity of the substantia nigra. It consists of longitudinal corticifugal fibres which arise in the cells of the cerebral cortex. These fibres form two groups—pyramidal and cortico-pontine.

The pyramidal fibres form the motor tract from the precentral motor region of the cortex of the frontal lobe, and they arise for the



1552


A MANUAL OF ANATOMY


most part from the cells of that region, which control the voluntary muscles of the body.

The cortico-pontine fibres lie on each side of the pyramidal tract, those coming from the frontal region of the cortex being on the medial and those from the temporal region on the lateral side; the basis of each peduncle, therefore, is formed, from within outward, by frontopontine, pyramidal, and temporo-pontine tracts (see Fig. 958).

Tegmentum.— The tegmentum is continuous interiorly with the formatio reticularis of the dorsal portion of the pons, which in turn is continuous interiorly with the formatio reticularis of the medulla oblongata; the upward prolongation of the tegmentum makes the tegmental subthalamic region. The two tegmenta, right and left, are separated from each other by a median raphe, which is continuous with


Nucl. of Inf. Corpus Quadrigeminum


Fibres of Lat. Lemn.


Fourth N. Nucl.


Pallido-rubro-olivary

Tract

Medial longit. Bundle


Rubro-spinal Tr.

Fibres of Basis Pedunculi


Decuss. of Sup. Cerebell. Peduncle

Medial Lemniscus


Substantia Nigra


Pons


Fig. 955

Section through Inferior Corpora Quadrigemina.


that of the pons. In the lower part of the mesencephalon this raphe is indistinct on account of the decussation which takes place across the median plane between the superior cerebellar peduncles, underneath the lower pair of quadrigeminal bodies.

The dorsal surface of each tegmentum extends on either side of the grey matter of the aqueduct, and becomes continuous with the basal parts of the upper and lower quadrigeminal bodies of the corresponding side, which constitute the tectum. The ventral surface is separated from the crusta by the substantia nigra.

Structure of the Tegmentum. —Each tegmentum, besides being continuous interiorly with the formatio reticularis of the dorsal portion of the pons, consists of bundles of longitudinal and transverse fibres, the intervals between which are occupied by grey matter.










THE NERVOUS SYSTEM


1553


Grey Matter. —The grey matter of the tegmentum contains the red nucleus.

The red nucleus (nucleus ruber) is a round reddish mass, which is situated in the centre of the upper part of the tegmentum, and lies in the path of the superior cerebellar peduncle of the opposite side. It is on the same level as the upper quadrigeminal body, and is prolonged upwards into the subthalamic tegmental region. Some of the fibres of the superior cerebellar peduncle of the opposite side surround the red nucleus in the form of a capsule on their way to the thalamus. Other fibres of that peduncle enter the red nucleus, and terminate in arborizations around its cells.

The axons of the cells of the red nucleus form two sets of nervefibres—ascending and descending. The ascending fibres pass to the thalamus in company with those fibres of the superior cerebellar peduncle which encapsule the red nucleus. These ascending fibres form relays which carry on those fibres of the superior cerebellar peduncle which terminate within the red nucleus, that nucleus being a cell-station in their path. The descending fibres constitute the rubro-spinal tract (or bundle of Monakow). The fibres of this tract cross the median plane in the raphe, and by their decussation with those of the opposite side they constitute the ventral fountain decussation [of Foret), in contradistinction to the dorsal fountain decussation (of Meynert). The latter decussation is on a higher level, and involves the fibres of the ventral longitudinal bundles, or tecto-spinal tracts, which derive their fibres from the cells of the stratum opticum and stratum lemnisci of the upper quadrigeminal bodies. The rubrospinal tract of either side descends through the pons and medulla oblongata into the lateral column of the spinal cord, where each constitutes the prepyramidal tract, which lies on the ventro-lateral aspect of the lateral cortico-spinal tract. (The tecto-spinal tract, or ventral longitudinal bundle, on either side descends into the anterior column of the spinal cord.)

White Matter of the Tegmentum. —The principal tracts of the white matter on either side are as follows:

1. Superior cerebellar peduncle.

2. Medial (posterior) longitudinal bundle.

3. Ventral longitudinal bundle, or tecto-spinal tract.

4. Pallido-rubro-olivary tract.

5. Rubro-spinal tract.

6. Medial lemniscus (chief sensory tract).

7. Lateral (acoustic) lemniscus.

Superior Cerebellar Peduncle.— The fibres of this peduncle emerge for the most part through the hilum of the nucleus dentatus in the cerebellar hemisphere. The two peduncles, right and left, having emerged from the hemispheres, pass upwards on the lateral parts of the dorsal surface of the pons in a converging manner towards the lower pair of the quadrigeminal bodies, being connected

98


1554


A MANUAL OF ANATOMY



by the superior medullary velum. On entering the mesencephalon, the two peduncles decussate across the raphe beneath the lower quadrigeminal bodies. This decussation extends as high as the upper quadrigeminal bodies, and it involves almost all the fibres of the two peduncles. Each peduncle, having gained the opposite side, ascends in the upper part of the tegmentum as a longitudinal tract,

and soon comes into contact with the red nucleus. Many of its fibres enter this nucleus and terminate in arborizations around its cells. Other fibres of the peduncle encapsule the nucleus, and then ascend through the subthalamic tegmental region to the anterior part of the thalamus, within which they terminate in arborizations around the cells of the anterior nuclear area (chief sensory nucleus). From the cells of the red nucleus relays of fibres proceed upwards, which carry on those fibres of the peduncle which terminate within the nucleus, and these relays ascend with those fibres of the peduncle which encapsule the red nucleus to the thalamus.

The superior cerebellar peduncle of one side connects the cerebellar hemisphere of that side with the postcentral in the Mid-brain as it re gion of the cerebral cortex of the

. opposite side, the red nucleus and the

lbres su P enor peduncles thalamus being cell-stations in the path are shown decussating and f gu & r

rprl thipIahc '-'■l tilt; IlDlCS.


Fig. 956.—Plan of Relations of Certain Fibre-tracts


reaching red nucleus, from which the rubro-spinal tracts emerge, decussate, and pass below the cerebellar fibres to enter the pons laterally. On the right the pallidorubro-olivary tract is shown in white, indicating its relation to the peduncular fibres; it sinks deeply in the pons, turning somewhat laterally. It is not shown in front of the red nucleus.


A few of the fibres of each superior cerebellar peduncle do not take part in the decussation beneath the lower pair of quadrigeminal bodies, but ascend to the red nucleus of their own side.


Before the fibres decussate, or after the decussation has taken place, each of them furnishes a descending branch.

These descending branches form the descending cerebellar bundle (of Cajal), which traverses the dorsal part of the pons and the medulla oblongata, giving off collaterals to the motor nuclei of these parts. According to Cajal, the fibres of this bundle enter the anterior column of the spinal cord, and are connected with the cells of the ventral column of grey matter.

The superior cerebellar peduncle also contains the indirect or ventral spino-cerebellar tract (of Gowers).









THE NERVOUS SYSTEM


1555


Medial or Posterior Longitudinal Bundle. —This bundle occupies he dorsal part of the tegmentum, and is intimately related to the

rey matter which forms the ventral wall or floor of the aqueduct, t lies close to the median raphe, as does its fellow of the opposite ide, and across the raphe an interchange of fibres takes place between he two bundles. In the spinal cord it is represented by the anterior ntersegmental fibres and the anterior marginal bundle (of Lowenthal). n the medulla oblongata it traverses the dorsal part of the pyramid, >eing separated from the pyramidal (motor) fibres by the medial emniscus or chief sensory tract. Thereafter it traverses the formatio eticularis of the dorsal part of the pons, and is continued upwards is one of the tracts of the tegmentum of the crus cerebri.

The fibres of the medial longitudinal bundle are regarded as being

he axons of cells belonging to (1) the nucleus of Deiters, which is me of the terminal nuclei of the vestibular root of the auditory nerve,

2) the formatio reticularis of the medulla oblongata and pons,

3) the formatio reticularis of the tegmentum, (4) the sensory nucleus )f the fifth cranial nerve, and (5) the nucleus of the longitudinal rundle. Inferiorly the fibres of the bundle ramify within the interior column of the spinal cord in connection with the motor cells )f the ventral horn of grey matter. Superiorly its fibres are intimately •elated to the following important nuclei—namely, (1) the oculonotor nucleus, or nucleus of the third cranial nerve; (2) the trochlear lucleus, or nucleus of the fourth cranial nerve; and (3) the abducent mcleus, or nucleus of the sixth cranial nerve, these being the nuclei which control the muscles of the eyeball and upper eyelid. The Dundle furnishes numerous collaterals to each of these nuclei, which

erminate in arborizations around their cells. The bundle also estabishes connections with the motor nuclei in the pons and medulla iblongata.

The medial longitudinal bundle extends as high as a special nucleus, called the nucleus of the posterior longitudinal bundle, which is situated n the grey matter of the ventro-lateral portion of the third ventricle rear the upper opening of the aqueduct, from the cells of which nucleus

some of its fibres arise. . .

The bundle consists of ascending and descending association fibres, which form connections between the important nuclei just referred to. Probably the chief use of the bundle is to maintain a functional association between these nuclei, and insure harmonious action of the muscles which are supplied by the nerves arising from them.

Tecto-spinal Tract (Ventral Longitudinal Bundle). This bundle ties on the ventral aspect of the medial longitudinal bundle.. Its fibres are derived from the stratum opticum and stratum lemnisci of the upper quadrigeminal body of the opposite side. These fibres, as stated in connection with the upper quadrigeminal bodies, descend beside t e °Tey matter round the aqueduct, cross the median plane, and decussate with the corresponding fibres of the opposite side, the decussation being known as the dorsal fountain decussation (of Meynert). Ihe


j 556


A MANUAL OF ANATOMY


fibres descend through the tegmentum, lying close to the red nucleus, to which they furnish collaterals. Thereafter they traverse the formatio reticularis of the pons and medulla oblongata, still lying on the ventral aspect of the medial longitudinal bundle. From the medulla oblongata the fibres pass into the anterior ground-bundle of the lateral column of the spinal cord, where they lie just in front of the rubro-spinal tract, and they form arborizations around the motor cells of the ventral horn of grey matter.

The pallido-rubro-olivary tract (Figs. 955 and 956) is a well-formed and marked bundle of fibres which can be found in sections through the mid-brain and pons. The fibres lie between the red nucleus and the olive, above the medial part of the red nucleus and the cerebellar peduncular fibres in the crura, within the concavity of these fibres as they pass between their decussation and the superior peduncle, more laterally in the middle and lower parts of the pontine tegmentum, and gain the inferior olive just below the lower border of the pons; they may be visible in part on the surface here (Fig. 883). The exact path of the tract between the red nucleus and the globus pallidus is not certainly known; the tract is probably in great part interrupted at the red nucleus, from which the rubro-olivary fibres take origin, but direct pallido-olivary fibres are known to be present also.

This tract is essentially a structure belonging to the brains of the higher vertebrates, in which the inferior olive replaces or reinforces the primitive olivary formation; this is represented in the human brain by the medial and other accessory olives.

Rubro-spinal Tract (or Bundle of Monakow). —The fibres of this tract are derived, as previously stated, from the axons of the cells of the red nucleus. They cross the median plane, decussating with the corresponding fibres of the opposite side, and constituting the ventral fountain decussation [of Foret). The tract then descends through the pons and medulla oblongata into the lateral column of the spinal cord, in which it constitutes the prepyramidal tract on the ventrolateral aspect of the lateral cortico-spinal tract.

Medial Lemniscus. —The medial or main lemniscus of either side begins in the lower part of the medulla oblongata. It is here the only lemniscus on either side, and its fibres are derived from the deep lemniscal arcuate fibres, which arise from the cells of the nucleus gracilis and nucleus cuneatus of the opposite side. The main lemniscus therefore represents the upward continuation of the posterior column of the spinal cord (gracile and cuneate fasciculi), and it is spoken of as the chief sensory tract. The deep lemniscal arcuate fibres cross the median plane directly above the decussation of the pyramids [motor decussation) , thus constituting the decussation of the lemnisci (main fillets or chief sensory tracts), or the sensory decussation. The fibres, after crossing the median plane, form the tract of the side to which they have crossed. In the medulla oblongata it lies close to the median raphe, and at first is in front of the medial longitudinal bundle, and



THE NERVOUS SYSTEM


1557


iirectly behind the pyramid. The main lemniscus then ascends through

he dorsal part of the pons, its relative position remaining unchanged, [n this situation the lateral lemniscus, to be presently described, takes jp its position on the outer or lateral aspect of the main or medial Dand. The main tract, on leaving the pons, enters the ventral part i)f the tegmentum, still having the lateral tract on its outer side. As t encounters the red nucleus it is displaced laterally and backwards, md then occupies the dorso-lateral part of the tegmentum, lying almost beneath the medial geniculate body.

The fibres of the main or medial lemniscus (chief sensory tract) terminate in two ways: (1) Some enter the upper quadrigeminal body, md these probably terminate in the stratum lemnisci; (2) others traverse the subthalamic tegmental region, and enter the anterior part of the thalamus, within which they terminate in arborizations around the cells of the ventro-lateral nuclear area (chief sensory nucleus). From these cells relays of thalamo-corticipetal fibres proceed to the cerebral cortex.

It is convenient to refer to the main or medial band as the sensory lemniscus.

Lateral Lemniscus. —The main or medial band being the sensory lemniscus, it is convenient to refer to the lateral one as the auditory fillet or lemniscus. The fibres of this fillet are derived from the following sources: (1) The corpus trapezoides, the fibres of which come from the ventral cochlear nucleus, the nucleus trapezoides, and the superior olive of the opposite side, as well as from the superior olive of the same side; (2) the auditory striae, which are derived from the lateral cochlear nucleus of the opposite side; and (3) the nucleus of the lateral lemniscus.

The fibres of the right and left lateral lemnisci decussate across the median plane. Having crossed to the opposite side, the fibres become longitudinal and form a well-marked ascending tract in the dorsal part of the pons, which takes up a position on the lateral or outer side of the main or medial tract. In this part of its course the lateral lemniscus encounters a collection of grey matter, called its nucleus. Some of its fibres end in this nucleus. Others pursue their upward couise, and are reinforced by relays of fibres which arise from the nerve-cells of the nucleus On leaving the pons the lateral band enters the tegmentum, and its fibres terminate in (1) the nuclei of the lower quadrigeminal body, and (2) the cells of the medial geniculate body. The fibres destined for the lower quadrigeminal body, having curved round the lateral aspect of- the superior cerebellar peduncle, become superficial on the outer side of the tegmentum. The fibres destined for the medial geniculate body reach it through the inferior brachium partly directly and partly through the intervention of the lower quadrigeminal body. The axons of the cells of the geniculate body form corticipetal fibres which pass to the cortex of the first or superior

temporal gyrus of the temporal lobe. .

The lateral lemniscus, therefore, is associated with the auditory apparatus. It is chiefly composed of ascending fibres. There are,


A MANUAL OF ANATOMY


I55 8

however, some descending fibres which are probably derived from the lower quadrigeminal bodies.

In addition to the foregoing, there are other tracts.

Fasciculus Retroflexus. —The fibres of this bundle, already described, arise from the cells of the ganglion habenulce. They descend in the upper part of the tegmentum internal to the red nucleus, and they terminate in arborizations around the cells of the interpeduncular ganglion.

Bundle of Munzer. —The fibres of this tract descend from the lower quadrigeminal body to the formatio reticularis of the lateral part of the pons.

Spino-thalamic Tract. —The fibres of this tract, as stated in connection with the tracts of the spinal cord, arise from the cells of the dorsal grey column of the opposite side. Having crossed in the ventral or white commissure, they enter the antero-lateral or indirect cerebellar tract (tract of Gowers), in which they ascend through the medulla oblongata, pons, and tegmentum of the crus cerebri to the thalamus of the side to which they have crossed.


Fig. 957.—Diagram to show Position and Relations of Structures in

Tegmental Subthalamus.

Supposed to be viewed from the medial aspect. R, red nucleus. The subthalamic nucleus is shown antero-lateral to this. Dotted line shows course of fasciculus retroflexus from habenula to interpeduncular ganglion. Course of anterior pillar of fornix is indicated, also mamillo-thalamic tract (bundle of Vicq d’Azyr) passing up medial to front part of subthalamic nucleus. Substantia nigra is seen near pontine level, but passes upwards and laterally out of the section higher up.

Subthalamic Tegmental Region. —This region represents the upward prolongation of the tegmentum of the crus cerebri beneath the posterior part of the inferior or ventral surface of the thalamus. The prolongation contains (1) an upward extension of the red nucleus of the tegmentum, (2) the fibres of the superior peduncle of the cerebellum, and (3) the main or medial lemniscus (chief sensory tract).

The upward extension of the red nucleus ceases about the level of the corresponding corpus mamillare. Some of the fibres of the superior peduncle of the cerebellum terminate, as stated, in the red nucleus, and others encapsule it, as they do in the tegmentum. Many fibres issue from the cells of the red nucleus, and these, along with the investing fibres of the superior cerebellar peduncle, enter the inferior or ventral surface of the thalamus.


THE NERVOUS SYSTEM


1559


The main lemniscus (chief sensory tract), which lies on the dorsolateral aspect of the red nucleus, also enters the inferior or ventral surface of the thalamus.

Development of the Crura Cerebri. —The crura cerebri are developed in the ventral wall of the mesencephalon.

Basis Pedunculi (Crusta or Pes). —The basis is the ventral portion of the crus cerebri, and is separated from the tegmentum of the crus by a mass of dark grey matter, called the substantia nigra, which is situated in the interior. Externally the separation is indicated on the outer aspect by the lateral sulcus, and on the inner aspect by the oculomotor sulcus, through which the fasciculi of the oculo-motor nerve emerge. The basis is continuous with the internal capsule of the corpus striatum, and it consists of longitudinal centrifugal fibres, which arise in the cells of the cerebral cortex. These fibres are arranged in two sets, pyramidal and cortico-pontine. The pyramidal fibres form


supr cerebellar peduncle — red nucleus^ fillet-_ posi c long 1 , bundlenucleus of 3 rd nerve

supr corpus_

quadrigeminum


corpus mammilare 3 r - d cranial nerve -optic tract -fronto pontine fibres

--pyramidal tract _ temporo pontine fibres _ -substantia nigra

-exT geniculate body

-inti- do. do. --fimbria -—dentate fissure -optic radiations

aqueduct


F IG . 958.—The Crura Cerebri and their Relations.


the motor tract from the precentral region of the cerebral cortex, and the cortico-pontine fibres are arranged in two strands—namely, frontopontine and temporo-pontine.

The pyramidal fibres form the motor tract from the precentral region of the cerebral cortex. T hey traverse the lenticular portion of the posterior limb of the internal capsule, and then occupy the middle three-fifths of the crusta. Thereafter they descend through the ventral portion of the pons and the pyramid of the medulla oblongata. In the lower part of the pyramid they give rise to the crossed and direct pyramidal tracts. The crossed pyramidal tract, having taken part in the decussation of the pyramids, descends in the spinal cord as the lateral corticospinal tract, occupying the posterior part of the lateral column of the opposite side The direct pyramidal tract descends (anterior cortico-spmal tract) in the spinal cord, occupying the medial part of the anterior column of the same side. Its fibres however, cross at intervals to the opposite side.




1560


A MANUAL OF ANATOMY


As the pyramidal tract descends through the pons and medulla oblongata, some of its fibres pass to the motor nuclei of the cranial nerves in these regions.

The cortico-pontine fibres are arranged in two strands, frontopontine and temporo-pontine. The fibres of the fronto-pontine strand arise from the cells of the cortex of the anterior part of the frontal lobe, and, having traversed the anterior limb of the internal capsule, they are regarded as occupying the medial fifth of the basis pedunculi. The fibres of the temporo-pontine strand arise from the cells of the cortex of the temporal lobe, and having traversed the postlenticular part of the internal capsule, they occupy the lateral fifth of the crusta. In the ventral part of the pons both the fronto-pontine and the temporo-pontine fibres terminate in arborizations around the cells of the nucleus pontis, whereas the pyramidal fibres pass uninterruptedly through the ventral part of the pons.

Substantia Nigra. — t his is a mass of dark grey matter which is situated between the tegmentum and the basis of the crus cerebri. Like the basis, it is semilunar or crescentic, as seen in transverse section. It contains many multipolar nerve-cells, which are deeply pigmented, and it extends from the upper border of the pons into the subthalamic tegmental region. Laterally it reaches the lateral sulcus on the lateral aspect of the crus, where it is thin, and the oculo-motor sulcus on the medial aspect, where it is thick, and is traversed by the fasciculi of the third cranial or oculo-motor nerve. Its tegmental surface is concave, and the surface directed towards the basis is convex. From the latter surface prolongations extend into the basis.

1 he substantia nigra does not acquire its pigment before the second or third year after birth.

Aqueduct of Mid-brain. —The aqueduct is the narrow passage which leads through the mesencephalon from the third to the fourth ventricle (iter a tertio ad quartum ventriculum ). It lies nearer the dorsal than the ventral aspect of the mesencephalon; its direction is from above downwards, and its length is rather more than J inch. Its upper opening is situated on the posterior boundary of the third ventricle immediately underneath the posterior commissure, and its lower opening occupies the superior median angle of the floor of the fourth ventricle. In transverse section the aqueduct is T-shaped in its upper part near the third ventricle, and triangular in its lower part near the fourth ventricle. The passage is lined with ciliated columnar epithelium, external to which there is a thick layer of grey matter, which is spoken of as the central (Sylvian) grey matter. This is continuous superiorly with the grey matter of the floor and lateral walls of the third ventricle, and inferiorly with that which covers the floor of the fourth ventricle. It contains numerous nerve-cells disposed in a scattered manner, but, in addition to these, there are certain definite cell-groups.. These groups constitute the nuclei of origin of the following cranial nerves: the third or oculo-motor, the


THE NERVOUS SYSTEM


1561


fourth or trochlear, and the mesencephalic root of the fifth nerve. The oculo-motor nucleus is situated in the ventral portion of the grey matter underneath the upper quadrigeminal body, and it extends upwards into the layer of grey matter on the adjacent portion of the lateral wall of the third ventricle. The trochlear nucleus is also situated in the ventral portion of the grey matter, but at a lower level than the oculo-motor nucleus, being placed underneath the upper part of the lower quadrigeminal body. The nucleus of the mesencephalic root of the fifth nerve is extensive, and is situated in the lateral portion of the grey matter.

Development. —The aqueduct is the persistent remains of the cavity of the mesencephalon.

Posterior Perforated Substance. —This area has been previously described in a general way in connection with the base of the encephalon. It will here be considered more fulfy. It lies at the bottom of a deep depression, called the interpeduncular fossa, which forms the back part of the interpeduncular space. The fossa is bounded posteriorly by the median portion of the upper border of the pons, and laterally by the crura cerebri. Anteriorly it is limited by the corpora mamillaria.

The locus perforatus is a perforated lamina of grey matter which forms the floor of the interpeduncular fossa, the openings being for the passage of the postero-medial ganglionic branches of the posterior cerebral arteries. This grey lamina extends between the tegmenta of the crura cerebri.

Ganglion Interpedunculare. —This is a collection of nerve-cells situated medially in the lower part of the grey lamina which constitutes the posterior perforated substance. On either side it receives the fibres of the fasciculus retroflexus , which are derived from the ganglion habenulae.


Structure of the Cerebral Hemispheres.


The cerebral hemisphere is composed of grey and white matter. The grey matter is disposed externally, and forms the cerebral cortex, which, with the exception of the rhinencephalon, is known as the neopallium. The white matter occupies the interior, and constitutes the

medullary centre.

Cerebral Cortex.— The grey matter forms a continuous covering to the entire hemisphere, dipping into the sulci, so as to cover the opposed surfaces of the gyri, as well as the bottom of the sulci, is thicker over the superficial surfaces of the gyri than at the bottom of the sulci, and attains its greatest thickness over the upper portions of the precentral and postcentral gyri, whilst it is thinnest over the


The cerebral cortex is indistinctly divided into strata by means of layers of a whitish substance. When examined m section it therefore presents a stratified appearance, and is seen to consist of successive


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A MANUAL OF ANATOMY


grey and white layers alternating with each other. In most parts of the cerebral cortex there are four superimposed strata; but in certain situations— e.g., over the precentral gyrus—there are as many as six. These strata are as follows, from without inwards:


1. Molecular layer, a superficial

white layer (pale and narrow).

2. Superficial grey layer, the outer

granular.

3. Outer white band of Baillarger.

4. Middle grey layer.


5. Inner white band of Baillarger.

6. Inner or deep grey layer, the

polymorphous layer, subjacent to which is the white matter of the medullary centre.


Layers 3, 4, and 5 are included in the pyramidal layer, in which the cells tend to increase in size as they lie more deeply; the largest lie over the inner band of Baillarger.

The medullated fibres of the medullary centre pass into the stratified grey cortex in a radiating manner, and within the cortex their course for the most part is perpendicular to the superficial surface, and between the component cells of the cortex.

Minute Structure of the Cerebral Cortex. —The cortex is composed of nerve-cells and nerve-fibres.

Nerve-cells. —These are arranged in four layers, which are, from without inwards: (1) the molecular layer; (2) the layer of small pyramidal cells; (3) the layer of large pyramidal cells; and (4) the layer of polymorphous cells.

The molecular or plexiform layer, which is the most superficial, is thin, and consists of cells and fibres. Many of the cells are neurogliacells, the others being nerve-cells. These nerve-cells are for the most part fusiform, and are disposed horizontally. They are known as the horizontal cells of Cajal. Their dendrons and axons are long, the latter forming medullated fibres which are disposed horizontally or parallel to the surface. These furnish minute branches which pass vertically towards the surface. The horizontal cells, according to Cajal, receive impulses from the corticipetal fibres which extend from the thalamus to the cerebral cortex.

In addition to these fibres there are many others which enter the molecular layer from deeper sources, and form a dense interlacement by their ramifications. The sources from which these extraneous fibres are derived are: (1) the terminal ramifications of the apical dendrons of the pyramidal cells (small and large); (2) the axons of the cells of Martinotti, which lie in the polymorphous layer; and (3) corticipetal fibres derived from the medullary centre of the gyrus.

The pyramidal layers represent the second and third layers, and are composed of characteristic pyramidal cells which are peculiar to the cerebral cortex, those of the second layer being small, whilst those of the third layer are large. The layer of small pyramidal cells is narrow, but the layer of large pyramidal cells is of considerable thickness. The giant pyramidal cells of the motor cortex are known as cells of Betz. There is no well-marked line of demarcation between


THE NERVOUS SYSTEM 1563

these two layers, the one passing imperceptibly into the other. They constitute the chief part of the cerebral cortex.


Molecular j Plexiiorm Layer


Layer of Small £ Pyramidal Cells


Layer of Large Pyramidal ° Cells


Layer of . Polymorphous ** Cells


Plexus of Exne r (.Superficial Tangential Fibres)


Band ot Bechterew


Outer Band of Bail larger


Radiating

Vertical

Fibres

Inner Band of Baillarger


Deep Tangential Fibres


•White Medullary Centre


Fig , W -Diagram showing the Minute Structure of the Cerebral J Cortex (Poirier).

The fibres are shown on the right, and the cells on the left.


The apex of each pyramidal cell is directed towards the surface of the evrus and is prolonged into a long tapering dendrite which (Fig. 960) passes into the molecular layer, giving off delicate collatera s




















































































564

A MANUAL OF ANATOMY


Terminal Ramifications


in its course. Near the surface of the molecular layer it divides into terminal filaments, which are disposed horizontally and mingle with the tangential fibres. The base of the pyramidal cell is directed towards the medullary centre of the gyrus, and from its centre an axon is given off, which enters the medullary centre, giving off collaterals in its course. From each side of the body of the cell, as well as from each lateral angle of its base, dendrites are given off.

The polymorphous layer is the deepest layer, and is composed of cells which have different shapes. Each cell gives off several dendrites, which pass towards, but do not enter, the molecular layer. The axon of each cell enters the medullary centre as a nerve-fibre.

In addition to the foregoing cells of the cerebral cortex, two other kinds of cells are met with amongst the pyramidal and polymorphous cells—namely, the cells of Golgi and the cells of Martinotti. The cells of Golgi are characterized by the fact that the axon of each almost immediately divides into several branches, which pass towards the surface, but soon terminate without entering the molecular layer. The cells of Martinotti are chiefly met with in the polymorphous layer. The axon of each cell passes towards the surface, and enters the molecular


Human Cerebral Cortex (Ramon y Cajal).


layer, where it divides into terminal branches, which form part of the tangential fibres of this layer.


Nerve-fibres of the Cortex. —1 hese are arranged in two groups— vertical and tangential.

The vertical (radial) fibres are disposed in radiating bundles, which issue from the medullary centre, and traverse the polymorphous and large pyramidaf layers, after which they become indistinguishable. The polymorphous and large pyramidal cells lie m the spaces between these bundles, and assume a columnar arrangement. The fibres of the radiating bundles gradually become less numerous, some of them becoming the axons of the polymorphous cells, but most of them becoming the axons of the large pyramidal cells. The radiating bundles contain centripetal cortical fibres, which pass into the molecular layer and end in terminal ramifications, forming part of its tangential fibres.



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The tangential fibres are disposed horizontally at different levels, and form the following strata: (1) the superficial tangential fibres (plexus of Exner), which occupy the superficial part of the molecular layer; (2) the band of Bechterew, which is situated in the superficial part of the small pyramidal layer; (3) the outer band of Baillarger (band of Vicq d’Azyr), which intersects the large pyramidal layer; (4) the inner band of Baillarger, which is situated between the large pyramidal and polymorphous layers; and (3) the deep tangential fibres (intracortical association fibres), which are situated in the deep part of the polymorphous layer.

The tangential fibres are formed by (1) the collaterals of the polymorphous and pyramidal cells and of the cells of Martinotti; (2) the ramifications of the axons of the cells of Golgi; and (3) association fibres.

Medullary Centre of Cerebral Hemisphere. —The white matter of the medullary centre consists of medullated nerve-fibres, which pursue different courses, and are arranged in three groups—namely, projection fibres, commissural fibres, and association fibres.

The projection fibres connect the cerebral cortex with parts at a lower level, and they are of two kinds—corticipetal or afferent, and corticifugal or efferent. The commissural fibres pass from one hemisphere to the other, and connect portions of the cerebral cortex of opposite hemispheres. The association fibres are confined to one side of the median plane, and they bring different parts of the cerebral cortex of the same hemisphere into association with one another.

Projection Fibres. —These fibres, as stated, are both corticipetal and corticifugal, and the majority of them constitute the internalcapsule of the corpus striatum, and the diverging arrangement of its fibres known as the corona radiata, which passes to all parts of the cerebral cortex. Some projection fibres, however, do not traverse the internal capsule and corona radiata—£.g., the fibres of the ansa peduncularis.

Corticifugal Fibres.— The corticifugal or efferent fibres constitute the following tracts:

1. Pyramidal or motor. 3 - Fronto-pontine.

2. Cortico-thalamic. 4- Temporo-pontine.

5. Optic radiation (portion).

The pyramidal or motor tract derives its fibres from the axons of the pyramidal cells of the cortex of the precentral gyrus, which is situated in front of the central fissure. Having traversed the corona radiata, these fibres pass in succession through (1) the posterior limb of the internal capsule, (2) the middle three-fifths of the basis of the crus cerebri, (3) the ventral portion of the pons, and (4) the pyramid of the medulla oblongata. The motor strand enters the spinal cord in three ways—partly as the direct or ventral cortico-spinal tract, partly as the uncrossed lateral tract, but chiefly as the crossed lateral


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A MANUAL OF ANATOMY


corticospinal tract. Ultimately the fibres terminate at different levels in arborizations around the motor-cells of the ventral column of grey matter of the opposite side, from which cells the fibres of the motor nerve-roots proceed.

The efferent fibres which pass to the motor nuclei of the cranial nerves do not, as a whole, run in the cortico-spinal pathway through the basis pedunculi and basis pontis. They leave this path, usually in the upper part of the mid-brain, and pass down (Fig. 961) in the tegmentum of the mid-brain and pons. They reach the tegmentum also at lower levels, passing usually either medial or lateral to the substantia nigra, but they are not constant in this matter. These cortico-pontine or

cortico-bulbar fibres are thus aberrant or extra-pyramidal fibres. The figure, which is modified from Dejerine, shows the nuclei supplied by this group, the remnant of which rejoins the main tract in the medullary pyramid. Each central supply to the nuclei decussates, crossing the middle line nearly at the level of the nucleus to which it is going.

The cortico-thalamic tract extends only between the cerebral cortex and the thalamus. Its fibres arise as the axons of the pyramidal cells of various parts of the cerebral cortex, and they terminate in arborizations around the cells of the thalamus.

The fronto-pontine tract does not extend lower than the pons. It consists of fibres which arise as the axons of the pyramidal cells of the cortex of the prefrontal region—that is to say, the region of the frontal lobe in front of the precentral sulcus. These fibres traverse the anterior limb of the internal capsule, and then descend through the inner or medial fifth of the basis of the crus cerebri into the pons. Within the pons they terminate in arborizations around the cells of the nucleus pontis.

The temporo-pontine tract, like the preceding, does not extend lower than the pons. It consists of fibres which arise as the axons of the pyramidal cells of the cortex of the first and second temporal gyri. These fibres traverse the postlenticular part of the posterior limb of the internal capsule, and then descend through the outer fifth of the basis of the crus cerebri into the ventral part of the pons. Within this part of the pons they terminate in arborizations around the cells of the nucleus pontis.

The corticifugal fibres of the optic radiation consist of fibres which arise as the axons of the pyramidal fibres of the cortex of the occipital


N. III.


' ~ ' N. IV.


N. V.

N. VI.

N. VII.

N. Amb. N. XII.


Fig. 961. — Plan of Extrapyramidal Fibres running in the Tegmentum to Nuclei of Cranial Nerves (modified from Dejerine).







THE NERVOUS SYSTEM


1567

lobe. Ihey traverse the post lenticular part of the posterior limb of the internal capsule, and thereafter pass to the lower visual centres— namely, the lateral geniculate body and the upper quadrigeminal body. Within these bodies they terminate in arborizations around their component cells.

Corticipetal Fibres. —The corticipetal or afferent fibres belong to the following tracts:

1. Medial lemniscus. 3. Thalamic radiation.

2. Superior cerebellar peduncle. 4. Auditory radiation.

5. Optic radiation.

The medial lemniscus, or principal sensory tract , arises from the nucleus gracilis and nucleus cuneatus of the medulla oblongata, and is the upward prolongation of the posterior column of the spinal cord. Having decussated with its fellow, it ascends through the dorsal part of the pons through the tegmentum of the crus cerebri, and through the subthalamic tegmental region to the thalamus. Within this body its fibres terminate in arborizations around the thalamic cells. As the medial lemniscus ascends towards the thalamus some of its fibres enter the upper quadrigeminal body, in which they end. From the thalamus the fillet-fibres are continued to the cerebral cortex by relays of thalamo-cortical fibres.

The superior cerebellar peduncle, having decussated with its fellow, soon comes into contact with the red nucleus. Many of the fibres of the peduncle enter this nucleus and terminate in arborizations around its cells. Numerous fibres encapsule the nucleus, and continue their course upwards, traversing the subthalamic tegmental region, and finally entering the ventral aspect of the thalamus, within which they terminate in arborizations around the thalamic cells. As in the case of the fillet-fibres, they are continued to the cerebral cortex by relays of thalamo-cortical fibres.

The thalamic radiation is composed of thalamo-cortical fibres which arise as the axons of the cells within the thalamus, that body being regarded as an aggregation of cell-stations in the path of such corticipetal fibres as those of the medial lemniscus and superior cerebellar peduncle. These thalamo-cortical fibres, as stated in the description of the thalamus, issue from that body in four groups or stalks—frontal, parietal, occipital, and inferior or ventral. The fibres of the frontal stalk traverse the anterior limb of the internal capsule, and most of them pass to the cortex of the frontal lobe. The fibres of the parietal stalk pass partly through the internal capsule and partly through the external capsule to the cortex of the parietal lobe and of the central region of the frontal lobe. The fibres of the occipital stalk belong to the optic radiation, to be presently described. The fibres of the inferior or ventral stalk form the ansa lenticularis and ansa peduncularis. The ansa lenticularis enters the nucleus. lentiformis, within which its fibres terminate. the ansa peduncularis passes beneath the nucleus lentiformis and traverses the external capsule, the


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A MANUAL OF ANATOMY


destination of its fibres being the cortex of the temporal lobe and insula.

The auditory radiation consists of fibres which arise as the axons of the cells of the medial geniculate body. Having issued from that body, they traverse the postlenticular part of the posterior limb of the internal capsule, and pass to the cortex of the middle part of the first temporal gyrus of the temporal lobe.

The corticipetal fibres of the optic radiation are associated with the corticifugal fibres, already described. The corticipetal fibres arise as the axons of the cells of the corpus geniculatum laterale and upper quadrigeminal body. They traverse the postlenticular part of the posterior limb of the internal capsule, and then pass to the cortex of the occipital lobe.

Commissural Fibres. —These fibres are disposed transversely, and serve to connect the grey cortex of one hemisphere with that of the other. They constitute the following commissures: (1) the corpus callosum ; (2) the anterior commissure ; and (3) the lyra, which is known as the hippocampal commissure. The fibres of the corpus callosum, as they enter each hemisphere, are disposed so as to form an extensive callosal radiation, and serve to connect the cortex of one hemisphere with that of the other. The individual portions of cortex so connected may be symmetrical, but to a large extent are not. The fibres arise on one side as (1) the axons of pyramidal or of polymorphous cells, or (2) collaterals of projection or of association fibres; and on the opposite side they terminate in delicate arborizations.

The anterior commissure, which crosses from side to side in front of the anterior pillars of the fornix, divides on either side into two parts, olfactory and temporal. The olfactory portion enters the olfactory tract. Some of its fibres serve to connect the olfactory bulb of one side with that of the other side; and other fibres connect the olfactory bulb of one side with the temporal lobe of the opposite. The temporal portion enters the white matter of the temporal lobe on either side.

The hippocampal commissure lies below the splenial portion of the corpus callosum, and is separated from the roof of the third ventricle by the tela chorioidea. It is a thin layer of arched fibres connecting the posterior pillars and sides of the fornix, and derived mainly from the hippocampus of each side; it is shown in Fig. 933. The commissure, which is not well developed in the human brain, was known as the ‘ lyra ’ in former days.

The corpus callosum is the great commissure of the neopallium; the hippocampal and anterior commissures, phylogenetically much older, are connections of the rhinencephalon, and hence archipallial.

Association Fibres. —These fibres serve to connect different parts of the cortex of the same hemisphere, and they are of two kinds, short and long.

The short association fibres pass between neighbouring gyri, extending in their course across the bottom of the sulci. Some of them


/


THE NERVOUS SYSTEM 1569

lie beneath the grey cortex, whilst others are contained within its deep part.

The long association fibres pass between portions of the grey cortex, which are at some distance from each other. They are arranged in bundles, the chief of which are as follows: (1) the superior longitudinal fasciculus; (2) the interior longitudinal fasciculus; (3) the perpendicular fasciculus; (4) the uncinate fasciculus; (5) the cingulum; (6) the occipitofrontal fasciculus; and (7) the fornix.

The superior longitudinal fasciculus consists of fibres which extend from the frontal to the occipital lobe. Posteriorly many of its fibres sweep downwards and forwards into the temporal lobe, and from this circumstance it is sometimes spoken of as the arcuate fasciculus.

The inferior longitudinal fasciculus connects the occipital and temporal lobes, its fibres being disposed upon the lateral walls of the posterior and inferior horns of the lateral ventricle.

The perpendicular fasciculus connects the inferior parietal lobule with the occipito-temporal gyrus.

The uncinate fasciculus crosses the stem of the lateral fissure, and connects the frontal and temporal lobes.

The cingulum is connected with the rhinencephalon, and lies upon the under surface of the callosal gyrus and the upper surface of the hippocampal gyrus. Its fibres connect the gyri of the lobe with the cerebral cortex.

The occipito-frontal fasciculus connects the frontal with the occipital and temporal lobes. It lies internal to the corona radiata, in intimate relation to the nucleus caudatus, and as the fibres pass backwards they lie on the outer walls of the inferior and posterior horns of the lateral ventricle.

The fornix connects the hippocampus major of one side with the corresponding corpus mamillare, and through the latter with the thalamus by means of the mamillo-thalainic tract (bundle of Vicq d’Azyr), the fibres of which arise in the corpus mamillare.

Peculiarities of the Cerebral Cortex-— 1. Calcarine Area.— This area is situated on the medial surface of the occipital lobe in close proximity to the calcarine fissure, and it is known as the visual area. In this region the outer band of Baillarger is very conspicuous, and is known as the white band of Gennari, whilst the inner band of Baillarger is absent.

2. Central Area. —In this region, more especially in the cortex of the precentral gyrus, there are groups of very large pyramidal cells, which are known as the giant-cells of Betz, and nerve-fibres are present in large numbers.

3. Hippocampal Area. —The hippocampus corresponds to the hippocampal or dentate fissure, and is produced by an infolding of the cerebral cortex. It is therefore composed chiefly of grey matter, and is covered superficially by a thin layer of white matter, called the alveus, which is continuous with the fimbria. The hippocampus is composed of the following layers, named in order from the ventricular surface outwards: (1) the alveus, composed of white matter, and covered by the ventricular ependyma; (2) neuroglial layer, consisting of neuroglia fibres and cells; (3) pyramidal layer, composed of large pyramidal cells; (4) stratum radiatum, which is the outer part of the pyramidal layer, and is composed of the dendrites of the apical parts of the pyramidal cells, being thereby rendered striated in appearance; (5) stratum laciniosum, composed of the ramifications

99


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A MANUAL OF ANATOMY


of the foregoing apical dendrites intimately intermixed; (6) stratum granulosum, composed of many small cells; and (7) the involuted medullary lamina, consisting of white fibres.

Olfactory Tract and Olfactory Bulb. —These are developed as a hollow outgrowth from the anterior cerebral vesicle, more particularly from the part of it which ultimately gives rise to the lateral ventricle, and is known as the telencephalon. In many animals the central cavity persists, and maintains its connection with the lateral ventricle; but in man the cavity disappears, though traces of its ependymal lining remain. External to the vestigial ependyma there is a layer of white matter, and superficial to this there is a layer of grey matter. In the olfactory tract the layer of grey matter is very thin over the ventral or inferior aspect, but over the dorsal or superior aspect it is fairly thick. In the bulb the reverse is the case, the grey matter being thick over the ventral aspect, where it receives the olfactory filaments, but thin over the dorsal aspect.

Structure of the Ventral Grey Matter of the Olfactory Bulb. —The

grey matter consists of three layers—namely, (1) the nerve-fibre layer, (2) the glomerular layer, and (3) the granular layer.

The nerve-fibre layer is the most superficial layer, and is composed of olfactory nerve-fibres. These fibres are non-medullated, and arise as the axons of the olfactory cells of the olfactory mucous membrane of the nasal fossa. Having passed through the foramina of one half of the cribriform plate of the ethmoid bone, they enter the grey matter on the ventral aspect of the bulb, where they break up and form arborizations. These intermingle with the arborizations formed by the dendrites of the mitral cells, to be presently described.

The glomerular layer is composed of round bodies or glomeruli, which are formed by the interlacements between the arborizations of the olfactory nerve-fibres and those of the dendrites of the mitral cells.

The granular layer lies next to the layer of white matter, and is chiefly characterized by the presence of large mitral cells. These are pyramidal, and one dendrite from each cell passes into the glomerular layer, where it gives rise to a glomerulus in the manner just described in connection with the glomerular layer. Other dendrites intermingle with those of adjacent mitral cells. The axon of each mitral cell enters the white layer of the bulb, and passes along the olfactory tract to the cerebrum.

Weight of the Brain. —The average weight of the brain of the adult male is about 48 ounces (1,360 grammes), and that of the adult female about 44 ounces.

Arteries of the Encephalon.

Cerebral Part of the Internal Carotid Artery. —The internal carotid artery, having pierced the roof of the cavernous sinus internal to the anterior clinoid process of the sphenoid bone, ascends between the second and third cranial nerves to the inner end of the stem of the


THE NERVOUS SYSTEM 1571

lateral fissure. Here it divides into its terminal branches, anterior and middle cerebral.

Branches are posterior communicating, anterior choroidal, anterior cerebral, and middle cerebral.

The posterior communicating artery arises from the back part of the internal carotid, and passes backwards to anastomose with the


Fig. 962.—Arteries on the Base of the Brain.


posterior cerebral artery. It is usually small, but is often larger on one side than the other. Occasionally it is absent.

The anterior choroidal artery arises from the back part of the internal carotid close to its termination. It passes backwards and outwards between the crus cerebri and the "hippocampal gyrus, and enters the lower and anterior extremity of the descending horn of the



A MANUAL OF ANATOMY


U572

lateral ventricle by passing through the choroidal fissure. In its course it passes just above the uncus, gives twigs to the hippocampal gyrus and crus cerebri, and terminates in the choroid plexus of the lateral ventricle.

Anterior Cerebral Artery. —This is the smaller of the two terminal branches of the internal carotid artery, and it has a more limited distribution than the other terminal branch—namely, the middle cerebral artery. It passes forwards and inwards above the optic nerve, and just internal to the roots of the olfactory tract, to the commencement of the great longitudinal fissure, where it is connected with its fellow of the opposite side by a short transverse vessel, called the anterior communicating artery. After this it enters the great longitudinal fissure, turns round the genu of the corpus callosum, and passes back


Fig. 963.—Median Section of Brain with Distribution of Anterior

Cerebral Artery.

wards over the upper surface of that body to the splenium, where it anastomoses with the posterior cerebral artery.

Branches .—These are arranged in two groups, antero-medial or ganglionic and cortical.

The antero-medial or ganglionic branches, small in size, pass through the lamina cinerea along with twigs from the anterior communicating artery, and supply the front part of the caudate nucleus.

The cortical branches are as follows: (1) medial orbital, to the medial orbital gyrus, olfactory lobe, and gyrus rectus; (2) anterior medial frontal, to the superior frontal gyrus, the anterior two-thirds of the middle frontal gyrus, and the anterior part of the marginal gyrus; (3) middle frontal, to the collosal gyrus, the posterior part of the marginal gyrus, and the upper part of the precentral gyrus; and (4) posterior



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frontal, to the precuneus and the corpus callosum, the branch of the latter being known as the artery of the corpus callosum.

Anterior Communicating Artery. —This vessel connects the two anterior cerebral arteries at the entrance to the great longitudinal fissure, and lies over the lamina terminalis in front of the optic commissure. It gives off a few twigs, which accompany the antero-medial branches of each anterior cerebral artery.

Middle Cerebral Artery. —This is the larger of the two terminal branches of the internal carotid artery, and it has a wider distribution than the anterior cerebral artery. It enters the stem of the lateral fissure, in which it passes outwards.

Branches .—These are arranged in two groups, antero-lateral or ganglionic and cortical. The antero-lateral or ganglionic branches


Pig. 964._Distribution of the Left Middle Cerebral Artery (Charcot).


F.i. Superior Frontal Gyrus F.2. Middle Frontal Gyrus F.3. Inferior Frontal Gyrus F.A. Ascending Frontal Gyrus P.A. Ascending Parietal Gyrus L.P.S. Superior Parietal Gyrus L.P.I. Inferior Parietal Gyrus P.C. Angular Gyrus L.O. Occipital Lobe


L.T. Temporal Lobe

S. Middle Cerebral Artery entering Lateral Fissure

P. Lenticulo-striate Arteries

1. Artery to Inferior Frontal Gyrus

2. Ascending Frontal Artery

3. Ascending Parietal Artery

4. Parieto-Temporal Artery

5. Arteries to Temporal Lobe


pass through the anterior perforated substance, and foim two sets, medial striate and lateral striate. The medial striate arteries repiesem the ‘ lenticular arteries * of Duret, and they supply the globus pallidus (inner part) of the lentiform nucleus, the internal capsule, and the caudate nucleus. Ihe lateral striate arteries represent the. lenticulostriate ’ and ‘ lenticulo-optic ’ arteries of Duret. The lenticulo-striate arteries supply the putamen (outer part) of the lentiform nucleus and the external capsule. One of the lenticulo-striate aiteries is said to be larger than the others, and is subject to rupture in cases of cerebral haemorrhage. It is often termed the artery of cerebral hcemorrhage (Charcot). Its course is laterally and upwards round the outer aspect of the lentiform nucleus, between it and the external capsule, and then through the internal capsule to the caudate nucleus. The lenticulo



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A MANUAL OF ANATOMY


thalamic arteries supply the posterior part of the lentiform nucleus and the anterior part of the thalamus on its lateral aspect.

The most important point to notice about all the ganglionic arteries of the brain is that they are ‘ end arteries/ and their branches, once having divided, never anastomose again; hence, if one of them is blocked by an embolus, which is often a piece of fibrin from a diseased heart valve, the area of brain supplied will be cut off from all bloodsupply, and the clinical effects may be very grave.

The cortical branches arise in the vicinity of the insula, and are: (i) inferior lateral orbital, to the orbital surface of the frontal lobe lateral to the internal orbital sulcus, and to the inferior frontal gyrus;


Fig. 965.- —Coronal Section of the Cerebral Hemispheres made One Centimetre behind the Optic Commissure (Charcot, from Duret).


The arteries of this region are shown.


I. Area of Cerebral Artery

II. Area of Middle Cerebral Artery

III. Area of Posterior Cerebral Artery V.V. Sections of Anterior Cornua of Lateral Ventricles

P.P. Anterior Pillars of Fornix C.N. Caudate Nucleus L.S.A. Lenticulo-Striate Arteries CL. Claustrum I.R. Insula E.C. External Capsule


L. N. Lentiform Nucleus I.C. Internal Capsule O.T. Optic Tract (cut)

G.M. Grey Matter of Third Ventricle O.C. Optic Chiasma O.N. Optic Nerve A.C. Anterior Cerebral Artery C. Internal Carot'd Artery

M. C. Middle Cerebral Artery L.A. Lenticular Arteries

A.C.H. Artery of Cerebral Hemorrhage


(2) ascending frontal, to the posterior third of the middle frontal gyrus, and to the lower and greater part of the precentral gyrus; (3) ascending parietal, to the postcentral gyrus and superior parietal gyrus; (4) parietotemporal, which traverses the posterior horizontal limb of the lateral fissure, and supplies the inferior parietal lobule and the posterior parts of the superior and middle temporal gyri; and (5) the temporal branches, which emerge from the posterior horizontal limb of the fissure, and supply the anterior and greater parts of the superior and middle temporal gyri.

Fourth or Intracranial Part of the Vertebral Artery. —The vertebral artery, on leaving the suboccipital triangle, pierces the dura mater and arachnoid, and enters the cranial cavity through the foramen












THE NERVOUS SYSTEM


1575


magnum. As it ascends, it lies at first on the side of the medulla oblongata, between the hypoglossal nerve and the anterior root of the suboccipital nerve. It then passes upwards on the ventral surface of the medulla, and on reaching the lower border of the pons it unites with its fellow of the opposite side to form the basilar artery. It will generally be found that one vertebral artery is larger than the other.

Branches. —Posterior meningeal, posterior spinal, posterior inferior cerebellar, anterior spinal, and bulbar.

The posterior meningeal artery arises from the vertebral artery just before it pierces the dura mater, and it enters the cerebellar fossa of the occipital bone where it supplies the dura mater.

The posterior spinal artery arises from the vertebral artery immediately after it has pierced the dura mater. It descends upon the side of the spinal cord in front of the posterior roots of the spinal nerves, and it gives off a branch which descends behind these roots. These two arteries, in themselves small, are reinforced by the spinal branches of the second part of the vertebral artery and of the dorsal branches of the intercostal arteries. In this manner lateral anastomotic arterial chains are formed upon each side of the spinal cord in front of and behind the posterior nerve-roots.

The posterior inferior cerebellar artery, of large size, arises a little above the preceding branch. It passes backwards between the vagus and accessory nerves, and then over the restiform body to the vallecula of the cerebellum, where it divides into branches. Some of these supply the inferior vermis, and others ramify on the inferior surface of the cerebellar hemisphere, at the periphery of which they anastomose with branches of the superior cerebellar artery. The artery furnishes branches to the corresponding choroid plexus of the fourth ventricle and to the medulla oblongata.

The anterior spinal branch of the vertebral artery arises from that vessel near its termination. It passes obliquely downwards and inwards over the ventral aspect of the medulla oblongata, and at the median line it unites with its fellow of the opposite side to form the anterior spinal artery of the spinal cord. It furnishes twigs to the medulla oblongata.

The bulbar branches are distributed to the medulla oblongata.

Basilar Artery. —This vessel is formed by the union of the two vertebral arteries. It extends from the lower to the upper border of the pons, occupying the median basilar groove on its ventral surface. It lies deep to the arachnoid membrane within the cisterna pontis, and at the upper border of the pons it divides into the two posterior cerebral arteries.

Branches. —These are as follows, on either side: transverse, internal auditory, anterior inferior cerebellar, superior cerebellar, and posterior cerebral.

The transverse or pontine arteries are numerous, and pass outwards on either side to supply the pons, the sensory and motor roots of the fifth cranial nerve, and the middle peduncle of the cerebellum.



1576 A MANUAL OF ANATOMY


The internal auditory artery, long and slender, accompanies the auditory nerve through the internal auditory meatus, and is distributed to the internal ear.

The anterior inferior cerebellar artery arises from the centre of the basilar, and passes backwards to be distributed to the anterior part of the inferior surface of the cerebellar hemisphere. It anastomoses

with the posterior inferior


cerebellar artery, which is a branch of the vertebral.

The superior cerebellar artery arises from the basilar close to its termination. It passes outwards parallel to the posterior cerebral artery, from which it is separated by the third cranial nerve. It then winds round the outer side of the crus cerebri below the fourth cranial nerve, and so reaches the superior surface of the cerebellar hemisphere, where it divides into branches. These supply the superior vermis, the upper medullary velum, the tela chorioidea, and the superior surface of the cerebellar hemisphere, at the periphery of which they anastomose with branches of the inferior cerebellar arteries.

Posterior Cerebral Artery. —This is one of the terminal branches of the basilar artery at the upper border of the pons. It passes at first outwards beneath the crus cerebri, and parallel to the superior cerebellar artery, from which it is separated by the third cranial nerve. It then winds round the outer side of the crus cerebri, lying between it and the hippocampal gyrus, and above the fourth cranial nerve. In this manner it reaches the tentorial or inferior and medial surfaces of the occipital lobe of the cerebral hemisphere. It receives, not far from its commencement, the posterior communicating artery.

Branches .—These are arranged in three groups—ganglionic, choroidal, and cortical.

The ganglionic group includes two sets of branches, postero-medial and postero-lateral.


Fig. 966. —The Areas of Distribution on the Surface of the Three Main Cerebral Arteries.







THE NERVOUS SYSTEM 1577

The postero-medial ganglionic arteries pass medial to the crus cerebri, and pierce the posterior perforated substance. They supply the inner part of the crus cerebri and the posterior part of the thalamus.

The postero-lateral ganglionic arteries pass on the lateral side of the crus cerebri, and supply the outer part of the crus, the posterior part of the thalamus, the corpora quadrigemina, and the corpora geniculata.

The posterior choroidal arteries are two or three in number, and pass through the choroidal fissure to the tela chorioidea, which they supply, together with the choroid plexus of the lateral ventricle, and the corresponding choroid plexus of the third ventricle.

The cortical branches are: (i) anterior temporal, to the anterior parts of the occipito-temporal and hippocampal gyri; (2) posterior temporal, to the posterior parts of the occipito-temporal and hippocampal gyri, and the inferior temporal gyrus; and (3) occipital, to the occipital lobe. One of the occipital branches is called the calcarine artery. It lies in the calcarine fissure, and supplies the lingual or infracalcarine gyrus and the cuneus.

Circulus Arteriosus (Circle of Willis).—This circle or (to be more exact) heptagon is formed in front by the anterior communicating artery,

which connects the two .

anterior cerebral arteries; behind by the basilar artery as it divides into the two posterior cerebral arteries; and on either side by (1) the anterior cerebral artery, (2) the trunk of the internal carotid aitery,

(3) the posterior communicating artery, and (4) the posterior cerebra artery, in this order from before backwards. The cncle furnishes twigs to the grey cortex of the interpeduncular region. It serves to equalize the blood-pressure in the cerebral arteries, and it also provides for the regular supply of blood to the brain m cases where one of the main arterial trunks may be obstructed.

The following parts are contained within the circulus arteriosus, in order from behind forwards: (1) the posterior perforated area; (2) the corpora mamillaria; (3) the tuber cinereum and infundibulum; and

(4) the optic chiasma.


13 15 1*


x. Internal Carotid

2. Middle Cerebral

3. Anterior Cerebral _

4. Anterior Communicating

5. Posterior Communicating

6. Posterior Cerebral

7. Basilar

8 Superior Cerebellar 9. Transverse Pontine

10. Internal Auditory

11. Anterior Inferior Cerebellar

12. Posterior Inferior Cerebellar

13. Vertebral

14. Anterior Spinal

15. Posterior Spinal

16. Anterior Choroid

17. Posterior Choroid

18. Central or Ganglionic

19. Central or Ganglionic

20. Central or Ganglionic

(Postero-mesia!)

21. Central or Ganglionic

(Postero-lateral)


Fig. 967. —The Arteries at the Base of the Brain, and the Arterial Circle.




A MANUAL OF ANATOMY


1578

Veins of the Encephalon.

The cerebral veins are arranged in two groups—superficial and deep.

The superficial cerebral veins are divided into two sets—superior and inferior.

The superior cerebral veins return the blood from the upper parts of the outer surfaces of the cerebral hemispheres. They lie in the pia mater, and pierce the arachnoid membrane and inner layer of the dura mater, after which they open into the superior longitudinal sinus, having previously received the veins from the medial surface of either hemisphere. Their direction for the most part is forwards and medially, whilst the direction of the blood-current in the superior longitudinal sinus is backwards.

The inferior cerebral veins return the blood from the lower parts of the cerebral hemispheres, and they terminate in the cavernous, superior petrosal, and lateral sinuses. One of these veins is known as the superficial middle cerebral or superficial Sylvian vein. It passes along the lateral fissure, and opens into the front part of the cavernous sinus or else into the spheno-parietal sinus. This vein communicates posteriorly with (1) the superior longitudinal sinus by means of the great or superior anastomotic vein (of Trolard); and (2) the transverse sinus by means of the inferior anastomotic vein (of Labbe).

the deep cerebral veins are as follows: (1) the choroidal veins; (2) the veins of the corpora striata; (3) the internal cerebral veins (veins of Galen); (4) the anterior cerebral veins; (5) the deep middle cerebral veins; and (6) the basilar veins.

The choroidal vein of each side begins in the choroid plexus of the inferior horn of the lateral ventricle. It ascends at first, and then passes forwards in the lateral margin of the tela chorioidea to the interventricular foramen, where it unites with the vein of the corpus striatum to form the corresponding internal cerebral vein (vein of Galen).

The vein of the corpus striatum is formed by branches which issue from the corpus striatum and thalamus. It runs forwards in the groove between these two bodies, lying superficial to the taenia semicircularis, and at the interventricular foramen it joins the choroidal vein to form the corresponding internal cerebral vein.

The internal cerebral vein (vein of Galen) of each side is formed close to the interventricular foramen by the union of the choroidal vein, the vein of the corpus striatum, and the vein of the septum lucidum. The two veins, right and left, pass backwards between the two layers of the tela chorioidea, and they unite beneath the splenium of the corpus callosum to form one vessel, called the great cerebral vein, which opens into the front part of the straight sinus. Each vein receives tributaries from the thalamus, the corresponding choroid plexus of the third ventricle, the corpus callosum, and the corpora quadrigemina; and, before joining its fellow, it takes up the basilar vein of its own side. The great cerebral vein receives tributaries from the upper surface of the cerebellum.


THE NERVOUS SYSTEM


1579


The anterior cerebral vein of each side is situated in the great longitudinal fissure, along with the corresponding artery. Having curved round the genu of the corpus callosum, it passes to the anterior perforated region, where it joins the deep middle cerebral vein to form the basilar vein.

The deep middle cerebral vein returns the blood from the insula and lies deeply within the stem of the lateral fissure.

The basilar vein of each side begins at the anterior perforated area, where it is formed by the union of the anterior cerebral and deep middle cerebral veins. It passes backwards round the crus cerebri, and opens into the internal cerebral vein of its own side just before that vessel joins its fellow to form the great cerebral vein. The basilar vein receives, close to its commencement, one or more inferior striate veins, which descend from the corpus striatum through the anterior perforated substance. It also receives tributaries from the parts within the interpeduncular space.

The cerebellar veins are arranged in two groups—superior and

inferior.

The superior cerebellar veins terminate in the great cerebral vein, and in the straight, transverse, and superior petrosal sinuses. The inferior cerebellar veins pass to the sigmoid, inferior petrosal, and occipital sinuses.


Blood-supply of the Different Parts of the Encephalon.

The medulla oblongata is supplied by the vertebral, anterior spinal, and

posterior inferior cerebellar arteries. .,

The pons is supplied by the transverse or pontine branches of the basilar

The cerebellum is supplied inferiorly by the posterior inferior cerebellar branches of the vertebral arteries, and the anterior inferior cerebellar branches of the basilar artery. Superiorly it is supplied by the superior cerebellar branches of the basilar artery, and to a limited extent by the posterior inferior cerebellar

arteries

The crus cerebri is supplied by the postero-medial and postero-lateral branches of the posterior cerebral artery, and by the posterior communicating

Th’e posterior perforated substance is pierced by the postero-medial branches

of the posterior cerebral arteries. , , . v

The corpora quadrigemina are supplied by the postero-lateral ganglionic

branches of the posterior cerebral arteries. , +

The thalamus is supplied posteriorly by the postero-meclial and posterolateral ganglionic branches of the posterior cerebral artery. Anteriorly its outer part is supplied by the lenticulo-thalamic branches of the middle cerebral artery,

and its inner part by the posterior communicating artery. .

The anterior perforated substance is pierced by the antero-lateral ganglionic

branches of the middle cerebral artery. , ., . . c ,,

Frontal Lobe— The superior frontal gyrus, the anterior two-thirds of the middle frontal gyrus, and the upper portion of the precentral gyrus are supplied bv cortical branches of the anterior cerebral artery. The posterior third of the

middle frontal gyrus, the inferior frontal gyrus and the .lower “h^ce-ebra* of the precentral gyrus are supplied by cortical branches of the^middle <cerebral „ rfpr * Q n t h e orbital surface the internal orbital gyrus, olfactory lobe, ana gyrus rectus are supplied by the anterior cerebral artery, whilst the remainder is supplied by the middle cerebral artery.


1580 A MANUAL OF ANATOMY

Parietal Lobe. —The whole of this lobe, practically, is supplied by the middle cerebral artery.

Occipital Lobe. —This lobe is supplied by the posterior cerebral artery.

Temporal Lobe. —The superior and middle temporal gyri and the pole are supplied by the middle cerebral artery, and the remainder is supplied by the posterior cerebral artery.

Medial Surface of the Cerebral Hemisphere. —The anterior cerebral artery has an extensive distribution to this surface, which it supplies as far back as the internal part of the parieto-occipital fissure. The parts behind this fissure— namely, the cuneus and the parts around the calcarine fissure—are supplied by the posterior cerebral artery.

The corpus callosum is supplied by the anterior cerebral arteries.

Corpus Striatum. —The nucleus caudatus and nucleus lentiformis are supplied for the most part by the antero-lateral or ganglionic branches of the middle cerebral artery, which pass through the anterior perforated substance. According to Duret they form three sets—lenticular, lenticulo-striate, and lenticulo-thalamic. The lenticular (internal striate) arteries supply the globus pallidus (inner part)


Fig. 968. —Brain of an Embryo about Four Weeks Old (from

Quain’s ‘ Anatomy ’) (His).

1. Telencephalon 4. Metencephalon 7. Spinal Cord

2. Thalamencephalon 5. Myelencephalon 8. Pontine Flexure

3. Mesencephalon 6. Cervical Flexure 9. Olfactory Lobe

1, 2=prosencephalon; 3 = mesencephalon; 4, 5 = rhombencephalon

of the lentiform nucleus, the internal capsule, and the caudate nucleus. The lenticulo-striate (external striate) arteries supply the putamen (outer part) of the lenticular nucleus, and the external capsule. One of the lenticulo-striate arteries is larger than the others, and is subject to rupture in cases of cerebral haemorrhage. It is known as the artery of cerebral hcemorrhage (Charcot). Its course is outwards and upwards round the lateral aspect of the lentiform nucleus, between it and the external capsule, and then through the internal capsule to the caudate nucleus. The lenticulo-thalamic arteries supply the posterior part of the lentiform nucleus, and the anterior part of the thalamus on its lateral aspect.

1 he front part of the caudate nucleus is supplied by the antero-medial ganglionic branches of the anterior cerebral and anterior communicating arteries.

The larger arteries occupy the subarachnoid space, where they divide into branches which enter the pia mater. These in turn give off smaller branches, which enter the cerebral substance, some of them being cortical and others medullary in their distribution.

Blood-supply of the Choroid Plexuses. —The choroid plexus of the lateral ventricle derives its blood from (i) the anterior choroidal artery, which is a


THE NERVOUS SYSTEM


1581


branch of the internal carotid or the middle cerebral; and (2) the posterior choroidal arteries, which are branches of the posterior cerebral. The choroid plexuses Of the third ventricle derive their blood from the posterior choroidal arteries. The choroid plexuses of the fourth ventricle are supplied from the posterior inferior cerebellar arteries.

The tela chorioidea derives its blood from the posterior choroidal arteries and from the superior cerebellar arteries.

Lymphatics of the Brain. —There are no lymphatic vessels in the brain. Their place is taken by spaces in the outer coat of the arteries, called perivascular spaces, which are in communication with the subarachnoid space.


Development of the Encephalon.

A brief outline of the formation of the brain has been given on pp. 55-59* in which can be followed the development of the primary cerebral divisions into fore-, mid- and hind-brain, the formation of the cerebral vesicles, the appearance of the brain flexures, and in general the changes which lead to the existence in their proper positions of the main structures in the brain. The student is advised to read this general account before proceeding to the following descriptions, which deal with the conditions in the developing brain in a more detailed manner.


Metamorphoses of the Hind-brain.

The pontine flexure begins to show about the beginning of the fourth week, is well marked at the beginning of the second month, and has its two limbs very close to one another by the end of this month. The posterior limb of the flexure, down to the nuchal bend, is termed the myelencephalon, the anterior limb the metencephalon, and the narrow junction with the mid-brain is the

isthmus.

Myelencephalon. —The walls, opened out in their upper parts, with a wide roof-plate, give origin to the medulla oblongata, and the cavity forms the lowei half of the fourth ventricle ; at the extreme lower end the cavity is not enlarged. The development of this lower or hinder portion of the myelencephalon proceeds in general on the lines of development already described for the spinal cord, with certain modifications due to the opening out and change in direction in certain tracts passing to the brain, the crossing of pyramidal fibres, and the presence of certain tracts and nuclei (to be described later) associated with the

existence of visceral arches. , ,

A little higher up the myelencephalon broadens to make the lower part of

the fourth ventricle. The alar and basal laminae are now in the floor of the wide cavity. The broad roof-plate, a single layer of epithelium, is attached at the sides to an everted edge of the alar lamina, known as the rhombic lip, an over hanging the outer parts of the laminae. Such a definite rhombic lip however, is only found in the cranial part of the myelencephalon, where, as will be seen later, the great enlargements appear which are due to growth of the vestibular nuclei.

The widened roof-plate is covered by vascular mesenchyme, representing pia mater. At the level of the pontine flexure the ependymal or epithelial roof is invaginated into the fourth ventricle in the form of a transverse fold —plica chorioidea, containing pia mater—which extends between the lateral recesses of the ventricle From this transverse fold two vertical folds —phcce chorioidea, e wise containing pia mater—extend vertically downwards into the ventricle close to the median line. These ependymal folds, containing pia mater, form the two

choroid nlexuses of the lateral ventricle. . , .

At a comparatively early stage the afferent fibres of the seventh, ninth and

tenth nerves pass into the marginal zone of the alar lamina, and form a bund here i ZZausTolitarius; this bundle becomes deeply buried by subsequent

thl °Thehvno!gloss afnucleus deveiops, as has been mentioned already on p. 1440 praetor witWnth"ndymal P zone, in the upper part of the column of


1582


A MANUAL OF ANATOMY


loosened nuclei seen here in the cervical region formed from the ependymal zone ventro-laterally. The sixth nucleus possibly arises from the extreme cranial end of the same column, but this is not certainly known. In the young embryo the efferent nuclei (except that of the hypoglossal) lie in the basal lamina, where they cause internal depressions by their rapid growth in the thin wall (Fig. 970). These depressions are known as neuromeres, and are a marked though temporary feature of most embryonic brains. In the illustration they are seen from within, and have a curious distribution, in that the sixth neuromere is placed behind the seventh. The fifth has two neuromeres, of which the most cranial is much the deeper; the line of flexure of the hind-brain, which has not yet begun in this specimen, will pass through this deep neuromere of the fifth nerve.


Fig. 969.—Lateral View of Brain, End of Second Month.

Cbl, cerebellar rudiment; P, N, pontine and nuchal flexures; M, mid-brain flexure; Pit, hypophysis; Cpb, corpus ponto-bulbare. Roof-plate of hindbrain is only shown in outline.

A little later a neuromere will mark, rather indefinitely, the ninth efferent nucleus, but is not seen in the figure.

It must be understood that the neuromeres are present only in the basal lamina; this, for practical purposes, is the one seen in the figure, the thin and narrow strip (D) being the only representative of the alar lamina.

Behind the region of neuromeres the myelencephalon narrows down to its continuity with the spinal cord, and it is here, extending cranially, that the olive is laid down.

The inferior olive, with its medial and dorsal accessory formations, is developed as a modification of the upper part of the ventro-lateral column of neuroblasts in the mantle zone, from which the ventral column is formed at a lower level. The early stages of this development are shown in Fig. 971, while Fig. 972


THE NERVOUS SYSTEM


1583



Fig. 970.

A, sagittal section of brain of 4-9 mm. embryo; B, section through two adjacent

neuromeres.


Fig. 971. —Hind-brain, 13-5 Mm. Embryo.

Outline of longitudinal median section on right below; m this cr. is the cranial slope and sp. the spinal cord; TS, tractus solitanus; vl ventro-lateral nuclei. Planes of sections a, b, and c correspond with those showm on the outline.




A MANUAL OF ANATOMY


1584

gives the appearances in subsequent stages. In these it can be seen that the median accessory olive (m.o.) is first defined, the main olivary mass being constructed from the more lateral condensations.

It must be said here that the classical and accepted account of olivary development refers it to the ventral migration of neuroblasts from the ‘ rhombic lip/ The account given above is put forward because it is in accord with observed facts, whereas the older story is very unsatisfactory in several particulars.


Fig. 972.

Transverse sections through olivary region in embryos of 15 and 16 mm.; horizontal sections, 18, 21, and 28 mm. in neighbourhood of nuchal flexure.

The cuneate and gracile nuclei are formed directly from the dorsal neuroblasts.

Each pyramid is a ventral bulging of that part of the basal lamina which is on the mesial side of the olivary body, and it is produced by the motor tracts as they descend in the marginal layer from the central area of the cerebral cortex through the pons. This begins in the fourth month.

The ponto-bulbar body is represented in the embryo by a collection of small and darkly-staining nuclei lying below the caudal part of the ‘ rhombic lip/ It appears in the latter part of the second month, and its nuclei spread fairly rapidly over the surface of the myelencephalon, especially ventrally and cranially; here they lay down the beginnings of the pontine nuclei. On the myelencephalon further back they appear to be responsible for the various small superficial




THE NERVOUS SYSTEM


1585


arcuate nuclei which may be found on the surface of the pyramid, etc. There is also a possibility that the lateral accessory cochlear nucleus may possess a similar origin.

As the pyramids, right and left, bulge ventralwards, the floor-plate, which connects the basal laminaj, sinks, and the anterior median fissure is formed, as in the development of the spinal cord. The spongioblastic floor-plate is now invaded by nerve-fibres, most of which cross from one side to the other, these fibres being (1) the anterior superficial arcuate fibres, (2) the deep arcuate fibres, and (3) the cerebello-olivary fibres. In this manner the raphe of the bulb is formed, as in the development of the anterior or white commissure of the spinal cord.

In the more cranial portion of the myelencephalon, where the efferent nuclei have been laid down in the neuromeres, the subsequent growth of the alar lamina affords opportunity for the development of the vestibular nuclei, which thus lie just cranial to the great dorsal masses of the gracile and cuneate nuclei. The


Fig. 973._ Plans to show Areas in Floor of Fourth Ventricle, with

Position of Developing Structures.

changes which take place in the floor of the ventricle in this part are verv com plicated and not by any means understood; the plans in Fig. 973 are attempts to show the results of the changes. The first figure gives the positions of the neuromeres on the left, with the sites of the afferent nuclei on the r t§tit. n . e second and third the vestibular masses are associated with considerable widening but there is apparently a marked forward upgrowth of the floor in the basal area which carries the sixth and seventh nuclei forward and brings them against the metencephalic surface. This is no doubt associated with the curious relations between the two nuclei and nerves, but the way in which it comes about, as well as the reason for its occurrence, is not known. The positions of the various nuclei are given approximately in the plans, and a general idea of their origins and changes can be obtained by a study of the figures.

Metencephalon.— -From this are developed the pons, cerebellum, its upper and middle peduncles, and the superior and inferior medullary vela Its cavity forms the upper part of the fourth ventricle .


100







1586


A MANUAL OF ANATOMY


1 he pons develops as a ventral thickening on the lower end of the region, immediately cranial to the pontine flexure. Its nuclei appear to owe their origin to the neuroblasts which have spread over the surface from the ponto-bulbar body; presumably they increase subsequently in situ, but no definite indications of mitotic activity have been


Fig. 974.— Semi-diagrammatic Figures showing Cerebellar Rudiments.


found among them. The down-growing cerebro-spinal fibres find their way into and among these nuclei in the third month and subsequently.

The cerebellum is developed from the alar laminae of the metencephalon, the thickening involving the roof-plate in its growth. The

two lateral cerebellar plates formed by the laminae are at first inclined to each other at an angle (Fig. 976), but as the angle of the pontine flexure becomes more closed and the metencephalon widens, the paired cerebellar rudiments come nearly into line with one another (Figs. 974, 977). The lateral plates, being thickenings in the floor of the cavity, project at first into the cavity, covered by the roof-plate, which is attached to the margins outside them ; subsequently the attachment of the roof-plate is turned in (Fig. 977) below the bulging lateral formations, so that it becomes attached, descriptively, to the anterior and lower aspect of the transversely disposed cerebellar rudiment. It is in the taenial fold to which the roof-plate is attached here that the floccular enlargement occurs a little later.

In some lower vertebrates the cerebellum develops altogether within the cavity of the hind-brain, as in the early human stage; the later change in the human conditions enables the structure to expand freely outside the ventricle.

The upper part of the roof-plate of the rhombencephalon, at the angle of junction of the lateral plates, is invaded by them, and forms


Fig. 975.— Brain of Third Month Embryo seen

FROM BEHIND, TO SHOW *

the Transverse Cerebellar Rudiment.

This stage is between those shown in the previous figure.


/




THE NERVOUS SYSTEM


1587


the basis in which the vermis develops; growth here is slow, and the lateral lobes in their enlargement come to overlap it and cover it in.

The lateral plates come into evidence during the second month; the transversely disposed plate (Fig. 975) is well marked in the middle of the third month, when the flocculus may be recognized, and after this the development proceeds slowly. After the third month the vermis shows transverse fissures, and in the fifth month these are found also on the lateral lobes.


Fig 076_Ventricular Aspect of Cranial Slope of Hind-brain in Em bryos OF 12, 16, AND 28 Mm., TO SHOW BEGINNINGS OF CEREBELLAR FOLD.


The fissure cutting off the flocculus extends across the region of the vermis, marking off the nodule here. At the end of the third month a fissura prima forms across the vermis, making the lowei edge of the future culmen, and a little later a fissura secunda forms below the future pyramid . Other secondary fissures follow, and some ol the fissures of the vermis extend into the lateral lobes, but most of the

fissures here are separate local formations. , A

Cerebellar thickening, spreading into the roof-plate above and below the main development, forms the upper and lower medullary vela, the first extending (valve of Vieussens) to the closed isthmus, the




1588


A MANUAL OF ANATOMY


second being continuous below with the undeveloped ependymal or epithelial roof-plate of the myelencephalon.

On either side of the valve of Vieussens the roof-plate is thickened by the superior cerebellar peduncles.


Fig. 977.—Views from behind of the Hind-brain in Embryos of 35

and 48 Mm.

EV, IV, extra- and intra-ventricular surfaces.

The superior peduncles of the cerebellum, right and left, arise from the cells

of the nucleus dentatus of the corresponding lateral cerebellar hemisphere. Emerging from the anterior parts of the dentate nuclei, the peduncles give rise to two thickenings of the roof-plate of the metencephalon, one on either side of the valve of Vieussens. Thereafter they enter the mesencephalon or mid-brain,


THE NERVOUS SYSTEM


1589


and, after decussating, each passes to the corresponding red nucleus of the tegmentum of either crus cerebri, which constitutes its lower cell-station.

The middle peduncles of the cerebellum ( brachia pontis) are developed from the cells of the pontine nuclei of the pons right and left. The fibres of each peduncle issue from the lateral portion of the corresponding pontine nucleus, and enter the adjacent cerebellar hemisphere.

Mesencephalon. —This portion makes a sharp curve (Fig. 969) as it develops. It has a large cavity, which is slowdy lessened in size by the growing thickness of the walls, finally remaining as the small aqueduct. The thickened walls around this are composed of a floor lamina and roof lamina (tectum); in the former are formed the crura cerebri, while the tectum gives rise to the corpora quadrigemina.

The isthmus is a part of some interest; it is essentially a derivative of the hind-brain, so that its name isthmus rhombencephali is correct. The isthmus is produced at the spot where the two regions of growth, metencephalic and mesencephalic, meet each other, but there is in addition a definite forward extension of the basal lamina from the hind-brain into the opening, of which it makes the immediate wall. This extension carries with it the trochlear nucleus, which develops in the basal lamina of the hind-brain just above the opening; the decussation of the nerves, originally in the floor-plate of the hind-brain, is carried down also and lies therefore on the dorsum of the isthmus.


Fig. 978._Schemes to show the Parts of the Third Ventricle formed

FROM THE THALAMENCEPHALON (Bi.ACK LlNE) AND TELENCEPHALON (INTERRUPTED Line).

A little later there is an extension forwards from the alar lamina of the hindbrain, passing on the outer side of the basal extension, and carrying with it (sensory) nuclei from the upper part of the trigeminal nucleus; this appears to be the beginning of the mesencephalic root of the nerve, further short connections

developing subsequently. .

The mid-brain presents dorsally a median longitudinal groove, which separates two rounded eminences, known as the corpora bigemina . At a latei period a transverse groove appears, which divides each of the corpoia bigemina into two, thus giving rise to the corpora quadrigemina.

The third nucleus forms in the ventral mantle zone, and the fourth nucleus o-ains its position here secondarily. The red nucleus is probably, formed in situ, from the mantle zone of the floor lamina. I he corpora quadrigemina, formed from the alar laminae (and probably from the roof-plate secondarily involved), are hollow at first. -They become solid in the fourth and fifth months. The bases peduneulornm begin to appear in the fourth month in the central

parts of the marginal zone. . . . . . ,

Diencephalon or Thalamencephalon.— This is the anterior primary vesicle (Fig 978) Its cavity forms the greater part of the third ventricle (the anterior portion being derived from the secondary outgrowth, the telencephalon). Its walls develop into the thalamus, corpora mamillaria, tuber cmereum, mfundibulum, and posterior lobe of pituitary, and from the roof-plate grow the pineal


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A MANUAL OF ANATOMY


body and the ependymal roof of the ventricle. Moreover, when it is first formed, the thalamencephalon gives origin to the optic outgrowth on each side.

The pineal body, or epiphysis cerebri, is developed from the dorsal part of the ependymal roof of the third ventricle. It appears as a diverticulum of the ependymal roof close to the mesencephalon, and it is directed backwards, so that it comes to lie over the corpora quadrigemina. The distal end is blind, and in connection with it a number of closed follicles are formed which contain calcareous particles forming the acervulus cerebri, or brain-sand. The proximal part of the diverticulum forms the stalk of the pineal body, which contains the pineal recess and opens into the third ventricle.

The basal laminae, smaller than the alar, give origin to the tuber cinereum and the outgrowth which makes the infundibular process of the pituitary formation (see p. 1171). These structures are (Fig. 978) on the lower part of the


Fig. 979.— Diagram showing Parts of the Fore-brain and Structures

Derived from These.

D, diencephalon; Tel, telencephalon; T, E, M, are thalamus, epithalamus, and metathalamus; H is hypothalamus; CV, cerebral vesicle; CS, corpus striatum.

posterior aspect of the fore-brain immediately behind the site of the optic outgrowth. Corpora mamillaria are formed from basal laminae just behind and above the tuber. The position of these structures is due to the length and curve of the mid-brain; when this gets relatively shorter, and the nasal fossae grow upwards from below, the fore-brain is rotated upwards to some extent, and the structures thus come to lie more below the third ventricle.

The optic outgrowths, although they are actually low down with reference to the fore-brain, are derivatives from its alar region, and the interlaminar sulcus (big. 978) reaches its lowest or most cephalic point between the optic pouch and the infundibulum. This sulcus is here the hypothalamic sulcus, but the continuation of the ‘ sulcus of Monro ’ towards the foramen is a secondary effec t produced by the growth of the thalamus.

For development of the eye, see next chapter.

The thalamus begins early in the second month as an enlargement in




THE NERVOUS SYSTEM


i 59 i


the anterior part of the dorsal lamina due to growth of the mantle cells. It increases rapidly and narrows the ventricle, so that in the fourth or fifth month the two bodies come into contact and fuse to a small extent, leaving as an attachment between them when they draw apart again the connexus thalami or massa intermedia.



Fig. 980. —Schemes to show how the Corpus Striatum alters by its Growth the Shape of the Cavity.

occ, occipital growth, with formation of posterior horn; i.c., line in which pyramidal motor fibres issue through corpus striatum.


Later, in the fourth to fifth month, growth of the alar laminae farther back than the thalamic formation makes the pulvinar and geniculate bodies, and becomes secondarily continuous with the thalamus.

Telencephalon. —This (Fig. 979) is an outgrowth or forward bulging of the terminal part of the thalamencephalon; it gives off on each side the two cerebral vesicles. These hollow vesicles have cavities widely open into that of the telencephalon, and thus into the third ventricle, of which the telencephalon makes the most anterior part ..

The cerebral vesicles, growing fairly rapidly, stand up above the general level of the fore-brain, being separated from each other by the rudiment of the great longitudinal fissure. The direction of their main growth is upwards and backwards from the interventricular foramen, which is the opening from the telencephalon into a vesicle. They also grow forward for a little distance in front of the foramen, and of course their increasing size leads to an increasing prominence laterally. Thus they cover successively the diencephalon, the mid-bram, and finally the hind-brain and its formations, so that these are not to be seen from above. The vesicles begin to cover the mid-brain (Fig. 981) during the third month, and grow over the cerebellum in the fifth month. .

The interventricular foramen does not increase in size pari passti with the growing brain, hence

becomes relatively very small.

The anterior wall of the central unpaired or original telencephalic growth is the lamina terminalis. This is the direct path from one cerebral

fn the other whence it becomes the path .

Ilong which commissural fibres between the hemispheres will proceed in their

P " Thl V w fl S n a s S of'the cerebral vesicles are very thin at first, and thicken slowly. But the thkkening of the corpus striatum is visible at an early stage m each vesicle bednrdng to stand up into the cavity as a growthi ofthe floor and lower partof the outer wall. It grows rapidly, so that in the third month (Fig. 981)


Fig. 981. —Brain of Embryo in Middle of Third Month seen from

ABOVE, THE UPPER PART of the Right Cerebral Vesicle being removed to expose the Corpus Striatum (CS), Thalamus (OT), Midbrain (MB).




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A MANUAL OF ANATOMY


it makes a marked prominence, while the greater part of the remaining wall of the vesicle is still thin. The corpus striatum, thus forming a floor for the lateral ventricle, is the cause, by its further growth, of alteration in the shape of that cavity. This is shown in Fig. 980. The mass, at first low, grows in an upward and backward direction, projecting into the cavity, and thus leading to the formation of an inferior horn ; the forward-turned end of this horn is the result of further growth of the body. The deep cleft seen on the inner side of the mass in Fig. 981 is obliterated by fusion following on growth of the corpus on one side and of the thalamus on the other, and the floor of the ‘ body ’ of the cavity is raised accordingly.

The corpus striatum is one of the primitive formations in the brain connected with its primitive functions. A section across the fore-brain and vesicles in the second month is represented diagrammatically in Fig. 982, where the thick mass of the corpus striatum is in contrast with the thin wall of the rest of the vesicle. This thin wall is the rudimentary neopallium, that part of the cerebrum which in man is associated with the higher functions of the brain, and will, when it begins to grow, completely overshadow in size the original portion. The result of neopallial growth is shown in the second diagram; the mass of the corpus is not affected, while the rapid increase of the area of the neopallium leads to its overlapping the inert striate mass. Overlapping cannot take place on the inner and basal aspects, but growth outwards and backwards and downwards


Fig. 982. —Schematic Sections to show how the Corpus Striatum (CS) is

OVERLAPPED BY THE GROWTH OF THE NEOPALLIUM, AND IS DIVIDED INTO CAUDATE and Lenticular Parts by the Pyramidal Fibres. OT, thalamus.

is unrestricted, so we find the surface area corresponding with the corpus striatum is overlapped by opercula from behind and above and in front. The surface area corresponding with the striate body is the insula, and the opercula covering it in make by their presence the lateral fissure.

Growth of the neopallial area implies formation of processes from its nervecells, and during the third month the pyramidal motor fibres begin to extend down toward the lower regions. They pass, as indicated in Fig. 982, through the corpus striatum to reach the marginal region beside the thalamus, and m doing so divide the corpus into caudate and lentiform masses, the caudate mass lying between them (1 internal capsule) and the ventricular cavity, the lentiform ganglion between them and the surface; hence the lentiform and caudate nuclei are always separated from one another by fibres of the internal capsule, and these fibres are always separated from the lateral ventricle by the caudate mass, and from the surface by the lentiform body. The mass of fibres, however, passing out of the striate body below and behind, come to separate the lenticular part here from the thalamus, with which they are coming into relation.

The capsular fibres, passing through the corpus striatum, are affected by its upward growth, and thus make their passage and exit in a line (Fig. 980) curved like the surface of the striate body; this being so, it is easily understood that a section downwards, as along the arrow, would cut, in order from above, neopallium, cavity, caudate nucleus, internal capsule, lentiform nucleus, capsule, caudate nucleus, cavity, and finally neopallium again,





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1593


In tig. 982 the medial wall of the cavity of the vesicle is seen to remain thin. This wall is invaginated into the cavity as the ependymal covering of the choroid plexus of the lateral ventricle. The line of this thin wall lies just above the floor thickening of the corpus striatum, and is affected by the growth of this body, so that it assumes a curve corresponding with the curved shape of the ventricle, being invaginated into this throughout its length. This thin inner wall is continuous with the roof-plate of the third ventricle at the interventricular foramen, and the invaginations which cover the plexuses of the ventricles thus become continuous at this point.

The anterior part of the hemisphere vesicle corresponds to the frontal lobe; the lower part, as far forwards as the stem of the lateral fissure, becomes the temporal lobe ; and the upper and posterior part represents the parietal lobe.


Fig. 983.—Side Views of Left Hemisphere at Beginning of Fifth and End of Sixth and Seventh Months (modified from Koli.mann).

The occipital lobe is formed at a later period as the hemisphere grows backwards. The limbic lobe is developed in connection with the medial surface of the hemisphere. The olfactory lobe is developed as a hollow protrusion from the ventral aspect of the frontal portion of the hemisphere vesicle.

The surface of each hemisphere becomes very much broken up into gyri or convolutions, this being effected bv means of fissures. I he fissures are of two kinds—complete and incomplete/ The incomplete fissures are merely sulci produced by the growth of the gyri, and they do not involve the entire thickness of the walls of the cerebral hemispheres. The complete or primary fissures are infoldings of the walls of the cerebral hemispheres, and involve their entire thickness. They consequently give rise internally to certain prominences upon the wall of each lateral ventricle—namely, the lateral choroid plexus, hippo


1594 A MANUAL OF ANATOMY

campus, calcar avis, and eminentia collaterals. The primary fissures are as follows:

Lateral. Calcarine (anterior part).

Choroidal. Parieto-occipital.

Hippocampal. Collateral (central part).

With the exception of the lateral fissure, already described, the other complete fissures appear on the medial surface of the vesicle of the cerebral hemisphere.

The choroidal fissure is not really a fissure, but merely a groove or sulcus produced by an infolding of the vesicular wall, which is here composed entirely of ependyma. It commences above and behind the interventricular foramen of the corresponding side, and it terminates close to the tip of the temporal pole, where the inferior or descending horn of the lateral ventricle ends. Between these two points it passes backwards, downwards, and then forwards into the future temporal lobe in a crescentic manner, so as to embrace the stalk of the cerebral hemisphere. After the ependymal infolding has taken place, vascular mesenchyme dips in between its two folds, and so a plica chorioidea is formed. From this choroidal fold the lateral choroid plexus of the corresponding side is formed. This plexus projects into the lateral ventricle, but is excluded from the ventricular cavity by the ependyma of the wall, previously infolded. When the lateral choroid plexus is withdrawn, the thin ependymal covering of the plexus comes away with it, or is broken down. Under "these circumstances the choroidal fissure is really a fissure, inasmuch as the lateral ventricle now opens upon the surface through it. The internal prominence produced by the choroidal fissure is the lateral choroid plexus covered by ependyma, this prominence being verv conspicuous.

The other complete fissures will be found described in connection with the cerebral hemispheres. All the primary fissures are formed before the fourth month.

The incomplete fissures are very numerous. The first two to make their appearance are the calloso-marginal fissure, or sulcus cinguli, on the medial surface of the cerebral hemisphere, and the central sulcus, or fissure of Rolando, on the external surface. These, along with the other incomplete fissures, will be found described in connection with the cerebral hemispheres. - These are developed in the later foetal months, and inconstant tertiary fissures appear for years after birth.

The cavity of the vesicle of the cerebral hemisphere forms on either side the lateral ventricle, which is very much curtailed by the thickening undergone by the vesicular walls, and the internal prominences produced by the complete fissures. As the frontal lobe undergoes development the body of the ventricle extends forwards into it, and so the anterior horn of the ventricle is formed. As the occipital lobe becomes developed at a later period the body of the ventricle extends backwards into it, and so the posterior horn is formed. Meanwhile the interventricular foramen on either side, originally large, is being gradually much diminished in size.

Basal Ganglia. —The basal ganglia of each cerebral hemisphere are as follows:

Corpus striatum.

Claustrum.

Amygdaloid nucleus.

ihey are all developed from the deep part of the much thickened cortical substance which forms the floor of the lateral fossa. The claustrum and amygdaloid nucleus remain of small size, but the nucleus caudatus of the corpus striatum forms a conspicuous prominence in the lateral ventricle as it bulges into that cavity.

Commissures. —The commissures are as follows:

1. Corpus callosum. 4. Anterior.

2. Fornix. 5. Posterior.

3. Hippocampal. 6. Middle.


7. Habenular.


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1595


At an early period in the development of the cerebral hemispheres the interhemispherical (great longitudinal) fissure leads directly down to the roof of the diencephalon. At a later period the roof of the diencephalon is separated from the great longitudinal fissure by two commissures placed one above the other. The upper commissure is the corpus callosum, and the lower one is the fornix. These may be looked on as drawn back from the lamina terminalis by the growing vesicles, although this is not quite an accurate statement of their formation.


Fig. 984.—Schematic Figure to show the General Formation of

the Main Commissures.

The thalamus (OT) is supposed to be cut away in part, exposing the region of the corpus striatum; this has grown up "(producing the curved form of ventricle already described), and the choroidal fissure (, ch.f .) is therefore a curved line. Fibres of the internal capsule ( i.c.) also come through the mass of the corpus striatum in a curved line, and lie between the thalamus and lenticular part of corpus striatum. Hippocampal formations lie on the other side of the choroidal fissure, and association fibres here make the fimbria ( fimb .) and fornix (/). Commissural fibres cross the middle line and are cut at h (hippocampal commissure) and ac (anterior commissure) ; these are in the lamina terminalis. As the neopallium grows, its commissure, the corpus callosum, begins to form; it is at first in the lamina terminalis (cc.), but with increasing growth it extends forwards (A) and backwards (P). The rostrum, shown by the dotted line, is subsequently formed by fusion of the walls of the two'vesicles, the septum lucidum being that part of the wall between this and A. The backward extension carries with it the hippocampal commissure; these backward movements, involving also the hippocampus, are associated with great growth of the front part of the brain.

Fornix—Anterior Commissure— These appear in the third month. In Fig 984 is shown the inner aspect of a cerebral vesicle in which the lamina terminalis is visible. Thickenings begin in this from fibres crossing between the olfactory and insular regions', these make the anterior commissure. . A little later fibres extend from each hippocampus to the sides of the lamina, where they turn into the side walls of the telencephalon, and reach much later the basal laminae of the thalamencephalon, constituting the anterior




1.596 A MANUAL OF ANATOMY

pillars of the fornix. The rest of the fornix is carried back with the growing vesicle.

Corpus Callosum. —Some time later, as the neopallium grows, its commissural fibres begin to become evident as the corpus callosum. These at first make use of the path already utilized by the earlier commissural fibres, and are found crossing at and above the upper part of the lamina terminalis as far back as the level of the interventricular foramen, as seen in the figure. As the neopallium grows, however, its commissural fibres become too numerous to be confined to this area, and their line of crossing extends backwards and forwards. The hinder extension necessarily lies with the fornix above the choroidal fissure, while the forward extension is between the two anterior expansions of the vesicles or hemispheres. These anterior fibres of the corpus callosum come from the frontal lobe above the anterior horn of the ventricle, hence that part of the wall of the hemisphere below these fibres is the medial wall of this horn. The hemispheres become approximated and fused below this small area of medial wall as the result of growth of neopallium round it, and commissural fibres now find their way through the line of fusion below the medial wall of the anterior horn. This makes the rostrum, and its continuity in front and above with the original forward extension of the corpus callosum cuts off the two areas of medial wall from the rest of this wall in the longitudinal fissure; they now form the two layers of the septum lucidum, and the cavity between them is only the corresponding part of the space of the fissure, similarly cut off. The fusion between the medial walls is not confined to the region just considered, but is found behind this in the neighbourhood of the callosal fibres and the fornix; this explains the posterior extension of the region of the septum lucidum.

The hippocampal commissure appears on the back part of the ventral aspect of the plate formed by the fused areas of the cerebral hemispheres, to which position it has been carried by the backgrowth of the corpus callosum. Its fibres pass across from one hippocampus to the other, and they correspond to the region known as the lyra or psalterium.

The posterior commissure is formed in connection with the back part of the roof of the diencephalon behind the pineal diverticulum.

The so-called middle commissure is not a commissure properly so called, but is formed by the fusion over a limited area of the grey matter of the medial surfaces of the thalami, and properly termed connexus thalami.

The habenular commissure is produced by the decussating fibres of the thalamic stria?, these fibres, as they decussate, forming the dorsal part of the pedicle of the pineal body.

Meninges of the Encephalon. —The walls of all the cerebral vesicles are invested by mesoderm, and this tissue becomes differentiated into the three meninges—namely, the dura mater, arachnoid, and pia mater.

Choroid Plexuses. —The choroid plexuses of the two lateral, third, and fourth ventricles are developed as infoldings of the ependymal walls of the ventricles. Vascular mesenchyme (mesoderm) dips in between the two layers of each infolding, and in this manner plicce c-horioidece are formed. These choroidal folds give rise to the choroid plexuses, which as they project into the ventricles carry the ependymal walls, already infolded, before them.

Tela Chorioidea. —The vascular mesoderm ( pia mater) investing the neural tube is converted into a double layer interposed between fore-brain and cerebral vesicles as a result of the backgrowth of the latter. It is clear that this velum interpositum extends to the interventricular foramen, where its two layers are continuous, and where vessels of the cerebral layer can join those of the earlier one. The dotted lines in Fig. 982 show the position of these two layers on section; it can be seen that the ‘ cerebral ’ layer reaches out to the thin medial wall of the vesicle, and its marginal vessels can invaginate this to form the choroid plexus of the lateral ventricle, while the deeper layer rests on the roof of the third ventricle, and makes its choroid plexus. The continuity of the two layers at the foramen explains why the lateral vein runs there to join the internal cerebral vein, which is in the lower layer.


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Development of the Peripheral Nervous System.

The peripheral nerves are arranged in two groups—namely, spinal, which are derived from the spinal cord; and cranial, which arise from the brain.

The spinal nerves are composed of two kinds of fibres—efferent, centrifugal, or motor; and afferent, centripetal, or sensory.

A motor spinal nerve-fibre arises as the axon of a neuroblast or nerve-cell in the mantle layer of the neural tube (see Development of the Spinal Cord).

A sensory spinal nerve-fibre is developed from a cell of a spinal ganglion, and these ganglia are developed from the corresponding neural crest.

Neural Crests. —The neural or ganglionic crests, right and left, are ridges of ectodermic cells which lie on either side of the neural tube. They are derived from a single crest of ectoderm, which is formed by the fusion of the ectoderm over each neural fold, this single crest being situated medially on the dorsal aspect of the neural tube along che line of fusion of the neural folds to close the tube. Subsequently the medial crest divides into right and left halves, which cover the dorsolateral aspects of the neural tube.

Each neural crest becomes broken up into a number of segments, or ganglia, there being four pairs for the head region, and thirty-one pairs for the region of the trunk.

Spinal Ganglia. —The spinal ganglia are arranged in thirtyone pairs, right and left.

Each cell of a ganglion acquires two poles — afferent or centripetal, and efferent or centrifugal—and at this stage it is consequently a bipolar cell.

The centripetal or proximal pole, which is the axon of a ganglionic cell, grows into the dorsal part of the wall of the neural tube and forms part of the dorsal or posterior root of a spinal nerve. Within the marginal layer of the neural tube the centripetal pole or nerve-fibre divides into tw r o branches, ascending and descending, which give off collaterals and terminate in arborizations. The centrifugal or distal pole joins the ventral or anterior nerve-root of the same segment of the spinal cord on the distal side of the ganglion to form a spinal nerve.

Though the cells of a spinal ganglion are originally bipolar, they become m the course of growth unipolar, the single pole having a T-shape. d his is brought about by an excessive growth of one wall of the bipolar cell, which biings the two original poles into contact, when they fuse, and are now connected with the cell by one stalk or pole, which divides into a centripetal and a centrifugal process.

Whilst the fibres of the dorsal or posterior roots of the spinal nerves grow into the mantle layer of the neural tube from the cells of the spinal ganglia, the fibres of the ventral or anterior roots arise within the mantle lacei as the axons of its neuroblasts or nerve-cells. The fibres of the anterior roots therefore grow out from the neural tube.

Cranial Nerves. _The development of the cranial nerves, with the exception

of the olfactory and optic nerves, corresponds for the most part with the development of the spinal nerves. The motor cranial nerve-fibres arise as the axons of nerve-cells of the brain, and groiv into the brain, whereas the sensory cranial nerve-fibres grow into the brain from the cells of the cephalic ganglia.


Neural Crest (Ectoderm)


Fig. 985. —Development of the Neural or Ganglion Crest (Keibel and Mall) (after Von Lenhossek and Koi.lmann).




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A MANUAL OF ANATOMY


Cephalic Ganglia. —The cephalic ganglia, like the spinal ganglia, are developed from the neural crests, and they constitute four pairs of ganglionic groups— namely, trigeminal, acoustico-facial, glosso-pharyngeal, and vagal. They are all comparable to the spinal ganglia.

The trigeminal ganglion is connected with the sensory root of the fifth cranial nerve. The centripetal poles of its nerve-cells pass inwards into the brain, forming the large sensory root of the nerve, and the centrifugal poles of its cells pass peripherally, forming the ophthalmic, superior maxillary, and sensory part of the inferior maxillary nerves. The trigeminal ganglion is thus clearly comparable to a spinal ganglion. The small motor root of the fifth nerve is homologous to the motor or anterior root of a spinal nerve, inasmuch as its fibres arise as the axons of nerve-cells within the brain.

The acoustico-facial ganglion resolves itself into facial and acoustic parts.

The facial ganglion, known as the geniculate ganglion, is connected with the genu of the facial nerve in the aqueduct of Fallopius. The centripetal poles of the nerve-cells of this ganglion form the sensory root of the facial nerve—the pars intermedia of Wrisberg —which passes inwards to the fasciculus solitarius and upper part of the glosso-pharyngeal nucleus. Many of the centrifugal poles of the cells issue from the ganglion as the chorda tympani nerve, which passes to the anterior two-thirds of the tongue as a nerve of special sense.

The acoustic ganglion is represented by the vestibular and cochlear ganglia in connection with the auditory nerve.

The vestibular ganglion is connected with the vestibular division of the auditory nerve in the internal auditory meatus. As in the other ganglia, the centripetal poles of the cells of this ganglion form the centripetal fibres of the vestibular nerve, which pass inwards to the brain. The centrifugal poles of the cells leave the ganglion, and form the peripheral part of the nerve as regards its distribution.

The cochlear ganglion, known as the ganglion spirale, is connected with the cochlear division of the auditory nerve, and is situated in the spiral canal of the modiolus. Its nerve-cells are related to nerve-fibres, as in the case of the vestibular ganglion.

The glosso-pharyngeal ganglion, which is broken up into a superior (jugular) ganglion and an inferior (petrous) ganglion, is comparable to a spinal ganglion. The centripetal poles of the nerve-cells, which issue from the ganglion, represent the centripetal sensory fibres of the glosso-pharyngeal nerve, passing into the brain. The centrifugal poles of the nerve-cells, issuing from the ganglion, represent the peripheral sensory fibres of the nerve.

The vagal ganglion, which is broken up into the upper ganglion (of the root) and the lower ganglion (of the trunk), is disposed towards the sensory fibres of the vagus nerve, as in the case of the other ganglia.

MENINGES OF THE ENCEPHALON.

Dura Mater.

The dura mater is a strong fibrous membrane which surrounds the encephalon, and is composed of two layers—outer and inner. The outer or endosteal layer serves as the internal periosteum or endocranium of the cranial bones, and the inner or sustentacular layer supports the encephalon. It is more firmly adherent to the bones forming the base of the skull than to those over the cranial vault, and it is also firmly attached along the course of the sutures. At the various openings it is prolonged outwards, blending with the sheaths of the transmitted nerves, and also becoming continuous with the external periosteum or pericranium. At the sphenoidal fissure it passes into the orbit


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to form the orbital periosteum. At the lower margin of the foramen magnum the two layers of which the dura mater is composed separate. The external layer blends with the periosteum of the occipital bone around the margin of the foramen magnum. The internal layer is prolonged into the spinal canal, and forms the theca of the spinal cord. The outer surface of the dura mater is rough and flocculent, owing to fibrous processes which connect it to the inner surfaces of the cranial bones. Its inner surface is smooth and covered by endothelium. Superiorly, on either side of the superior longitudinal sinus there are several small granular nodules, called arachnoid granulations, which are best marked in old age. They indent the parietal bone, and protrude into the superior longitudinal sinus, carrying with them prolongations from the endothelial lining of the sinus, which separate them from the blood.

The bodies are enlargements of the villi of the arachnoid membrane (see p. 1609).

Structure. —The dura mater consists of fibrous and elastic tissues arranged as parallel bundles.

The intracranial dura mater differs from the dura mater of the spinal cord in the following respects: (1) it consists of two layers— outer or periosteal, and inner or sustentacular—whereas the spinal dura mater has only one layer, representing the sustentacular layer; (2) it furnishes certain processes or septa, which project into the cranial cavity, and separate parts of the encephalon from each other, whereas the spinal dura mater sends no septa into the spinal cord; and (3) it contains venous sinuses, which are absent in the spinal dura mater, or are represented by the extradural venous plexuses (see p. 1434) •

Subdural Space.—This is the interval between the dura mater and the arachnoid membrane. There is really no space, but simply sufficient interval to contain a minute quantity of serous fluid for lubricating purposes. The dura mater and the arachnoid are therefore practically in contact with each other.

Blood-supply. —The cranial dura mater is supplied by the meningeal arteries, which are extradural and supply the inner table of the cranial bones. These vessels are very numerous, and the chief are as follows on either side, from before backwards: (1) anterior meningeal, two in number, from the anterior and posterior ethmoidal arteries; (2) meningeal, from the cavernous part of the internal carotid artery; (3) small meningeal, from the middle meningeal, or from the first part of the maxillary artery; (4) middle meningeal, from the first part of the internal maxillary; (5) meningeal branches of the ascending pharyngeal artery, (6) posterior meningeal branch of the occipital artery; and (7) posterior meningeal, from

the vertebral artery. . , . ,

The anterior meningeal branch of the anterior ethmoidal artery arises from that vessel as it accompanies the nasal nerve on the cribriform plate of the ethmoid bone, and it takes part in the supply of the dura mater of the anterior

fossa

The anterior meningeal branch of the posterior ethmoidal artery arises from that vessel after it has entered the cranial cavity through a minute foramen between the cribriform plate of the ethmoid and the sphenoid. It has a limited distribution to the dura mater in this region.


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A MANUAL OF ANATOMY


The meningeal branch of the internal carotid artery arises from the cavernous part of that vessel, and enters the middle fossa to supply the dura mater.

The small meningeal artery is usually a branch of the middle meningeal, but it may arise from the first part of the maxillary artery. It enters the cranial cavity through the foramen ovale, and supplies the adjacent dura mater and the trigeminal ganglion.

The middle meningeal artery, as stated, is a branch of the first part of the maxillary artery. Its diameter is that of the foramen spinosum, through which it enters the cranial cavity. After passing into the cranium it divides into two branches, anterior and posterior. The anterior branch passes forwards, outwards, and upwards in a groove on the upper surface of the great wing of the sphenoid bone to the inner aspect of the antero-inferior angle of the parietal bone, where there is a groove, or sometimes a short canal. The position of the middle meningeal artery at this level is ascertained by taking a point on the exterior of the skull ii inches behind the zygomatic process of the frontal bone and inches above the zygomatic arch. From this point the artery ascends in a


Frontal

Diploic

Vein


T rx.

Anterior Temporal Diploic Vein


Posterior Temporal Diploic Vein


Occipital Diploic Vein


Fig. 986.— The Veins of the Diploe.


The outer tables of the cranial bones have been removed.


branching meningeal groove near the anterior border of the parietal bone as high as the superior longitudinal sinus. In this part of its course it furnishes numerous branches forwards and backwards.

The posterior branch passes backwards on the squamous part of the temporal bone, and then on to the inner aspect of the inferior border of the parietal bone, where there is a meningeal groove about the centre. From this point it ascends in a branching groove as high as the superior longitudinal sinus, giving off branches forwards and backwards.

The distribution of the middle meningeal artery extends as high as the superior longitudinal sinus forwards on to the frontal bone, and backwards on to the occipital bone. Besides supplying the dura mater and the inner table and diploe of the bones, the vessel furnishes the following branches: (1) Ganglionic to the trigeminal ganglion; (2) a petrosal brcinch, which passes through the hiatus (hallopii) to supply the facial nerve in its canal, and anastomose with the stylo-mastoid branch of the posterior auricular artery; and (3) an orbital branch, which enters the orbit through the sphenoidal fissure, and anastomoses with the ophthalmic artery.



THE NERVOUS SYSTEM


1601


The meningeal branches of the ascending pharyngeal artery, which is a branch of the external carotid, are three in number: One passes through the foramen lacerum into the middle fossa; another passes through the jugular foramen into' the posterior fossa; and the third passes through the anterior condylar foramen, likewise into the posterior fossa.

The posterior meningeal branch of the occipital artery passes through the jugular or mastoid foramen into the posterior fossa.

The posterior meningeal branch of the vertebral artery passes through the foramen magnum into the cerebellar fossa of the occipital bone.

As a rule, only one of these various posterior meningeal arteries is large enough to carry injection. .#•

Meningeal Veins. —The sinuses or veins with the middle meningeal artery are two in number. They pass through the foramen ovale, and terminate in the pterygoid plexus of veins. The other meningeal veins are disposed in one of two ways. Some of them accompany the corresponding arteries and terminate in extracranial veins; whilst others end in the various intracranial venous sinuses, in part directly and in part by means of venous lacunae.

Veins of the Diploe. —These vessels are situated in the cancellous tissue between the outer and inner plates of the cranial bones, and are exposed after removal of the outer plate. They are destitute of valves, and are arranged in the form of a network, from which the blood is returned by four diploic veins on either side—namely, frontal, anterior temporal, posterior temporal, and occipital. These terminate partly in extracranial veins, and partly in the intracranial venous sinuses and meningeal veins.

The frontal diploic vein passes downwards and escapes through an opening in the outer plate of the frontal bone at the supra-orbital notch, where it joins the communicating vein which passes between the supra-orbital and ophthalmic veins. It receives radicles from the frontal air-sinus of the same side.

The anterior temporal diploic vein is confined to the back part of the frontal and anterior part of the parietal regions, and descends to terminate in two ways. It partly joins one of the extracranial deep temporal veins by passing through an opening in the outer plate of the great wing of the sphenoid, and in part it ends in a meningeal vein, or in the spheno-parietal, or it may be the cavernous

sinus. „ , . , - ,

The posterior temporal diploic vein is confined to the parietal region, and descends to the postero-inferior angle of the parietal bone, where it pierces the inner plate of that bone, and terminates in the transverse sinus..

The occipital diploic vein occupies the occipital region, and pierces the inner table of the occipital bone, to terminate in the transverse or sigmoid sinus.

Nerves of the Dura Mater— The dura mater receives nerves from (i) the sympathetic plexuses which accompany the arteries; (2) the three divisions o the fifth cranial nerve; (3) the ganglion of the root of the va gus nerve , an (4) the hypoglossal nerve. Headache is said to be due to irritation of these nerves, especially to one of the branches of the fifth, known as the nervus tentorn.

Processes of the Dura Mater.— It has been already explained that the dura mater is composed of two layers outer or periosteal, and inner or sustentacular. The processes are formed by the inner or sustentacular layer, and are four in number the faix cerebri, tentorium cerebelli, falx cerebelli, and diaphragma sellse.

The falx cerebri is an extensive sickle-shaped process, which occupies the great longitudinal fissure, where it lies between the two cerebral hemispheres. Anteriorly it is almost pointed, and is attached to the crista salli of the ethmoid bone. Posteriorly it is broad, and is attached to the upper surface of the tentorium cerebelli along the median line the straight sinus being situated at the place of junction The supenor border is convex, and is attached m the median line to the frontal,

IOI


1602 A MANUAL OF ANATOMY

parietal, and occipital bones, extending upon the latter bone only as low as the internal occipital protuberance. The superior sagittal sinus is situated along this border. The inferior border is concave and free. It overhangs the upper surface of the corpus callosum, from which it is separated by a slight interval. The inferior sagittal sinus is situated within this border. The lateral surfaces face the medial surface of the cerebral hemispheres.

The tentorium cerebelli is an extensive crescentic sheet which covers the cerebellum. Superiorly it supports the posterior parts of the cerebral hemispheres, and is elevated along the median line, whence it slopes towards the attached borders. Anteriorly it presents a free,

Falx Cerebri

9

I


Fig. 987.—The Falx Cerebri, Tentorium Cerebelli, and Venous Sinuses

of the Dura Mater (Left View).

i- Superior Sagittal Sinus 4. Transverse Sinus

2. Inferior Sagittal Sinus 5. Superior Petrosal Sinus

3. Straight Sinus 6. Internal Jugular Vein

sharp, concave border, which forms, with the dorsum sellae of the sphenoid bone anteriorly, an oval opening, called the foramen ovale tentorii , within which the mesencephalon is placed. Posteriorly and laterally the tentorium cerebelli is convex, and is attached as follows, from behind forwards: (i) to the horizontal ridge on the inner surface of the tabular part of the occipital bone, where the process contains the transverse sinus; (2) to the inner aspect of the postero-inferior angle of the parietal bone, where the process also contains this sinus; and (3) 1° the superior border of the petrous part of the temporal bone, where the process contains the superior petrosal sinus. Close to the apex of th e pars petrosa the outer and anterior borders of the tentorium





THE NERVOUS SYSTEM


1603


cerebelli cross, the outer border passing inwards to be attached to the posterior clinoid process of the sphenoid bone, and the anterior border passing forwards to be attached to the anterior clinoid process of that bone.

In all the carnivora the tentorium is ossified.


The falx cerebelli extends forwards into the posterior cerebellar notch, where it lies between the two cerebellar hemispheres. It is falciform in outline. Superiorly it is attached to the posterior part of the inferior surface of the tentorium cerebelli in the median line. Posteriorly it is attached to the internal occipital crest, and this portion bifurcates interiorly, the two divisions being connected to the lateral margins of the vermiform fossa. Along this posterior attachment it contains the occipital sinus and its two divisions. Anteriorly it ends in a free border.

In the ornithorhynchus and many cetacea the falx is ossified.


The diaphragma sellse is a small circular fold horizontally placed, which forms a roof for the sella turcica or hypophysial fossa of the sphenoid bone, and almost entirely covers the hypophysis. At its centre there is a small opening for the passage of the infundibulum.

Venous Sinuses of the Dura Mater.—These are blood-channels or spaces situated between the two layers of the dura mater, and lined with endothelium. They are as follows:


Superior sagittal sinus. Inferior sagittal sinus. Straight sinus.

Transverse sinuses (two). Occipital sinus. Cavernous sinuses (two).


Spheno-parietal sinuses (two). Circular sinus.

Superior petrosal sinuses (two). Inferior petrosal sinuses (two). Basilar sinus.

Petro-squamous sinuses (two).


The superior sagittal sinus is situated in the median line within the superior convex border of the falx cerebri. It extends from the crista galli of the ethmoid bone to the internal occipital protuberance, where as a rule it turns sharply to the right, and opens into the right transverse sinus. It sometimes, however, turns to the left, and opens into the left transverse sinus. Its posterior extremity is dilated, and forms the confluens sinuum, which usually occupies a depression on the right side of the internal occipital protuberance, and is connected with the dilatation at the posterior extremity of the straight sinus by a transverse vessel. It increases in size as it passes backwards, and its shape is triangular in section, the base being directed towards the cranial vault. The apex is directed downwards, and in this region the sinus is crossed by a number of delicate fibrous bands. Opening into the sinus on either side there are venous spaces, called lacunae laterales, which are situated within the dura mater, and projecting into these lacunae from below, or into the sinus itself, there are seveial arachnoid granulations, covered by the endothelial lining. The sinus receives the superior cerebral veins and some of the meningeal veins of the falx


A MANUAL OF ANATOMY


1604

cerebri. The former open into it from behind forwards, so far at least as the more posterior vessels are concerned, so that the blood-flow in these veins is opposed to the current of blood in the sinus, which is from before backwards.

The superior sagittal sinus sometimes communicates anteriorly with the veins of the roof of the nose through the foramen ccecum, and


Frontal Air-Sinus


Anterior Fossa

/


Cerebellar Fossa


Accessory Nerv Hypoglossal Nerve


Spinal Cord ' Occipital Sinus


Optic Nerve ^ Ophthalmic Artery


Third Nerve

Fourth Nerve

Region of Meckel’s Cave

Fifth Nerve

Facial Nerve’

Auditory Nerve

Glosso-pharyngeal Nerve

Vagus Nerve


Transverse Sinus


Cerebral Fossa

^ Confluens Sinuum


Border of Small Wing ✓ ' of Sphenoid


Ant. Pt. of Circ. Sinus ^ (Ant. Intercav. Sinus)

Hypophysis


Cavernous Sinus Sixth Nerve


Middle Fossa


Basilar Sinus Sup. Petrosal Sinus - Inf. Petrosal Sinus

Sigmoid Sinus


Fig. 988.—Ihe Internal Uase or- the Skull, showing the Cranial

Nerves and Venous Sinuses.


it communicates with the veins of the scalp by means of an emissary vein, which passes through the parietal foramen of each side, when that is present.

The inferior sagittal sinus (vein) is of small size, and is situated in the lower free border of the falx cerebri over its posterior two-thirds. Its direction is backwards, and it opens into the front part of the straight sinus at the anterior margin of the tentorium cerebelli. It is circular, and increases in size as it passes backwards. Its tributaries are derived












THE NERVOUS SYSTEM


1605

from the lower part of the falx cerebri, and from the adjacent parts of the medial surfaces of the cerebral hemispheres.

The straight sinus is situated at the junction of the falx cerebri with the tentorium cerebelli, and is formed by the union between the inferior longitudinal sinus and the great (internal) cerebral vein at the anterior margin of the tentorium. It is triangular in section, and increases in size from before backwards. Its course is downwards and backwards in the median line to the left side, as a rule, of the internal occipital protuberance, where it becomes slightly dilated, and is continued into the left transverse sinus. Its terminal dilatation is connected with the confluens sinuum by a transverse vessel. In those cases where the superior longitudinal sinus passes into the left transverse sinus the straight sinus passes into the right transverse sinus. In addition to the inferior longitudinal sinus and the great cerebral vein, the straight sinus receives some of the superior cerebellar veins and tributaries from the falx cerebri and tentorium cerebelli.

The transverse sinuses extend on each side from the internal occipital protuberance to the postero-lateral compartment of the jugular foramen, through which it passes, to terminate in the bulb of the internal jugular vein. The sinus pursues a curved course. It passes outwards in the transverse groove on the inner surface of the tabular part of the occipital bone as far as the inner surface of the postero-inferior angle of the parietal bone. Here it ascends for a little in a groove, and, having described a sharp curve, it passes downwards and medially in the sigmoid groove on the inner surface of the mastoid portion of the temporal bone. Finally, it turns forwards in the groove on the upper surface of the jugular process of the occipital bone to the jugular foramen. As far as the postero-inferior angle of the parietal bone the transverse sinus is contained within the attached border of the tentorium cerebelli, and just before leaving this it receives the superior petrosal sinus. The right transverse sinus is, as a rule, formed by the superior longitudinal sinus, and in these circumstances is larger than the left, which is usually formed by the straight sinus. The reverse, however, may be the case. Before leaving the torcular the two sinuses communicate freely.

The transverse sinus of each side, in addition to the superior petrosal sinus, receives tributaries from the posterior part of the cerebrum and the superior and inferior surfaces of the cerebellum. It also receives the posterior temporal and occipital diploic veins, and the petrosquamous sinus, when that sinus is present. Near its termination it communicates with the marginal sinus of the same side. The sinus communicates with extracranial veins by means of two large emissary veins, posterior condylar and mastoid, which pass through the corresponding foramina. These foramina, however, being inconstant, the

emissary veins are sometimes wanting.

The name ‘ transverse ’ is strictly applicable only to that part ot the sinus which is contained within the attached border of the tentorium cerebelli , and extends from the internal occipital protuberance to the


i6o6


A MANUAL OF ANATOMY


postero-inferior angle of the parietal bone. Beyond this latter point the sinus is known as the sigmoid sinus.

The occipital sinus is situated within the falx cerebelli along its attachment to the internal occipital crest. It is of small size, and is formed inferiorly by the union of the two marginal sinuses (inferior occipital) which lie on either side of the vermiform fossa and foramen magnum, where they communicate with the posterior intraspinal veins and the terminal part of each lateral sinus. Superiorly it opens into the confluens sinuum. In certain cases the marginal or inferior occipital sinuses remain separate, and then each opens into the corresponding transverse sinus. The occipital sinus receives tributaries from the falx cerebelli and the inferior surface of the cerebellum, and it establishes a communication between the beginning and end of the transverse sinuses.

The cavernous sinuses are so named because the interior of each is broken up by fibrous filaments, which impart to it a reticular appearance. Each sinus is situated on the side of the body of the sphenoid bone, and extends from the inner extremity of the sphenoidal fissure to the apex of the petrous part of the temporal bone. Anteriorly it receives the ophthalmic vein or veins from the orbit, and posteriorly it terminates by dividing into the superior and inferior petrosal sinuses. In addition to the ophthalmic vein or veins, the cavernous sinus receives the spheno-parietal sinus and some of the inferior cerebral veins. It communicates with the angular vein of the face through the superior ophthalmic vein; with its fellow of the opposite side through the circular sinus; with the transverse sinus by the superior petrosal sinus; with the internal jugular vein by the inferior petrosal sinus; with the pterygoid plexus through the inferior ophthalmic vein, and by an emissary vein which passes through the foramen ovale, or through the foramen Vesalii ’; and with the pharyngeal plexus by an emissary vein which passes through the foramen lacerum medium. The internal carotid artery, with the cavernous sympathetic plexus, lies within the sinus, and the sixth cranial nerve (Fig. 989) is close to the outer side of the artery. In connection with the roof and outer wall of the sinus there are the third and fourth, as well as the ophthalmic and superior maxillary divisions of the fifth cranial nerves.

The spheno-parietal sinus is of small size, and is situated on the inferior surface of the lesser wing of the sphenoid bone. It generally begins in connection with the anterior temporal diploic vein, and ends in the anterior part of the cavernous sinus.

The circular sinus fills any small part of the hypophysial fossa which

is not occupied by the gland; on each side it opens freely into the cavernous sinus.

The superior petrosal sinus is situated along the superior border of the petrous part of the temporal bone, and lies within the attached margin of the tentorium cerebelli. It begins at the back part of the cavernous sinus, and, having passed laterally and backwards, it opens into the transverse sinus as that is about to enter the sigmoid groove


/


THE NERVOUS SYSTEM 1607

of the pars mastoidea. It receives tributaries from the cerebellum and tympanum.

The inferior petrosal sinus occupies the groove at the junction of the basilar process of the occipital bone with the petrous part of the temporal bone. It begins at the back part of the cavernous sinus, and, passing backwards and slightly laterally, it leaves the cranial cavity through the antero-medial compartment of the jugular foramen, to terminate in the bulb of the internal jugular vein. It receives tributaries from the inferior surface of the cerebellum, and from the internal ear.


Fig. 989._Right Internal Carotid put in Position on Base of Skull and

Crossed by Sixth Nerve.

Greater superficial petrosal nerve is also seen entering foramen lacerum.

The basilar sinus is really a plexus of veins situated within the dura mater over the basilar process of the occipital bone, extending as low as the anterior margin of the foramen magnum, where it communicates with the anterior intraspinal veins. It connects the anterior ends of the inferior petrosal sinuses with each other.

The petrosquamous sinus is situated along the junction of the petrous and squamous parts of the temporal bone. Its direction is backwards, and it opens into the transverse sinus as that is entering the

siermoid fossa of the pars mastoidea.

Each petro-squamous sinus represents the continuation of the transverse sinus in early life, to terminate in the primitive jugular vein. Often there are no traces of the petro-squamous sinus.




i6o8


A MANUAL OF ANATOMY


Emissary Veins. —These are vessels which pass through foramina in the cranial wall, and establish communications between the intracranial venous sinuses and the extracranial veins. The principal emissary veins are mastoid, nasal, posterior condylar, parietal, and occipital. In addition to these there are emissary veins, which pass through (i) the foramen ovale, (2) the foramen Vesalii (when present), and (3) the foramen lacerum and the carotid canal.

The transverse sinus in many cases has two emissary veins, mastoid and posterior condylar.

The mastoid emissary vein is of large size. It passes through the mastoid foramen, and usuallv opens into the posterior auricular vein.

The posterior condylar emissary vein passes through the posterior condylar foramen, and opens into the suboccipital plexus, from which the blood is carried away by the vertebral and deep cervical veins.

The mastoid and posterior condylar veins are not constant.

The superior sagittal sinus may have three emissary veins—parietal and occipital and nasal.

The parietal emissary vein passes through the parietal foramen, and opens into the occipital plexus, or into radicles of the superficial temporal vein.

The occipital emissary vein passes from the confluens sinuum to the occipital plexus, being transmitted through a minute foramen which pierces the external occipital protuberance.

The nasal emissary vein passes through the foramen caecum; like all other emissary veins it is often absent. It has been seen already that the cavernous sinus communicates with (1) the angular vein of the face, (2) the pterygoid plexus, and (3) the pharyngeal plexus. The marginal or inferior occipital sinus communicates with the posterior intraspinal veins, and the basilar sinus communicates with the anterior intraspinal veins.


Arachnoid Membrane.

This is a very delicate membrane which loosely surrounds the encephalon, and is situated between the dura mater and pia mater. It does not dip into the fissures, except in the case of the great longitudinal fissure, its general course being over the gyri and other eminences and depressions of the encephalon. It is conspicuous at the base of the encephalon in the region of the interpeduncular space, pons, and medulla oblongata. Its outer surface is practically in close contact with the inner surface of the dura mater, the extremely slight interval containing a very little lubricating serous fluid, and being known as the subdural space. Between the arachnoid and the pia mater there is the interval known as the subarachnoid space. The membrane furnishes sheaths to the various cranial nerves.

Subarachnoid Space.—This space lies between the arachnoid and the pia mater. It is crossed by delicate trabeculae of connective tissue, which pass between the two membranes, in a reticular manner. The meshes of this reticulum contain the cerebro-spinal fluid.

The subarachnoid space communicates with the ventricles of the brain by one main opening, the median aperture of the ventricle or foramen of Magendie, which is situated in the median line of the roof of the ventricle a little above the lower angle.* In some situations— as, for example, over the gyri—the arachnoid and pia mater are in close

The modern view is that there are also foramina at the lateral angles of the ventricle, lateral apertures.


/


THE NERVOUS SYSTEM


1609


contact, but in other localities the two membranes are more or less widely separated by intervals, called cisternas. The most important of these are the cisterna magna, cisterna pontis, cisterna basalis, and cisterna venae magnae.

The cisterna cerebello-medullaris (or magna) lies between the posterior part of the inferior surface of the cerebellum and the medulla oblongata. It is of large size, the arachnoid, which here passes from cerebellum to medulla, being widely separated from the pia mater. It is continuous through the foramen magnum with the posterior part of the subarachnoid space of the spinal cord.

The cisterna pontis is situated on the ventral aspect of the pons. Interiorly it is continuous with the anterior part of the subarachnoid space of the spinal cord, and in the region of the medulla oblongata it is continuous otherwise with the previous cisterna and interpeduncularis. It contains the basilar artery.

The cisterna basalis or interpeduncularis is situated in front of the pons, in which situation the arachnoid extends over the interpeduncular space from one temporal lobe to the other. It contains the arteries which form the circulus arteriosus. This cisterna is prolonged outwards on each side into the stem of the lateral fissure, each of these prolongations containing the middle cerebral artery. Anteriorly it extends in front of the optic commissure into the great longitudinal fissure over the upper surface of the corpus callosum, this prolongation containing the anterior cerebral arteries.

The cisterna venae magnae lies just behind the entrance of the great transverse fissure, between the splenium and the corpora quadrigemina, where the great cerebral vein comes out.

The subarachnoid fluid can be drained away in two directions. It can enter the lymph-spaces of the cranial nerves upon which the arachnoid is prolonged outwards in the form of sheaths; and it can enter the lacunae laterales, and through the intervention of the Pacchionian bodies make its way into the superior longitudinal sinus.

Structure of the Arachnoid Membrane. —The arachnoid consists of fine fibrous tissue arranged in interlacing bundles, the intervals between these bundles being occupied by delicate cellular membranes. Several such layers, intimately blended together, form the membrane.

Beneath the arachnoid, and constituting a part of it, there is a reticulum of subarachnoid trabeculce. These trabeculae consist, as in the case of the arachnoid proper, of fine fibrous tissue, but the intertrabecular spaces, instead of being occupied by cellular membranes, contain cerebro-spinal (subarachnoid) fluid. The trabecular reticulum connects the arachnoid with the subjacent pia mater. The superficial surface of the arachnoid is covered with a delicate layer

of endothelium.

Arachnoid Granulations— These are small granular bodies which are situated along the course of the superior longitudinal sinus, into which some of them project. They are seldom met with in adults in other sinuses— e.g., the lateral and straight sinuses. Each body is a villous projection of the arachnoid membrane, with which it is connected by a narrow pedicle. Some bodies project into the superior


i6io


A MANUAL OF ANATOMY


longitudinal sinus; others project from below into the lacunae laterales. In all cases the bodies pierce the dura mater and carry before them the lining of the sinus. Superficially the bodies give rise to the depressions on the internal surface of the parietal bone near the superior border. Each body contains a prolongation of the subarachnoid space and reticulum. This is surrounded by a prolongation of the arachnoid membrane, and external to this is the endothelial lining of the sinus or of the lacuna. The granulations probably are channels through which the subarachnoid fluid can be drained away from the subarachnoid space into the lacunae laterales, and thence into the superior sagittal sinus, as well as into the other sinuses— e.g., the transverse and straight sinuses. They are rarely met with in children under twelve, and then most commonly in the transverse sinus.

Pia Mater.

The pia mater is the most internal covering of the encephalon. It is a very vascular membrane, which invests and is closely adherent to the entire surface. From its internal surface delicate processes pass into the cerebral substance, which represent the minute bloodvessels, surrounded by pia-matral sheaths. The pia mater not only invests the external surface, but also dips into the sulci, and covers the opposed surfaces of the gyri. It also furnishes sheaths to the various cranial nerves, which blend with their perineurium. It gives rise to two weblike expansions—namely, the tela chorioidea inferior and tela chorioidea superior.

The tela chorioidea inferior is situated in the lower part of the roof of the fourth ventricle, and from it are derived the choroid plexuses of that ventricle. The tela chorioidea superior (or velum interpositum) is an invagination of the pia mater through the transverse fissure beneath the splenium of the corpus callosum. It lies underneath the body of the fornix, and its lower surface is covered by the ependymal lining of the third ventricle, the latter forming the roof of that cavity. The tela chorioidea superior furnishes the choroid plexuses of the two lateral and third ventricles.

The pia mater of the encephalon differs from the pia mater of the spinal cord in being thinner and less adherent to the nervous substance. The greater thinness is due to the fact that it is destitute of the outer layer which characterizes the spinal pia mater.

Structure. —The pia mater of the encephalon consists of a single layer of areolar tissue, which contains a great many small bloodvessels, these being derived from the larger vessels lying in the subarachnoid space.

The Cranial Nerves.

The cranial nerves are arranged in twelve pairs. They have received numerical names according to the order in which they leave the cranial cavity from before backwards, and they also have descrip

THE NERVOUS SYSTEM


1611


tive names. The different pairs of nerves are as follows, in order from before backwards:


First, or olfactory.

Second, or optic.

Third, or oculo-motor.

Fourth, or trochlear (pathetic). Fifth, or trigeminal (trifacial). Sixth, or abducent.


Seventh, or facial.

Eighth, or auditory.

Ninth, or glosso-pharyngeal. Tenth, or vagus.

Eleventh, or accessory. Twelfth, or hypoglossal.


The cranial nerves are connected to certain parts of the encephalon, and these connections constitute their superficial or apparent origins. The fibres, however, can be traced to certain collections of grey matter, which are called nuclei. From the deep positions occupied by these nuclei they constitute the deep origins of the nerves.

First or olfactory nerve consists of the olfactory filaments or nerves, which are about twenty in number.

The olfactory nerves are non-medullated. They arise as the axons of the olfactory cells of the olfactory mucous membrane of the nasal fossa; and enter the cranial cavity through the foramina of one half of the cribriform plate of the ethmoid bone. Thereafter they enter the grey matter on the ventral or inferior aspect of the olfactory bulb, and terminate in arborizations which intermingle with the arborizations formed by the dendrites of the mitral cells situated in the granular layer of the bulb (see p. 1570).

Second or Optic Nerve.—This nerve arises from the brain by means of the optic tract, the deep connections of which have been already described (p. 1545). Each optic tract passes forwards and inwards to the optic commissure or chiasma, which is situated in front of the interpeduncular space. 1 he optic nerve of each side arises from the anterior part of the optic chiasma. It courses forwards and outwards to the optic foramen, through which it passes into the orbit, piercing the dura mater, and receiving a sheath from it, as well as from the arachnoid membrane. Having reached the back part of the eyeball, it pierces the sclerotic and choroid coats ^ inch to the nasal or inner side of the axis of the eyeball, and terminates in an expansion which forms the most internal layer of the retina, called the nerve-fibre or optic layer.

Neither this nor the preceding is, strictly speaking, a nerve at all.

Third or Oculo-motor Nerve.—The fibres of this nerve arise from the oculo-motor nucleus, which is situated in the grey matter of the ventral aspect (floor) of the aqueduct on a level with the upper quadrigeminal body, and extends superiorly for a short distance on to the lateral wall of the third ventricle. The nucleus is intimately related to the medial longitudinal bundle, by means of which it is connected with the trochlear and abducent nuclei. All three nuclei receive collaterals from the bundle; and in this manner a functional association between these nuclei is maintained, and harmonious action is insure on the part of the muscles which are supplied by the nerves arising from them. It consists of several groups of cells. As many as seven groups are ascribed to each oculo-motor nucleus by Perlia, which correspond


1612


A MANUAL OF ANATOMY


to the seven muscles supplied by the oculo-motor nerve, and are disposed symmetrically. In addition to these, there is a medially-placed group, the cells of which furnish fibres to both oculo-motor nerves. The fibres which arise from the individual groups of each nucleus are regarded as supplying particular orbital muscles. Certain of the oculo-motor fibres of one side arise from the nucleus of the opposite side, the fibres from either side decussating at the median line. Moreover, each oculo-motor nerve is said to receive fibres from the abducent


Olfactory Bulb


Lateral Fissure Gyrus Rectus


Ant. Perforated Substance


Tuber Cinereum


Corpus Mamillare Crus Cerebri


Post. Perforated Substance


Pons


Olfactory Tract


Optic Nerve


'.Medulla Oblongata


Spinal Cord


Optic Chiasm a


Optic Tract Third Nerve


Fourth Nerve


Fifth Nerve Sixth Nerve Facial Nerve

Sensory Root ol VIJ.

Auditory Nerve Glosso-pharyngeal Nerve Vagus Nerve ' Accessory Nerve

Hypoglossal Nerve


Fig. 990. —The Base of the Encephalon, and the - Cranial Nerves. 1, frontal lobe (orbital surface); 2, temporal lobe; 3, cerebellum.


nucleus of the opposite side, which ascend in the posterior longitudinal bundle and cross to the other side.

Two views are entertained in regard to the nerve-supply of the medial rectus muscle. According to one view, the muscle of one side is supplied by those fibres which have crossed from the oculo-motor nucleus of the opposite side. The other view is that the muscle of one side is supplied by those fibres which have crossed from the abducent nucleus of the opposite side. According to this latter view,













THE NERVOUS SYSTEM


1613


the nerve-fibres which supply the lateral rectus muscle of one side and those which supply the medial rectus muscle of the opposite side arise from the same nucleus—namely, the abducent nucleus—and vice versa.

Probably the whole of the oculo-motor nucleus is not in series with the medial somatic group to which the fourth, sixth, and twelfth nuclei belong, but that some of it corresponds to the more lateral group containing the seventh, ninth, and tenth nuclei. Fibres from this part probably go to the ciliary muscle and iris.


Fig qqi._Scheme showing the Different Cell-groups which constitute,

'according to Perlia, the Nucleus of Origin of the Third Cranial

OR OCULO-MOTOR NERVE (FROM TESTUT, AFTER PERLIA).


1. Posterior Dorsal Nucleus 1'. Posterior Ventral Nucleus

2. Anterior Dorsal Nucleus 2'. Anterior Ventral Nucleus


3

4

5


Central Nucleus ,

Jucleus of Edinger and Westphal Lntero-medial Nucleus


6. Antero-lateral Nucleus

7. Trunk of Oculo-Motor Nerve

8. Crossed Fibres

9. Nucleus of Origin of Fourth Nerve

g'. Intercrossing of Fourth Cranial Nerves

10. Third Ventricle M. Middle Line.


The old view, that the orbicularis oculi is supplied from this nucleus, by the medial longitudinal bundle and the facial trunk, is no longer held by anatomists.

Course of the Fibres o£ the Third Nerve.— The fibres pass forwards from their origin through the tegmentum, the red nucleus, and the medial portion of the substantia nigra, and afterwards make their superficial appearance at the oculo-motor sulcus on the medial aspect of

the crus cerebri. .

The third or oculo-motor nerve supplies the following seven muscles.

the levator palpebne superioris; the superior, inferior, and internal



A MANUAL OF ANATOMY


1614

recti; the inferior oblique; the sphincter pupillae; and the ciliary muscle.

Fourth (Trochlear, or Pathetic) Nerve. —The fibres of this nerve arise from the trochlear nucleus, which is situated in the grey matter of the ventral aspect (floor) of the aqueduct on a level with the upper part of the lower quadrigeminal body. The nucleus is intimately related to the medial longitudinal bundle, by means of which it is connected with the oculo-motor nucleus.

The fibres are at first directed laterally and backwards, and then medially to the upper part of the superior medullary velum, which they enter. Here the nerve crosses to the opposite side, decussating


Fig. 992.—Deep Origins of Third, Fourth, and Fifth Cranial Nerves.

with its fellow, after which it emerges from the upper end of the superior medullary velum close below the lower quadrigeminal body, and by the side of the frenulum veli. After this the nerve turns over the superior peduncle of the cerebellum, and is then directed forwards, round the outer aspect of the crus cerebri, between which and the temporal lobe it makes its superficial appearance.

The fourth nerve supplies the superior oblique muscle of the eyeball.

Fifth Cranial, Trigeminal (or Trifacial) Nerve. —The fifth cranial nerve resembles a spinal nerve in having two roots—sensory and motor

the former being large, and having a ganglion, called the trigeminal ganglion.







THE NERVOUS SYSTEM


1615

Sensory Root. —The fibres of this root are derived from the central poles of the bipolar cells of the trigeminal ganglion. After entering the pons each fibre divides into two branches, ascending and descending, as in the case of the fibres of the dorsal or sensory root of a spinal nerve. The terminal nuclei of these ascending and descending sensory fibres are two in number—upper and lower.

♦The upper sensory nucleus is situated in the outer portion of the dorsal part of the pons, where it lies close to the lateral side of the pontine or principal motor nucleus of the nerve. The ascending sensory fibres, after a short course, enter this nucleus and terminate in arborizations around its cells.

The lower sensory nucleus, continuing the line of the upper sensory nucleus, is an upward prolongation of the substantia gelatinosa from the tubercle and funiculus gelatinosus in the medulla oblongata. The nucleus is traceable as low as the dorsal grey horn of the spinal cord on a level with the second cervical spinal nerve, where it is close to the substantia gelatinosa. The descending sensory fibres, which are numerous, and constitute the spinal root of the fifth nerve, pass downwards through the pons and medulla oblongata into the spinal cord as low as the level of the second cervical spinal nerve. They are accompanied by the lower sensory nucleus, and at different levels they enter this nucleus and terminate in arborizations around its cells.

The disposition of the fibres and cells within the spinal root is of a reversed order—that is, the ophthalmic nerve is associated with the lower part of the spinal root, above this the maxillary, with the mandibular at the upper end.

The ascending or mesencephalic nucleus of the fifth extends along the grey matter on the side of the aqueduct as far as the level of the lower part of the upper corpus quadrigeminum. Its lower limit is lateral to the substantia coerulea in the upper part of the fourth ventricle. The mesencephalic root has only been recognized as sensory within the last few years, and there is reason to suppose that it receives proprioceptive impulses from certain muscles.

Motor Nucleus. —This nucleus is placed in the lateral part of the tegmental region of the pons, deep to the floor of the upper or pontine portion of the fourth ventricle, and immediately medial to the upper sensory nucleus of the nerve. Its fibres run ventro-laterally to emerge as the small motor root of the nerve.

Most of the axons of the cells of the terminal sensory nuclei pass inwards to the raphe and cross to the opposite side. They then become longitudinal and ascend in company with the medial lemniscus or chief sensory tract, their destination being the thalamus of the side to which they have crossed. They thus constitute a trigemino-thalamic ascending tract. From these fibres collaterals are furnished to (1) the facial nucleus, and (2) the ventral vago-glosso-pharyngeal nucleus, or nucleus am

i6i6


A MANUAL OF ANATOMY


biguus, from the cells of which latter the efferent or motor fibres of the pneumogastric or vagus nerve arise.

A few of the axons, however, enter the pontine or chief motor nucleus, and also the mesencephalic sensory nucleus, of the nerve, and terminate in arborizations around its cells.

The large sensory and small motor roots appear close together, on the lateral aspect of the ventral surface of the pons, the motor root lying above and slightly internal to the sensory root. The sensory root enters and the motor root leaves the pons.

Distribution. —The fifth cranial nerve has an extensive distribution by means of its three divisions—ophthalmic, superior maxillary, and inferior maxillary.

Ophthalmic Nerve (Sensory). —(i) The front part of the cranium; (2) the integument of (a) the upper eyelid, and ( b ) the root and tip of the nose; (3) the anterior part of the nasal mucous membrane, and the conjunctiva; (4) the eyeball; and (5) the lacrimal gland.

Maxillary Nerve (Sensory). —(1) The integument of the zygomatic and anterior part of the temporal regions; (2) the integument of (, a) part of the lower eyelid, ( b) the side of the nose, (c) the upper lip, and (d) that part of the face between the lower eyelid and the upper lip; (3) the upper teeth, and the mucous membrane of the upper gum;

(4) a large part of the nasal mucous membrane; (5) the mucous membrane of the maxillary air-sinus (or antrum of Highmore); (6) the mucous membrane of (a) the naso-pharynx, and ( b ) the soft and hard palate and the tonsil.

Mandibular Nerve (Sensory and Motor). —The sensory distribution

of this nerve is as follows: (1) the integument of (a) the temporal region, ( b ) the outer surface of the pinna, and (c) the external auditory meatus;

(2) the integument of the lower lip, and that which covers the mandible; a recurrent branch runs along the petro-squamous suture, supplying the mucous membrane of the tympanum and of the mastoid antrum;

(3) the temporo-mandibular joint; (4) the parotid salivary gland;

(5) the mucous membrane lining the buccinator muscle, and the integument covering that muscle (by means of the long or sensory buccal nerve ); (6) the mucous membrane (fungiform and conical papillae) of the anterior two-thirds of the tongue (common sensation); (7) the submandibular and sublingual salivary glands; and (8) the pulps of the lower teeth, and the mucous membrane of the lower gum.

The motor distribution of the mandibular nerve is as follows: (1) The muscles of mastication—namely, (a) the masseter, ( b ) the temporal, and (c) the pterygoid muscles; (2) the mylo-hyoid muscle and anterior belly of the digastric; (3) the tensor tympani muscle by means of a branch from the otic ganglion; and (4) the tensor palati muscle through the otic ganglion.

Sixth or Abducent Nerve. —The fibres of this nerve arise from the abducent nucleus, which is situated in the dorsal part of the pons close to the median line. It lies above the striae acusticae on the floor of the


THE NERVOUS SYSTEM


1617

fourth ventricle subjacent to the eminentia teres. The fibres emerge from the inner part of the nucleus, and pass through the lower part of the pons in a forward and slightly downward and lateral direction to the lower border of the pons just lateral to the pyramid of the medulla oblongata, where the nerve makes its superficial appearance.

The abducent nucleus receives collaterals from the medial or posterior longitudinal bundle, and a functional connection is thereby established between that nucleus and the oculo-motor nucleus. The medial rectus muscle of one side and the lateral rectus of the other side are thus associated muscles.

The sixth nerve supplies the lateral rectus muscle of the eyeball.


Fig. 993. —Deep Origins of Sixth, Seventh, and Eighth

Cranial Nerves.

Seventh or Facial Nerve.— The facial nerve is composed of two parts. One of these consists of efferent or motor fibres, and is known as the facial nerve proper. The other part, of small size, consists of afferent or sensory fibres. The facial nerve proper arises from the facial nucleus , which is situated deeply in the dorsal part of the lower portion of the pons. The fibres of the nerve pursue an intricate course before appearing superficially. They at first pass backwards and inwards to the floor of the fourth ventricle. Here they turn upwards, lying close to the median line in the form of a single bundle. The nerve then makes a sharp bend laterally, and passes forwards through the pons in a downward and outward direction to its place of emergence.

T02






i6i8


A MANUAL OF ANATOMY


In its course within the pons the nerve is intimately related to the dorsal aspect of the abducent nucleus.

1 he intrapontine part of the facial nerve proper is intimately related to the following structures:

1. The abducent nucleus. 4. The spinal root of the fifth

2. The superior olive. nerve.

3. The corpus trapezoides. 5. The medial or posterior

longitudinal bundle.

The motor facial nucleus receives fibres from the following sources: (1) The corpus trapezoides, being thereby brought into connection with the cochlear division of the auditory nerve; (2) the spinal root of the


sup?- cerebellar^ peduncle.


emmeniia, teres.

nucleus of"., sixth n.

medial longitudbundle.

stalk of olive"' mediul fillet -


\- -V-inferiori cerebellar -ymiddle j peduncle.

_\_lai- vestibular

nucleus.


-/-spinal root of V f - h n.


facial nucleus

-facial nerve

^vestibular part '"of 8 f h nerve.

s . "'-corpus trapezoides

"'Supr. olivary nucleus

abducens nerve pyramidal tract.

Fig. 994.— Diagrammatic Section through the Pons, to show Deep Origins of Sixth (Red) and Seventh (Black) Cranial Nerves.


fifth cranial or trigeminal nerve, which is the sensory nerve of the face; and (3) the pyramidal tract of the opposite side, being thereby brought into connection with the precentral motor area of the cerebral cortex.

All the foregoing fibres terminate within the nucleus in arborizations around its component cells.

The sensory portion of the facial nerve arises from the central poles of the bipolar cells of the geniculate ganglion on the facial nerve in the facial canal. This ganglion resembles the ganglion of the fifth nerve and the spinal ganglia, and most of the peripheral poles of its bipolar cells give rise to the chorda tympani nerve. The pars intermedia passes from the facial canal into the internal auditory meatus, after leaving which it runs to the lower border of the pons, where it lies between the





THE NERVOUS SYSTEM


1619


facial nerve proper and the auditory nerve. The nerve then enters the medulla oblongata, and passes downwards to the upper part of the nucleus of the fasciculus solitarius (see Glosso-pharyngeal Nerve), and its fibres terminate in arborizations around the cells of the upper part of that nucleus. In this situation it is closely associated with the terminal afferent or sensory fibres of the glosso-pharyngeal nerve.

The facial nerve proper emerges from the brain at the lower border of the pons in front of, and internal to, the auditory nerve; and the sensory part enters between the facial nerve proper and the auditory nerve.


Fig. 995.—Terminal Nuclei of the Vestibular Nerve, with their Superior Connections (Schematic) (L. Testut’s * Anatomie Humaine ’).


x. Cochlear Root, with its Two Nuclei

2. Accessory Nucleus

3. Lateral Nucleus (or Tuberculum Acusticum)

4. Vestibular Root

5. Medial Nucleus

6. Lateral Vestibular Nucleus (or Nucleus of

Deiters)

7. Superior Nucleus (of Bechterew)

8. Inferior Root or Nucleus of Auditory Nerve

9. Ascending Cerebellar Fibres


10. Fibres passing to Raphe

11. Oblique Fibres

12. Lemniscus

13. Inferior Sensory Root of Fifth Cranial Nerve

14. Pyramidal Fibres

15. Raph6

16. Fourth Ventricle

17. Inferior Peduncle of Cerebellum (Restiform

Body)

18. Origin of Auditory Striae


Distribution—Motor Part (Facial Nerve Proper).— (1) The muscles of the face, including the buccinator ; (2) the occipito-frontalis; (3) the muscles of the auricle; (4) the posterior belly of the digastric and the stylo-hyoid; (5) the platysma myoides; and (6) the stapedius muscle within the tympanic cavity.

Sensory Part (Sensory Root and Chorda Tympani). —The anterior two-thirds of the tongue (sense of taste).

The chorda tympani nerve conveys secretory and vaso-dilator fibres from the facial nerve proper to the submandibular and sublingual salivary glands. .

The large superficial petrosal nerve from the geniculate ganglion

of the facial nerve is concerned in the supply of the mucous membrane



1620


A MANUAL OF ANATOMY


of the palate, the path being as follows: (i) Large superficial petrosal nerve (facial fibres); (2) the nerve of pterygoid canal; (3) spheno-palatine ganglion; and (4) the descending palatine nerves.

Eighth, Auditory, or Acoustic Nerve. —The auditory nerve is the nerve of hearing and of equilibrium. It is an afferent or centripetal nerve which conducts impressions from the membranous labyrinth (cochlea and vestibule) to the medulla oblongata and pons, and thence to the cerebrum and cerebellum. It consists of two divisions—namely, the cochlear nerve or root, and the vestibular nerve or root.

. The fibres of the cochlear nerve arise from the bipolar cells of the spiral ganglion in the spiral canal of the modiolus, the modiolus being the central pillar of the osseous cochlea. The fibres of the vestibular nerve arise from the bipolar cells of the vestibular ganglion (or ganglion of Scarpa) at the deep end of the internal auditory meatus.

The two nerves or roots reach the brain at the lower border of the pons lateral to the facial nerve and ventral to the restiform body. They have different central connections, and consequently take different courses. The cochlear nerve passes round the outer side of the restiform body, whilst the vestibular nerve passes backwards medial to that body, and each root has special terminal nuclei.

Cochlear Nerve. —The terminal nuclei of the cochlear nerve, which is the nerve of hearing, are two in number—ventral and lateral.

The ventral or accessory nucleus lies on the ventral aspect of the inferior peduncle between the cochlear and vestibular nerves. The lateral or dorsal nucleus, or tuberculum acusticum , is situated on the lateral and dorsal aspects of the peduncle. The fibres of the cochlear nerve or root enter these two nuclei, and terminate in arborizations around their component cells.

Central Connections of the Ventral and Lateral Cochlear Nuclei.—

The ventral and lateral nuclei constitute cell-stations in the path of the fibres of the cochlear nerve, and from these cell-stations two fresh nerve-tracts arise, one being ventral, which constitutes the corpus trapezoides, and the other dorsal, which forms the auditory striae.

Ventral Cochlear Tract. —The relays, or fresh supplies, of nervefibres for this tract are furnished by the cells of the ventral nucleus. The axons of these cells give rise to the trapezium, or corpus trapezoides, which is reinforced by the axons of the cells of the nucleus trapezoides, and fibres from the superior olive of the same side. The trapezoidal fibres cross the median plane, and thereafter constitute the lateral lemniscus, being further reinforced by fibres from the superior olive of the side to which they have crossed, and from the nucleus of the lateral lemniscus. The fibres of this fillet terminate in the lower

quadrigeminal body and internal geniculate body, both of the same side.

Dorsal Cochlear Tract. —The relays of fibres for this tract are furnished by the cells of the lateral nucleus. The axons of its cells form the auditory striae, which cross the dorsal aspect of the restiform body and the floor of the fourth ventricle. At the median line they pass


THE NERVOUS SYSTEM


1621


forwards, and then cross to the opposite side. Thereafter they join the lateral or acoustic lemniscus of the side to which they have crossed.

The lateral or acoustic lemniscus (see p. 1557) derives its fibres from the following sources: (1) The corpus trapezoides; (2) the auditory striae; (3) the superior olive of both sides; and (4) the nucleus of the


Fig. 996. —Terminal Nuclei of the Cochlear Nerve, with their Superior Connections (Schematic) (L. Testut’s ‘ Anatomie Humaine ’).

The vestibular root and its terminal nuclei, with the efferent fibres of these latter, have been suppressed. In order not to obscure the trapezoid body, the efferent fibres of the terminal nuclei of the right side have been in a great part of their extent resected. The trapezoid body is consequently composed of only half of its fibres—namely, those which come from the left.


t. Vestibular Root of Auditory Nerve

2. Cochlear Root

3. Ventral Nucleus of Auditory Nerve

4. Lateral Nucleus (or Tuberculum Acusticum)

5. Efferent Fibres of Ventral Nucleus

6. Efferent Fibres of Lateral Nucleus, forming

the Auditory Striae

6'. Direct Fibres of the Striae going to the Superior Olivary Body of the same side


6 ". Crossed Fibres of the Striae going to the Superior Olivary Body of the opposite side.

7. Superior Olivary Body

8. Corpus Trapezoides

9. Trapezoid Nucleus

10. Lateral Lemniscus 11. Raphe

12. Pyramidal Tract 13. Fourth Ventricle

14, Inferior Peduncle of Cerebellum (Restiform Body)


lateral lemniscus. It serves as a path of connection between the ventral and lateral cochlear nuclei of one side, and the lower Quadrigeminal body and medial geniculate body of the opposite side.

It is to be noted that the ventral and lateral cochlear nuclei are slightly connected with the lower quadrigeminal body of the same side but not with the corresponding medial geniculate body.

The axons of the cells of the medial geniculate body form a cork








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A MANUAL OF ANATOMY


cipetal tract, which passes to the cortex of the first or superior temporal gyrus of the temporal lobe of the brain.

The complex nervous chain associated with the cochlear nerve may be tabulated as follows:

1. The bipolar cells of the spiral ganglion.

2. The fibres of the cochlear nerve.

3. The ventral and lateral cochlear nuclei.

4. The fibres of the corpus trapezoides, reinforced as stated.

5. The auditory striae.

6 . The medial geniculate body.

7. The corticipetal tract from the medial geniculate body to the

superior temporal gyrus.

The cell-stations connected with this nervous chain are as follows:

1. The ventral cochlear nucleus. 5. The nucleus of the lateral lemnis 2. The lateral cochlear nucleus. cus.

3. The nucleus trapezoides. 6. The lower quadrigeminal body.

4. The superior olive of each side. 7. The medial geniculate body.

Some of the fibres of the chain terminate in these cell-stations, and others are derived from the axons of the cells which compose the stations.

Vestibular Nerve. —The terminal nuclei of the vestibular nerve, which is the nerve of equilibrium, are three in number—namely, (1) the dorsal or principal nucleus, (2) the descending nucleus, and

(3) the nucleus of Deiters, associated with which there is the nucleus of Bechterew.

The medial or principal nucleus is situated in the floor of the fourth ventricle underneath the area acustica and stria (see Fig. 995, 5). The inferior nucleus is continuous with the lower end of the dorsal nucleus, and it accompanies the descending fibres of the vestibular nerve into the medulla oblongata. The lateral nucleus (nucleus of Deiters) is situated lateral to the dorsal and descending nuclei, and the superior nucleus (nucleus of Bechterew) represents the upper and outer part of the nucleus of Deiters.

As the vestibular nerve passes backwards medial to the inferior peduncle, some of its fibres, to be presently described, turn downwards. The majority, however, pass to the principal nucleus, the lateral nucleus, and the superior nucleus, and terminate in arborizations around the cells of these nuclei. A few of the vestibular fibres are regarded as passing directly to the cerebellum (superior vermis).


Superior N. Lateral N. Medial N.


Fig. 997.—The Position of the Vestibular Nuclei in Relation to Floor of Fourth Ventricle (Semi-schematic).









THE NERVOUS SYSTEM


1623


Central Connections of the Nuclei of the Vestibular Nerve. —The

fibres of the vestibular nerve terminate in the nuclei just stated. The medial or principal nucleus and the other vestibular nuclei are intimately related to the superior vermis of the cerebellum, and especially to the roof-nucleus, by means of cerebellar fibres. This communication represents the direct sensory cerebellar tract of Edinger, and it is contained within the inferior peduncle of the cerebellum. The axons of many of the cells of the lateral nucleus and superior nucleus pass into the medial or posterior longitudinal bundle, within which they divide into ascending and descending branches. In this manner the medial longitudinal bundle is brought into communication with the vestibular nerve. By means of the medial longitudinal bundle, which represents the ground-bundles in the spinal cord, the nucleus of Deiters is brought into communication with the anterior or motor horns of the spinal cord. By means of this bundle the nucleus is also brought into communication with the nuclei which control the ocular muscles— namely, the oculo-motor, trochlear, and abducent nuclei.

The complex nervous chains associated with the vestibular nerve may be tabulated as follows:


Cerebellar Chain.


1. The bipolar cells of the vestibular ganglion.

2. The fibres of the vestibular nerve.

3. The medial vestibular nucleus, lateral nucleus, and superior nucleus.

4. The secondary cerebellar vestibular tract or the direct sensory cerebellar

tract of Edinger, leading to the superior vermis and roof-nucleus of the cerebellum.


Spinal and Oculo-motor Chains.


1. The bipolar cells of the vestibular ganglion.

2. The fibres of the vestibular nerve.

3. The lateral and superior vestibular nuclei. , . . .

4 The secondary vestibular tract from the nuclei to the posterior longitudinal

bundle, and thence to the motor horns of the spinal cord and the motor nuclei of the ocular muscles.

The lateral nucleus thus has important connections as follows:

1. The membranous 3 - The motor horns of the


spinal cord.


vestibule.

2. The cerebellum.


4. The motor nuclei of the ocular muscles.


nncnnnrfinff Fibres of the Vestibular Nerve. —As the vestibular nerve


usually regarded as terminating in arborizations around the cells of


1624


A MANUAL OF ANATOMY


that nucleus. Superiorly they are related to the lateral vestibular nucleus, and some authorities have regarded them as fibres passing between the nucleus and the cuneate nucleus.

The cochlear and vestibular nuclei originally form one acoustic or auditory nucleus, which is developed from the rhombic lip.

Ninth or Glosso-pharyngeal Nerve. —This nerve consists chiefly of afferent or sensory fibres, which grow into the medulla oblongata, but it also contains a few efferent or motor fibres, which arise within the medulla oblongata.

Afferent or Sensory Fibres. —These fibres arise from the central poles of the bipolar cells of the ganglia which are situated on the


Fig. 998.— Deep Origins of Ninth and Tenth Cranial Nerves.

glosso-pharyngeal nerve as it passes through the jugular foramen. These ganglia resemble the ganglion of the fifth nerve and the spinal ganglia. Having entered the medulla oblongata, the afferent fibres end in two terminal sensory nuclei—namely, the dorsal vago-pharyngeal nucleus, and the nucleus of the fasciculus solitarius.

The dorsal vago-pharyngeal nucleus consists of two parts—upper and lower. The upper part is situated in the grey matter of the floor of the lower or bulbar part of the fourth ventricle, underneath the superficial area known as the trigonum vagi , and immediately external to the hypoglossal nucleus. The lower part is situated in the lower or closed part of the bulb, and lies in the grey matter which forms the lateral





THE NERVOUS SYSTEM 1625

wall of the central canal of the bulb, being here situated behind the hypoglossal nucleus.

The highest part of the dorsal vago-glosso-pharyngeal nucleus represents the portion associated with the glosso-pharyngeal nerve, and the remaining and greater part belongs to the vagus nerve.

The nucleus of the fasciculus solitarius is a column of grey matter and nerve-cells which accompanies the fasciculus solitarius, to be presently described, throughout the whole length of the medulla oblongata (Figs. 889 and 998).

A few of the afferent fibres of the glosso-pharyngeal nerve enter the highest part of the dorsal vago-glosso-pharyngeal nucleus, and terminate in arborizations around its cells. Most of the afferent fibres, however, descend along with a few of the afferent fibres of the vagus nerve, the two sets of descending fibres constituting a strand, called the fasciculus solitarius (tractus solitarius). This strand descends throughout the whole length of the medulla oblongata. It lies lateral to the dorsal vago-glosso-pharyngeal nucleus, inclining towards the ventral aspect of the upper part of that nucleus, and towards the dorsal aspect of its lower part. It is accompanied throughout by the nucleus of the fasciculus solitarius, and the glosso-pharyngeal afferent fibres of the fasciculus solitarius terminate at different levels in arborizations around its cells. The fasciculus solitarius is formed chiefly, not entirely, by glosso-pharyngeal fibres: fibres from facial enter its upper end.

Efferent or Motor Fibres. —These fibres arise within the medulla oblongata as the axons of some of the cells of the ventral vago-glossopharyngeal nucleus or nucleus ambiguus. This nucleus is situated in the formatio reticularis grisea of the medulla oblongata, and is in line with the facial motor nucleus, which is placed in the dorsal part of the lower portion of the pons. The nucleus ambiguus is ventral in position to the dorsal vago-glosso-pharyngeal nucleus, and the axons of its cells, some of which form the glosso-pharyngeal efferent or motor fibres, pass dorsal wards towards the last named. They then alter their course, and, passing forwards and laterally, associate themselves with the afferent or sensory glosso-pharyngeal fibres.

The funiculi of the glosso-pharyngeal nerve appear in the dorsolateral sulcus of the medulla oblongata, between the olivary and restiform bodies, and immediately below the facial nerve.

Distribution.— The glosso-pharyngeal nerve is distributed to (1) the mucous membrane of the posterior third of the tongue, of which part it is the nerve of taste, as well as of common sensation; (2) the mucous membrane of the pharynx, tonsil, and fauces, (3) the mucous membrane of the tympanum; and (4) the stylo-pharyngeus muscle. It also furnishes secretory and vaso-dilator fibres to the parotid gland by means of (1) its tympanic branch (Jacobson’s nerve), (2) the tympanic plexus, (3) the small superficial petrosal nerve, (4) the otic ganglion, and (5) the auriculo-temporal nerve.

Tenth or Vagus Nerve. —This nerve consists of afferent or sensory


1626


A MANUAL OF ANATOMY


\


fibres, which grow into the medulla oblongata; and efferent or motor fibres, which arise within the medulla oblongata.

Afferent or Sensory Fibres. —These fibres arise from the central poles of the bipolar cells of the ganglion of the root and the ganglion of the trunk of the nerve, which resemble the glosso-pharyngeal ganglia, the ganglion of the fifth nerve, and the spinal ganglia. Having entered the medulla oblongata, the afferent fibres pass to the same two terminal sensory nuclei as do the afferent fibres of the glosso-pharyngeal nerve— namely, the dorsal vago-glosso-pharyngeal nucleus and the nucleus of the fasciculus solitarius —which have just been described in connection with the glosso-pharyngeal nerve. Most of the afferent fibres of the vagus nerve pass to the vagal portion of the dorsal vago-glossopharyngeal nucleus, which represents its greater and lower part, the highest part of the nucleus receiving, as stated, a few of the afferent fibres of the glosso-pharyngeal nerve. Within the vagal part of the nucleus the afferent fibres of the vagus terminate in arborizations around its cells. A few of the afferent fibres, however, descend along with most of the afferent fibres of the glosso-pharyngeal nerve, the two sets of descending fibres constituting the strand called the fasciculus solitarius, already described in connection with the glosso-pharyngeal nerve. These descending afferent vagal fibres terminate, like the corresponding glosso-pharyngeal fibres, in the nucleus of the fasciculus solitarius, which has been described in connection with the glossopharyngeal nerve.

The dorsal vago-glosso-pharyngeal sensory nucleus, and the nucleus of the fasciculus solitarius, also sensory, are therefore shared in common by the afferent or sensory fibres of the glosso-pharyngeal and vagus nerves, but in unequal proportions. Only a few glossopharyngeal afferent fibres go to the dorsal vago-glosso-pharyngeal nucleus, whereas most of the vagal afferent fibres pass to that nucleus. In the case of the nucleus of the fasciculus solitarius it is the reverse.

Efferent or Motor Fibres. —These fibres arise within the medulla oblongata as the axons of most of the cells of the ventral vago-glossopharyngeal nucleus or nucleus ambiguus, which has been described in connection with the glosso-pharyngeal nerve. The fibres pass dorsalwards to the more superficially placed dorsal vago-glosso-pharyngeal nucleus. They then alter their course, and, passing forwards and outwards, associate themselves with the afferent or sensory vagal fibres.

The ventral vago-glosso-pharyngeal nucleus or nucleus ambiguus, which is a motor nucleus, is shared in common by the efferent or motor fibres of the glosso-pharyngeal and spinal accessory nerves, especially the latter. The fibres from this nucleus join the vagus, and leave it as the pharyngeal and laryngeal branches.

The funiculi of the vagus nerve appear in the dorso-lateral sulcus of the medulla oblongata, between the olivary and restiform bodies, and immediately below the funiculi of the glosso-pharyngeal nerve.

Distribution. —The vagus nerve has a very extensive distribution on either side, of which the following is a summary:


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Motor Distribution. —(1) The muscles of the soft palate (except the tensor palati) ; (2) the constrictor muscles of the pharynx; (3) the intrinsic muscles of the larynx; (4) the muscular tissue of the oesophagus and stomach; and (5) the muscular tissue of (a) the trachea, (b) the bronchi, and (c) the bronchial tubes.

Sensory Distribution. —(1) The pharynx, oesophagus, and stomach; (2) the larynx, trachea, and bronchial tubes to their terminal ramifications; and (3) the skin on the cranial aspect of the pinna, as well as of the lower and back part of the external auditory meatus.

Cardiac Fibres. —The cardiac fibres of the nerve are inhibitory (efferent) and depressor (afferent).

The most important connection of the vagus nerve is that which is established with the bulbar or accessory portion of the accessory nerve.

Glosso-pharyngeal and Vagal Nuclei. —These two nerves, as stated, consist of afferent or sensory and efferent or motor fibres. The afferent fibres of both nerves share in common two terminal nuclei —namely, the dorsal vago-glosso-pharyngeal nucleus and the nucleus of the fasciculus solitarius. Most of the glosso-pharyngeal afferent fibres terminate in the nucleus of the fasciculus solitarius, and most of the vagal afferent fibres terminate in the dorsal vago-glosso-pharyngeal nucleus. According to the description which has been given of the dorsal vagoglosso-pharyngeal nucleus, it is a nucleus of termination , or sensory nucleus. According to certain authorities, however, it is a mixed nucleus —that is to say, it is both a nucleus of termination, or sensory nucleus, and a nucleus of origin, or motor nucleus. In accordance with this view, the nucleus contains two sets of cells —sensory and motor —some of the afferent glosso-pharyngeal and most of the afferent vagal fibres terminating in arborizations around the sensory cells, and some of the efferent fibres of each nerve arising as the axons of the motor cells.

The nucleus of the fasciculus solitarius is a nucleus of termination,

or sensory nucleus.

The fibres of the sensory portion of the facial nerve terminate m the upper part of the nucleus of the fasciculus solitarius.

The efferent or motor fibres of the glosso-pharyngeal and vagus nerves arise as the axons of the motor cells of the ventral vago-glossopharyngeal nucleus, or nucleus ambiguus, which is a nucleus of origin, or motor nucleus. According to the description which has been given of this nucleus, it gives origin to all the motor fibres of the two nerves. If, however, the dorsal vago-glosso-pharyngeal nucleus is a mixed nucleus, then some of the efferent or motor fibres of the two neives

arise as the axons of its motor cells.

The axons of the cells of the terminal sensory nuclei are disposed like those of the cells of the terminal sensory nuclei of the fifth nerve. They cross to the opposite side, become longitudinal, and ascend m company with the medial lemniscus or chief sensory tract to the thalamus of the side to which they have crossed. They constitute the vago-glosso-pharyngeal ascending thalamic tract.


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Eleventh or Accessory (Spinal Accessory) Nerve. —This is a motor nerve, which is partly a continuation of the vagus. Its spinal fibres arise from the accessory nucleus, which is situated to a small extent within the medulla oblongata or bulb, and mostly within the cervical part of the spinal cord. This nucleus consists of a column of large cells which is continuous with the dorsafivago-glosso-pharyngeal nucleus at the medullary level. The column extends from the level of the lower part of the olivary body to the level of the sixth cervical nerve. The bulbar termination of the nucleus is situated on the dorso-lateral aspect of the hypoglossal nucleus. The spinal portion is situated in the lateral part of the anterior grey horn of the cervical spinal cord, and its cells lie directly behind the motor cells which give origin to the anterior roots of the upper five cervical nerves.

The fibres which emerge superficially from the bulb constitute the bulbar part of the accessory nerve, and are accessory to the vagus nerve. The fibres which arise from the spinal cord constitute the spinal part of the accessory nerve, and are really distinct from the bulbar fibres.

Bulbar Part.— The fibres of this part arise as the axons of the cells of the nucleus ambiguus. They are directed at first dorsalwards, and then outwards through the lateral part of the medulla oblongata or bulb, from which they emerge, behind the olive, in the form of about five funiculi, placed below, and in line with the funiculi of the vagus nerve. They then pass outwards, lying within the cranial cavity, and join the spinal part of the accessory nerve (which has entered the cranial cavity through the foramen magnum). The accessory nerve afterwards leaves the cranial cavity through the jugular foramen.

Spinal Part.— The fibres of this part arise as the axons of the cells of the accessory nucleus in the cervical cord. They are directed at first backwards, and then outwards through the lateral column of the spinal cord, from which they emerge as a series of funiculi which succeed to the funiculi of the bulbar part, the lowest spinal funiculus being on a level with the fifth cervical nerve. The funiculi of the spinal part ascend, lying in the subdural space between the ligamentum denticulatum and the posterior roots of the upper five cervical nerves. They enter the cranial cavity through the foramen magnum, and join the bundles of the bulbar part, to form the accessory nerve.

Distribution of Accessory Nerve.— After leaving the jugular foramen, the accessory nerve divides into two branches—internal and external— the internal branch containing the fibres of the bulbar part, whilst the external branch contains the fibres of the spinal part.

Spinal Distribution.— The external or spinal branch supplies the sterno-cleido-mastoid and trapezius muscles.

Bulbar Distribution. —The internal or bulbar branch passes over, and in close contact with, the ganglion of the trunk of the vagus nerve. Its fibres are continued into (1) the pharyngeal and superior laryngeal branches of the ganglion of the trunk of the vagus, and (2) the trunk of the vagus beyond the ganglion.


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The bulbar fibres, through their connection with the vagus nerve, are probably distributed to (1) the muscles of the soft palate, excluding the tensor palati; (2) the constrictor muscles of the pharynx; and (3) the intrinsic muscles of the larynx. The bulbar fibres may also furnish (a) the inhibitory fibres to the heart, (b) the motor fibres to the oesophagus, and (c) the motor fibres to the stomach.

The bulbar fibres of the accessory nerve are regarded by some authorities as arising from the column of cells which constitutes the ventral vago-glosso-pharyngeal nucleus, or nucleus ambiguus.

Twelfth or Hypoglossal Nerve.—The fibres of the hypoglossal nerve arise from the axons of the cells of the hypoglossal nucleus, which is situated within the medulla oblongata This nucleus represents a column of large multipolar motor-cells, which extends from the level of the auditory striae superiorly to the level of the upper part of the decussation of the pyramids inferiorly. The lower part of the nucleus is situated within the lower or closed part of the medulla oblongata, and its upper part lies within the upper, open, or ventricular part. The lower part lies in the grey matter which forms the ventro-lateral aspect of the central canal of the medulla oblongata. The upper part lies in the grey matter which covers the bulbar part of the floor of the fourth ventricle, and is underneath the area known as the trigonum hypoglossi.

The nerve-fibres issue from the ventral aspect of the nucleus, and the nerve-funiculi pass through the medulla oblongata in a dorsoventral direction, lying between its anterior and lateral areas, and between the formatio reticularis alba and formatio reticularis grisea. Having reached the bottom of the ventro-lateral sulcus between the pyramid and the olive, they emerge from the medulla oblongata in line with the sixth cranial nerve superiorly.

The two hypoglossal nuclei, right and left, are connected with each other by commissural dendrons; and each nucleus receives collaterals from the pyramidal tract of the opposite side, being thereby brought into connection with the precentral motor area of the opposite cerebral hemisphere.

Distribution.—The hypoglossal nerve is the motor nerve of the

tongue, and supplies (1) the stylo-glossus, (2) the hyo-glossus, (3) the genio-hyo-glossus, and (4) the intrinsic muscles of the tongue.

Cranio-cerebral Topography (see Figs. 999 and 1000).

The auricular point is the centre of the orifice of the meatus auditorius externus.

The pre-auricular point is situated in the depression between the tragus of the auricle and the condyle of the mandible.

The bregma, or point of junction of the sagittal and coronal sutures, corresponds to the centre of a line connecting the two auricular points (the centre of the orifice of the meatus auditorius externus).

The lambda, or meeting of the sagittal and lamboidal sutures, is


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situated about 2§ inches or four fingers' breadth above the inion, or external occipital protuberance.

The pterion, or region of the spheno-parietal suture, is situated about i| inches behind the external angular process of the frontal bone, and about if inches above the zygomatic arch. Two fingers’ breadth above the middle of the zygoma forms quite a useful indication to it.

the asterion, or point where the parieto-mastoid, occipito-mastoid, and lambdoid sutures meet, is situated about 2 inches behind the


Fig. 999.— Diagram showing the Relations of the Chief Cerebral Fissures to the Exterior of the Head (Reid).


A. Glabella B. Inion C. Auricular Point

E. Pre-auricular Point

F. Superior Rolandic Point

G. Posterior Border of Root of Mastoid Process

H. Inferior Rolandic Point B.C. Transverse Fissure


D.E., F.G. Reid’s Perpendicular Lines Sy.Fis. Lateral Fissure Sy.a.Fis. Anterior Limb of Fissure Sy.h.Fis. Posterior Horizontal Limb of Fissure p.o.Fis. External Parieto-occipital Fissure + Parietal Eminence


auricular point very nearly in line with the upper border of the zvgomatic arch. J

The sagittal line represents the line which connects the nasion, or meeting of the two fronto-nasal sutures, with the inion.

The sagittal suture corresponds to that part of the sagittal line which extends from the lambda to the bregma.

The coronal suture is indicated on either side by a line extending from the bregma to the pterion.

The lambdoid suture (occipito-parietal) corresponds to a line extending from the lambda to the asterion.

The squamo-parietal suture, and its continuation backwards as the parieto-mastoid suture, are indicated by a curved line, with the con






THE NERVOUS SYSTEM 163!

vexity upwards, extending from the pterion to the asterion, the highest part of the curve being about 2 inches above the zygomatic arch.

The superior sagittal sinus corresponds to a line drawn from the glabella to the inion, or external occipital protuberance. For the most part it occupies the median line, but as it grooves the upper portion of the tabular part of the occipital bone it deviates to one side, most commonly the right side.

The confluens sinuum (or torcular Herophili) is usually situated on the right side of the inion.

The occipital sinus corresponds to a line drawn downwards from the inion.


Fig. 1000.—The Relations of the Brain to the Surface (modified

from Hermann).


The transverse sinus on either side is indicated by a line drawn outwards from a point immediately above, and external to, the inion to a point immediately above the asterion. This line is slightly curved, the convexity being upwards. The sinus grooves the inner surface of one-half of the tabular portion of the occipital bone, along the line of attachment of the tentorium cerebelli ; and in the region of the asterion, where the sinus, in altering its course, describes a curve, it grooves the inner aspect of the parietal bone, close to the postero-inferior angle, for a very short distance.

Inasmuch as the superior sagittal sinus usually opens into the right transverse sinus, the right sinus is usually larger than that of the

left side. . . ,

The sigmoid part of the transverse sinus grooves the inner surface

of the mastoid portion of the temporal bone, and the superior surface















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of the jugular process of the occipital bone. The following line indicates approximately the course of the sinus: (1) Draw a line horizontally forwards from the asterion for fully f inch to a point on the root of the mastoid process in line with the upper part of the meatus auditorius externus; (2) the line now curves and passes downwards and forwards on the front part of the mastoid process towards its tip for f inch, lying close to the groove between the back of the pinna and the mastoid process (the level to which this line descends is J inch below the lower margin of the orifice of the meatus auditorius externus); and (3) the line finally passes forwards for inch to meet the jugular foramen, through which the sigmoid part of the transverse sinus leaves the cranial cavity to become the internal jugular vein.

The sigmoid sinus lies directly behind the mastoid or tympanic antrum, being separated from it only by a very thin plate of bone. In pyogenic affections of the tympanum and antrum the sinus is consequently liable to become affected with thrombosis.

The convexity of the genu of the sigmoid sinus is on a level with the temporo-mandibular joint, and lies from J to J inch behind the base of the suprameatal triangle or £ inch behind the pre-auricular point.

The right sigmoid sinus is usually larger than the left. In many cases the genu is scarcely perceptible, and the horizontal and vertical limbs under these circumstances are practically almost in direct continuity with each other. The genu lies at a depth from the surface varying from J to J inch.

The middle meningeal artery corresponds to the centre of the zygomatic arch. At a point from \ to £ inch above the centre of the arch it divides into its two terminal branches—anterior and posterior.

The large anterior division of the middle meningeal artery, as it lies in the groove, or, it may be, short canal, on the internal aspect of the antero-inferior angle of the parietal bone, is indicated by taking a point i-J inches behind the external angular process of the frontal bone, and ij inches above the zygomatic arch. From this point it ascends almost vertically towards the sagittal suture, lying about | inch behind the coronal suture.

The Sylvian point coincides with the pterion, and is situated about ij inches behind the external angular process of the frontal bone, and about 2 inches above the zygomatic arch. It indicates the division of the stem of the lateral fissure into its three limbs—anterior, ascending, and posterior. The anterior limb of the fissure passes horizontally forwards for about 1 inch, and the ascending limb upwards and slightly forwards for a variable distance, from the pterion.

The posterior limb of the fissure is long, and is directed for the most part horizontally backwards for fully 2 inches, after which it turns upwards into the parietal lobe for a short distance. The line which indicates the course of the posterior limb is called the Sylvian or lateral line. It extends from the pterion backwards and slightly upwards towards the lambda for about 2 inches until it lies below the parietal


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eminence, when it turns directly upwards for J inch. The parietal lobe and a small portion of the frontal lobe lie above the Sylvian line, and the temporal lobe lies below it.

The superior Rolandic point is situated ^ inch behind the centre of the sagittal line, which connects the nasion and the inion. This point approximately represents the upper extremity of the central fissure.

The inferior Rolandic point is situated on the Sylvian line about 1 inch behind the Sylvian point, and 2 inches above the pre-auricular point. It indicates the point where the central fissure, if sufficiently prolonged, would meet the posterior limb of the stem of the lateral fissure.

The line of the central fissure is represented by a line connecting the superior and inferior Rolandic points.

The Rolandic angle is the angle which this line forms with the sagittal line. It ranges from 65 to 70 degrees. The line, if sufficiently prolonged, would cross the zygomatic arch at its centre. It indicates in a general way the course of the central fissure, but this fissure usually ceases at a point J inch above the Sylvian line. The cerebral convolutions directly in front of, and behind, the central line are (1) the precentral, or ascending frontal, convolution (motor area) in front; and (2) the postcentral, or ascending parietal, convolution (sensory area) behind. The line represents the boundary-line between the frontal and parietal lobes of the cerebral hemisphere.

The base-line of Reid is represented by a line drawn backwards from the centre of the infra-orbital margin through the pre-auricular and auricular points to the inion. The central fissure may be determined from Reid’s base-line in the following manner: Two lines are drawn upwards to the sagittal line perpendicular to the base-line, one from the pre-auricular point and the other from the posterior border of the mastoid process close to its root. These two lines, together with the sagittal and Sylvian lines, enclose a quadrilateral area, and the diagonal connecting the postero-superior and antero-inferior angles represents the fissure except at its superior and inferior limits.

The parietal eminence may usually be felt, if it cannot be seen, as the point of maximal convexity in the parietal region. If it cannot be felt, its position may be estimated by localizing the four angles of the parietal bone, bregma, lambda, pterion, and asterion, and taking the point where the diagonals joining them meet. It indicates the position of the supramarginal gyrus of the parietal lobe of the cerebral hemisphere.

The frontal eminence corresponds to the middle frontal convolution.

The foramen magnum, through which the medulla oblongata is continuous with the spinal cord, lies midway between the mastoid processes. Its posterior margin is 2 inches from the inion in a downward and forward direction.

The tentorium cerebelli and superior surface of the cerebellum

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practically coincide with the level of the transverse sinus, as indicated by a line slightly curved upwards connecting the inion and asterion.

The lower level of the cerebral hemisphere may be indicated by the following line: Commencing at a point \ inch external to the nasion, the line passes laterally in an arched manner, with the convexity upwards, lying about J inch above the centre of the supra-orbital arch. It then inclines downwards and crosses the temporal ridge of the frontal bone about \ inch above the fronto-malar suture, which is easily felt. After this the line passes backwards and slightly downwards to the pterion, and thence to the upper border of the posterior part of the zygomatic arch. From this point the line passes backwards, lying about \ inch above the upper margin of the orifice of the meatus auditorius externus. It then crosses the supramastoid crest (posterior root of the zygoma), and passes to the asterion. From this it nearly follows the line of the transverse sinus from the asterion to a point a little above and external to the inion. In other words, speaking generally, the cerebral hemisphere extends as low as the superior nuchal line of the occipital bone posteriorly , the upper border of the zygomatic arch laterally , and the upper part of the eyebrow anteriorly. The frontal lobe of the cerebral hemisphere is to a large extent in contact with the frontal portion of the frontal bone, but it is also related to the anterior part of the parietal bone as far back as the central fissural line.

The parietal lobe is related to the part of the parietal bone which lies behind the central line. The lobe extends as far back as the parieto-occipital fissure, which is usually situated opposite the lambda.

The occipital lobe occupies the cerebral fossa of the tabular part of the occipital bone, its limits being the level of the lambda superiorly and the level of the inion inferiorly.

The temporal lobe is under cover of the squamous portion of the temporal bone and the postero-inferior part of the parietal bone. The parallel sulcus, which separates the first and second temporal convolutions, is indicated by a line drawn from the lambda to the marginal tubercle on the posterior border of the malar bone about J inch below the fronto-malar suture.

Below it is the middle temporal gyrus, the centre of which corresponds to the inferior horn of the lateral ventricle. A perforation \ inch below the line just mentioned and ij inches behind the middle of the external auditory meatus would strike it with certainty if it were distended.

Autonomic System.

The autonomic nervous system, mentioned at the beginning of this chapter, was then stated to be composed of two differentiated groups of fibres which were termed sympathetic and parasympathetic, the former known by that name to generations of anatomists, but the latter including within its limits a more modern conception of certain fibres


/


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and functions which were not grouped in any way with the sympathetic by the older observers.

The sympathetic system comprises all that chain of nerve cords and ganglia which lies on each side and front of the vertebral column, with its connections and distributions. The detailed descriptions of the system in the different regions of the body have been given in previous chapters of this book, where they can be perused. It is not necessary, therefore, to enter on these details again, but something can be said about the system in its general aspect.

The ganglia which, with the connecting cords, make up the ‘ sympathetic chain ’ are probably modified in number from an original set which corresponded with the number of spinal nerves. This original number of ganglia has been lessened, however, by fusion of neighbouring masses, and sometimes by actual loss; for example, in the cervical region, the four uppermost ganglia have been fused into the single superior cervical ganglion, and the lower two have joined in the inferior ganglion—which itself shows signs of joining with the first thoracic— while the ‘ middle cervical ganglion ’ is frequently, if not usually, absent or very small. In the thoracic region the number is decreased frequently by fusion, and in the lumbar and sacral portions of the chain is very variable.

Both afferent and efferent (secretory and motor) impulses pass through the sympathetic system. The afferent fibres run through it without interruption, arising from the cells in the posterior root ganglia. Efferent fibres arise from the lateral grey matter of the spinal cord, and leave the cord through the anterior nerve roots of the thoracic and upper two or three lumbar nerves. They pass from these to the sympathetic chain by fine branches (white rami communicantes), and run in this chain to the particular ganglia with which they are concerned.

Rami communicantes are of two sorts, white (medullated) and grey (non-medullated). White rami bring the medullated fibres to the sympathetic chain from the nerves within which they emerged from the spinal cord, and are therefore confined to the thoracic and upper lumbar regions. Grey rami are fibres of sympathetic origin, arising from cells in the ganglia in which the white fibres have been interrupted, and passing for convenience of distribution to any and all of the spinal nerves; in this way they reach their objectives, and are found connecting the sympathetic chain with all the spinal nerves—not limited to particular regions like the white rami.

The rami are thus of two sorts, and both sorts are present in the thoracic and upper lumbar region, but it must not be imagined that a ramus of either sort to a nerve is always a single branch; it may be doubled or even trebled, and may reach a ganglion or the interganglionic trunk, but there are only two kinds of fibres represented at the most, and the white fibres, wherever they join the sympatheitc trunk, run in it until they reach the appropriate ganglia—which may be near, or far away.


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Afferent fibres from visceral structures pass from these through the sympathetic system, and through white rami to the spinal nerve and spinal ganglion concerned; some are said to go through grey rami. Those from the body-wall and limbs, and from the head and neck, run their courses within the spinal nerves themselves.

Efferent fibres are always interrupted once in their course, so that there are two relays of the efferent impulse. The first is represented

anatomically by the course of the issuing white fibre from the cord to its appropriate ganglion, the second by the course of the non-myelinated fibre arising in this ganglion and passing to its objective.

The ganglionic cell which interrupts the efferent impulse, and relays it secondarily, may be one of those in the sympathetic chain ganglia, or may lie at some distance from the chain, in one of the great plexuses (cardiac, cceliac, hypogastric), or even in some more remote and minute collection of nerve-cells in the wall of a viscus or on a bloodvessel. Wherever this ganglionic cell may be, the first relay of the impulse is carried by a fibre which is not interrupted before reaching it; this fibre is termed preganglionic. The second relay is carried by a postganglionic fibre, which is the axon of this ganglion cell, and is non-medullated.

From what has been said it can be understood that the efferent fibres, in white rami communicantes, may run even to the extreme end of the sympathetic chain, or to a distant plexus, before losing their preganglionic status, so that they will pass through any intervening ganglia without interruption. Afferent fibres always pass through such ganglia without interruption, to reach the posterior root ganglia. Thus the level of exit of an efferent preganglionic fibre has little to do with the level or position of its terminal ganglion, and the postganglionic fibres in a grey ramus may have come from a sympathetic ganglion some distance away.


Fig. iooi.—Approximate Levels in Cord of Centres of Origin of Sweat - fibres (Pre ganglionic) supplying (A) Head, Neck, and Upper Part of Thorax; B, Upper Limb ; C, Lower Limb ; D, External Genitals and Anal Region.


Course of Sympathetic Efferent Impulses.

Central Origin.—The cells of the intermedio-lateral region of the cord, approximately corresponding with the nerve-levels which give passage to the fibres issuing from the cord, and considered generally to










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be those from which these fibres take origin. There are probably higher centres in the nervous axis which exercise some controlling influence over these thoraco-lumbar cells; such centres are presumed to lie in the hypothalamus, their scattered fibres passing down in the tegmentum and formatio reticularis of the pons and medulla. Other control centres in the floor of the fourth ventricle are more doubtful.

Dilatation of Pupil. — Preganglionic fibres emerge through upper (? three) thoracic nerves and run up the gangliated cord. Cell-station , superior cervical ganglion. Postganglionic fibres pass through ciliary ganglion without interruption and enter short ciliary nerves. Their course to the ganglion is curious. They pass up in the plexus on the internal carotid, leave this by the carotico-tympanic filaments, and


Fig. 1002.—To show Course of Sympathetic (Red) and Parasympathetic (Blue) Fibres concerned in Certain Infra-orbital Actions (Preganglionic Fibres, Dotted; Postganglionic, Solid Lines).

A, the routes followed in the case of the pupil; B, those in the case of lacrimal secretion; G, superior cervical ganglion; T, C, tympanic and cavernous plexuses; N, naso-ciliary nerve; S, spheno-palatine ganglion; Z, zygomaticomalar nerve; L, lacrimal nerve; F, facial ganglion.

enter the middle ear. From this they pass up through the foramen lacerum and join the cavernous plexus, from which they find their way into the orbit with some of the branches of the fifth nerve, or on the ophthalmic artery (Fig. 1002).

Lacrimal Gland— Preganglionic to superior cervical ganglion. Postganglionic along internal carotid, and probably through ophthalmic nerve and its lacrimal branch.

Salivary Glands (Fig. 1003).— Submandibular.— Preganglionic from upper thoracic nerves. Cell-station , superior cervical ganglion. Postganglionic along external carotid and facial, fibres passing unchanged through submandibular ganglion.

Parotid. —Preganglionic and cell-station as last.

Postganglionic along external carotid and maxillary arteries, through otic ganglion unchanged, into auriculo-temporal nerve.





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Sweating.—The preganglionic fibres leave the spinal cord at varying levels according (Fig. 1001) to their final destinations. The cellstations are in the sympathetic ganglia appropriate to the levels to be supplied. The postganglionic fibres, arising in these ganglia, are distributed through the cutaneous nerves; they reach these usually through grey rami communicantes and the nerves concerned, but in the case of the head and face they reach the cutaneous nerves by passing first through the arterial plexuses.

The mammary gland, being a modified sweat-gland, is supplied in a similar way, the postganglionic fibres reaching the gland along the fourth, fifth, and sixth intercostal nerves.

The cardiac sympathetic preganglionic fibres leave the cord by the upper four or five thoracic nerves, and end in the corresponding thoracic ganglia; these are therefore the cell-stations. Postganglionic


Fig. 1003.—To show the Paths of Sympathetic and Parasympathetic Fibres to Salivary Glands (Colour Plans as Last Figure).

FA, M, facial and maxillary arteries; CH, chorda tympani; AT, auriculo-temporal; O, otic ganglion; G, superior cervical ganglion; T, tympanic plexus.

fibres arise from the cells in these ganglia, pass upwards through the cervical chain to come off as cervical cardiac branches, or run more directly from the thoracic ganglia to the deep cardiac plexus. All these fibres pass through the plexus without interruption to reach the heart.

It may be added, although it does not come under this heading, that afferent fibres from the heart probably run in all these cardiac sympathetic nerves except the superior cervical branch.

It follows from this description of the course of the sympathetic cardiac fibres that the cell-collections which form the scattered cardiac ganglia are cell-stations for the parasympathetic only (see later).

The splanchnic nerves, which arise from the lower six ganglia of the thoracic gangliated cord, carry mainly medullated preganglionic fibres which have left the spinal cord through the lower seven or eight thoracic nerves. Their cell-stations are in the coeliac ganglion.



THE NERVOUS SYSTEM 1639

The postganglionic fibres arise here, and are distributed in the branches of the coeliac plexus.

Many afferent fibres run up through the plexus into the splanchnic nerves.

The bladder is supplied by sympathetic fibres which arise in the upper lumbar segments. Some of the preganglionic fibres appear to end in lumbar ganglia, but most of them go into the aortic plexus or lie lateral to this, cross the bifurcation of the aorta and the common iliac vessels, pass through their own side of the hypogastric plexus (presacral nerve), where some of them terminate, while others pass on into the vesical part of the pelvic plexus. The cell-stations are therefore in the lumbar ganglia, in the hypogastric plexus, and in the vesical plexus, and from these cells the postganglionic fibres run, in the course indicated, to the bladder wall.

The Parasympathetic System.

This system, connected in distribution and in some structural points with the sympathetic, is distinguished from it by separate origin from the central axis, and by different and largely opposed function. The fibres come from the central axis at its extremities, while the sympathetic has a limited output in its middle part; in both cases the fibres emerge among the ordinary fibres of an efferent nerve, but whereas they leave this almost at once (white rami) in the case of the sympathetic, they run to their distribution (for a considerable distance usually) in the parasympathetic in the nerve within which they emerge.

The cranial parasympathetic is comprised in an outflow of special fibres running in the third , seventh , ninth , and tenth nerves.

The third or oculo-motor nerve has fibres which run to the ciliary muscle and pupillary sphincter, being concerned in contraction of the pupil and accommodation. The preganglionic fibres, arising in the mid-brain, probably from the Edinger-Westphal nucleus, pass out in the third nerve into its inferior division, to reach their terminations in the ciliary ganglion' this is therefore the cell-station. Postganglionic fibres arise in the ganglion and pass forward in the short ciliary nerves.

The seventh or facial nerve contains parasympathetic fibres which reach the lacrimal, submandibular, and sublingual glands (Figs. 1002

and 1003). *

Those to the lacrimal gland arise from nuclear material (? upper

salivatory nucleus) in the reticular formation of the pons, run in the issuing nerve, and leave it by the greater superficial petrosal branch, they pass in this to the pterygoid canal, and so to the spheno-palatine ganglion * this is therefore the cell-station. Postganglionic fibres, arising here, pass into the maxillary nerve, enter its zygomatico-temporal offset, and run through the junction between this nerve and the lacrimal

into this last-named nerve.

Those to the submandibular and sublingual glands come as pre 0


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A MANUAL OF ANATOMY


ganglionic fibres from the upper salivatory nucleus, pass into the facial nerve, and leave it in the chorda tympani. The cell-station comprises the submandibular ganglion (for sublingual) and ganglion cells embedded in the submandibular gland (for this gland). The postganglionic fibres are short, arising from these ganglia.

The ninth or glossopharyngeal carries preganglionic fibres (which have arisen from the lower salivatory nucleus) to end in the otic ganglion; they reach this ganglion by passing through the tympanic branch and tympanic plexus, then through the lesser superficial petrosal nerve. The cell-station is in the otic ganglion. Postganglionic fibres run in the auriculo-temporal nerve.

The tenth or vagus nerve has a large number of visceral branches which belong to the parasympathetic system. The preganglionic fibres arise mainly (apparently) from the dorsal nucleus of the vagus, run in the nerve, and leave it by various visceral branches. They end in [a) microscopic ganglia in the viscera, or (b) more apparent but scattered ganglia, as in the cardiac plexus.

The sacral parasympathetic outflow takes place in the second and third sacral nerves; the fourth nerve apparently contributes sometimes to the outflow. Visceral preganglionic fibres issue with these nerves as the pelvic splanchnic nerves, and pass through the pelvic plexuses. The cell-stations are generally in the visceral walls of the pelvic viscera, but may lie in minute ganglia in the plexuses. Postganglionic fibres are short, arising from these cells.


Development of the Sympathetic Nervous System.

The sympathetic ganglia may be regarded as being developed from the ventral aspects of the neural crests and spinal ganglia, and they are therefore of ectodermic origin, according to this view. Certain cells become detached from the neural crests and spinal ganglia, and migrate ventralwards towards the region of the aorta, where they form the ganglionic sympathetic chain. The ganglionic cells proliferate, and are furnished with processes which become fibrillar. These fibrillar processes give rise to the chain which connects the ganglia, and also to the grey rami communicantes and the various visceral branches.



Fig. 1004.—Development of Sympathetic Ganglia (Schematic).