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Embryology
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=Chapter XV The Nervous System=
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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 nervous system is arranged in two main divisions, cerebro-spina and autonomic; this last contains the sympathetic and the parasym pathetic.
  
Line 26: Line 16:
 
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 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==
  
THE SPINAL CORD.
+
Membranes of the Spinal Cord. — The membranes are three: th dura mater, the arachnoid membrane, and the pia mater.
 
 
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
 
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
+
Fig. 859. — The Cerebrospinal and Sympathetic Systems (Anterior View) (Hirschfeld and Leveille).
  
  
1411
 
  
 +
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.
  
Frontal Lobe
+
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.
  
Temporal Lobe
+
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.
  
/. Basilar Artery Vertebral Arteries
+
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.
  
[■six
+
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.
  
j -— Superior Cervical Sympathetic / Ganglion
+
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
  
1>. Cervical Plexus
 
  
■j -Vertebral Artery
 
  
i-Middle Cervical Ganglion
 
  
Brachial Plexus ist Thoracic Ganglion
+
Dura Mater (Theca)
  
  
4th Intercostal Nerve Anterior Spinal Artery
+
—-Arachnoid
  
  
U_ Thoracic Sympathetic Cord
+
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
  
. Spinal Ganglion
+
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
  
Subcostal Nerve
+
"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.
  
L-. Lumbar Sympathetic Cord
+
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
  
  
Lumbar Plexus
 
  
 +
Ligamentum Denticulatum
  
Pelvic Sympathetic Cord
+
-Anterior Nerve-Root
  
 +
Posterior Nerve-Root Pia Mater
  
^— Sacral Plexus
 
  
 +
_ Spinal Cord
  
Ganglion Impar
 
  
 +
Anterior Nerve-Root (cut)
  
Eig. 859.—The Cerebrospinal and Sympathetic Systems (Anterior
 
  
View) (Hirschfeld and Leveille).
+
e subarachnoid space communicate freely with each other.
  
 +
The subarachnoid space has no communication with the subdura ace.
  
  
  
 +
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 continuous 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.
  
  
  
  
 +
Fig. 86i.—A Portion of the Spinal Cord (Anterior View). The theca has been laid open, and the arachnoid membrane removed.
  
  
  
  
 +
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 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 irtially divides that space into two compartments—ventral and orsal
  
  
  
  
 +
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
  
  
  
 +
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. ,. ,
  
1412
+
Structure— Each ligamentum denticulatum consists of fibrous tissue, which
  
 +
continuous with the outer layer of the pia mater.
  
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
  
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.
+
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 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.
+
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.
  
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 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 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 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
  
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
+
Fig. 863. —Lumbar and Sacral Portions of Spinal Canal, showing Lateral View of Conus Medullaris, Filum Terminale, and Theca (Testut).
  
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
 
  
 +
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.
  
THE NERVOUS SYSTEM
+
[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.
  
T 4 X 3
+
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
  
Dura Mater (Theca)
+
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 ~
  
-Arachnoid
+
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 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.
  
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
+
_ Linea Splendens
  
id, and its dorsal part is
 
  
ntinuous superiorly with Fig - 86 °- Portion of the Spinal Cord,
+
Cauda Equina Conus Medullaris
  
- rerebello rnednllarv cis- showing the Membranes, Ligamenta o LEiEDEiio-iiiEuuiiaiy Lis Denticulata, and Roots of the Spinal
 
  
"na of the cranial sub- Nerves.
+
If
  
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
+
_Filum Terminale
  
  
 +
The posterior septum is marked on the surface by a median sulcus.
  
Ligamentum Denticulatum
+
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.
  
-Anterior Nerve-Root
+
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.
  
Posterior Nerve-Root Pia Mater
+
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 and 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
  
_ Spinal Cord
+
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
  
  
Anterior Nerve-Root (cut)
+
Fig. 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.
  
e subarachnoid space communicate freely with each other.
+
1, 1. Anterior Median Fissure 5 > 5 - Spinal Ganglion
  
The subarachnoid space has no communication with the subdura ace.
+
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,
  
  
  
  
 +
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.
  
  
1414
+
Posterior Surface.
  
 +
1. Ganglion on each posterior nerve root.
  
A MANUAL OF ANATOMY
+
2. Arterial anastomotic chain behind and
  
 +
in front of the posterior nerve-roots.
  
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.
+
3. Fasciculi of posterior nerve-roots lie
  
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.
+
in a straight line, and enter through postero-lateral sulcus.
  
 +
4. Presence of a posterior median sulcus
  
Spinal Pia Mater.—This is the deepest membrane of the spins cord. It is definitely fibrous and very vascular, and closely invest
+
which cannot be opened.
  
the cord. Superiorly it is con
+
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.
  
A] Dura Mater (Theca)
+
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 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 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
  
. Linea Splendens
 
  
  
Ligamentum Denticulatuir.
+
?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).
  
  
Pia Mater
 
  
 +
two cornua is called the body. ' ,
  
J_L \nterior Nerve-Root
+
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
  
Fig. 86i.—A Portion of the Spinal Cord (Anterior View).
+
ornu with the grey commissure. This projection is known as the 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 theca has been laid open, and the arachnoid membrane removed.
+
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.
  
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.
+
The horns of the crescents of grey matter vary in shape, as seen on section, in different regions. In the cervical region the anterior 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.
  
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
 
  
 +
Fig. 867. —Transverse Sections of the Spinal Cord in Different Regions.
  
prolonged into the cord, which carry with them portions of the glia sheath.
+
A, cervical region; B, mid-thoracic region; C, lumbar region; D, conus medullaris.
  
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
+
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 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.
  
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 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.
  
THE NERVOUS SYSTEM
+
Postero - intermediate Sulcus and Septum of pia mater, well marked.
  
  
1415
+
Thoracic Region.
  
 +
1. Circular.
  
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.
+
2. Anterior and Posterior
  
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.
+
Horns, both narrow, posterior more so than anterior.
  
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
+
3. Formatio Reticularis,
  
 +
not very distinct.
  
Dura Mater
+
4. Lateral Horn, con spicuous, especially in upper part
  
 +
5. White Matter, less in
  
 +
amount, but large in proportion to Grey Matter.
  
Fig. 862._Diagram of a Transverse Section of the Spinal Cord
+
6. Central Canal, nearer
  
and its Membranes.
+
the ventral than the dorsal surface.
  
 +
7. Postero - intermediate
  
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
+
Sulcus, absent, but Septum of pia mater recognizable.
  
  
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
+
Lumbar Region.
  
 +
1. Almost circular.
  
irtially divides that space into two compartments—ventral and orsal
+
2. Anterior and Posterior
  
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
+
Horns, both broad, anterior more so than posterior.
  
iygl
+
3. Formatio Reticularis,
  
The two ligamenta denticulata act as lateral supports to the spinal °rd. ,. ,
+
absent.
  
Structure— Each ligamentum denticulatum consists of fibrous tissue, which
+
4. Lateral Horn, merged
  
continuous with the outer layer of the pia mater.
+
into anterior.
  
  
 +
5. White Matter, small
  
 +
in amount, and Grey Matter, large.
  
 +
6. Central Canal, in the
  
 +
centre.
  
  
1416 A MANUAL OF ANATOMY
+
7. Postero - intermediate Sulcus and Septum of pia mater, absent.
  
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.
+
Minute Structure of the Spinal Cord—Grey Matter.—The grey matter consists of nerve-cells, nerve-fibres, and neuroglia, and is very vascular.
  
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 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 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 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. 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).
  
Fig. 863. —Lumbar and Sacral Portions of Spinal Canal, showing Lateral View of Conus Medullaris, Filum Terminale, and Theca (Testut).
 
  
 +
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
  
  
  
 +
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 NERVOUS SYSTEM
+
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.
  
1417
+
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.
  
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.
+
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
  
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.
+
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.
  
[t is about 10 inches in length. As low as the back of the body of
+
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
  
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.
+
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.
  
Its lower part is purely fibrous. Fig . 864 ._the terminal
+
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.
  
Cauda EQuina.—This is situated witmn Part of the Spinal Cord,
+
Summary of the Gelatinous Substances of the Grey Matter. — These are
  
the lower part of the theca. It consists and the Cauda Equina of the roots of the lumbar, sacral, and ~
+
(1) The substantia gelatinosa, which forms a cap for the caput cornu posterioris :
  
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 .
+
(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.
  
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
+
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 ventral horn of grey matter, after which each fibre acquires its imitive sheath or neurilemma.
  
_ Linea Splendens
+
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
  
Cauda Equina Conus Medullaris
 
  
  
If
 
  
 +
' IG . 869.— Course of Nerve-Fibres in the Spinal Cord (from Halliburton’s
  
_Filum Terminale
+
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
  
  
 +
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<
  
1418
 
  
  
A MANUAL OF ANATOMY
+
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.
  
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.
+
Tracts of the Spinal Cord.
  
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.
+
Posterior Column. —The tracts of this column are ascending and xending, and they are as follows:
  
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.
+
Ascending Tracts. Descending Tracts
  
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.
+
Fasciculus gracilis (Fig. 871, 1). Semilunar (comma) tract (Fig. 871, 9). Fasc. cuneatus (Fig. 871, 2). Septo-marginal bundle (Fig. 871, 10).
  
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
+
Postero-lateral tract (of Lissauer)
  
 +
(Fig. 871, 3).
  
THE NERVOUS SYSTEM
+
Lissauer’s tract also belongs to the lateral column.
  
  
1419
 
  
 +
FIG . 871.— The Tracts of the Spinal Cord.
  
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
+
Ascending.
  
ura mater.
+
Fasciculus gracilis (Goll’s column).
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
+
. Fasciculus cuneatus (Burdach’s column).
 +
Fasciculus postero-lateralis (Lissauer’s tract). _
  
2
+
|.. Posterior spino-cerebellar fasciculus (Flechsig’s tract).
  
 +
5. Anterior spino-cerebellar fasciculus (Gower’s tract).
  
 +
5 . Lateral spino-thalamic tract.
  
?IG. 865.—Two Segments of the Spinal Cord, showing the Attachments of the Anterior and Posterior Nerve-Roots, and the Spinal Ganglia.
+
7. Anterior spino-thalamic tract.
  
A, superior view; B, anterior view.
+
1 . Intersegmental tract (fasciculus proprius: ground bundle).
  
1, 1. Anterior Median Fissure 5 > 5 - Spinal Ganglion
 
  
2, 2. Posterior Median Sulcus 6, 6. Spinal Nerve
+
Descending.
  
a, a. Anterior or Motor Nerve-Root 7 , 7 - Anterior Primary Ramus
+
9. Semilunar (or comma) tract.
  
4, 4. Posterior or Sensory Nerve-Root 8, 8. Posterior Primary Ramus
+
10. Septo-marginal bundle.
  
9, 9. Medial and Lateral Branches of Posterior Primary Ramus
+
n. Lateral cerebro-spinal fasciculus (crossed pyramidal tract).
  
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.
+
12. Anterior cerebro-spinal fasciculus (direct pyra midal tract).
  
Each spinal nerve breaks up into an anterior and a posterior primary yamus
+
13. Lateral intersegmental tract.
  
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,
+
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.
  
  
  
 +
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).
  
1420
+
3. Intersegmental tract (Fig. 871, 13 and
  
 +
14).
  
A MANUAL OF ANATOMY
+
4. Rubrospinal tract (Fig. 871, 16).
  
 +
5. Vestibulo-spinal tract (Fig. 871, 19).
  
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.
+
6. Olivo-spinal tract (Fig. 871, 18).
  
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.
+
7. Tecto-spinal tract (Fig. 871, 17).
  
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.)
 
  
 +
Ascending Tracts.
  
Mode of distinguishing the Anterior and Posterior Surfaces of the Spinal Cord.—These surfaces may be recognized by attending to the following points:
+
1. Dorsal spino-cerebellar tract
  
 +
(Fig. 871, 4).
  
Anterior Surface.
+
2. Ventral spino-cerebellar tract
  
1. Linea splendens in median line,
+
(Fig. 871, 5).
  
especially in lower part.
+
3. Anterior spino-thalamic tract
  
2. Anterior spinal artery in
+
(Fig. 871, 7).
  
median line.
+
4. Lateral spino-thalamic tract
  
3. Fasciculi of anterior nerve roots spread over a wide area.
+
(Fig. 871, 6).
  
4. Presence of an anterior median
+
5. Spino-tectal tract (Fig. 871, 6).
  
sulcus which can be opened.
 
  
 +
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
  
Posterior Surface.
+
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
  
1. Ganglion on each posterior nerve root.
+
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.
  
2. Arterial anastomotic chain behind and
+
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.
  
in front of the posterior 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.
  
3. Fasciculi of posterior nerve-roots lie
+
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.
  
in a straight line, and enter through postero-lateral sulcus.
+
The vestibulo-spinal tract (fasciculus vestibulo-spinalis, Fig. 871,
  
4. Presence of a posterior median sulcus
 
  
which cannot be opened.
 
  
5. Presence of gracile and cuneate fasci culi in upper part.
+
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.
  
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.
+
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.
  
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 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 NERVOUS SYSTEM
+
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 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
  
1421
+
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.
  
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,
+
The tracts of the antero-lateral column may be otherwise arranged s follows:
  
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
 
  
 +
Ventral Column.
  
Central Canal
+
nterior cerebro-spinal tract (descending). (12).
  
 +
nterior intersegmental (descending)
  
 +
( 1 4 ) - . ;
  
?i G . 866._ Transverse Section of the Spinal Cord in the Upper Thoracic
+
ulco-marginal tract (descending)
  
Region, showing the Arrangement of the Grey Matter and Cells (Semi-diagrammatic) (after Poirier).
+
( 15 ) estibulo-spinal tract (descending)
  
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
+
( T 9);
  
two cornua is called the body. ' ,
+
nterior spino-thalamic tract (ascending) ( 7 ).
  
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
+
Lateral Column.
  
ornu with the grey commissure. This projection is known as the
+
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.
  
1422
+
The fibres of the entire antero-lateral ground-bundle are association or ngitudinal commissural fibres, which serve to connect the grey matter of suecessive 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.
  
A MANUAL OF ANATOMY
+
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.
  
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 anterior spinal branches of the vertebral arteries are seldom of equa size, and often only one is present.
  
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 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.
  
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.
+
It is only under very favourable conditions that all these arteries are injected equally.
  
The horns of the crescents of grey matter vary in shape, as seen on section, in different regions. In the cervical region the anterior
+
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:
  
  
A
 
  
 +
) 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.
  
Fig. 867. —Transverse Sections of the Spinal Cord in Different
+
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 foramina on the posterior surface of each vertebral body, open ito the transverse communicating branch between the two anterior •ngitudinal intraspinal veins.
  
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.
+
Fig. 872. — Schematic Sections to show Positions of Longitudinal Arterial and Venous Channels.
  
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 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
  
  
 +
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 t0 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 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 NERVOUS SYSTEM
+
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:
  
i 4 2 3
+
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.
  
sterior, but in the lumbar region it occupies the centre. In the aus medullaris it is near the posterior surface.
+
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
  
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.
+
Fig. 874. —Section across Cord, Semi-diagrammatic, about End of First Month. (Explanation in text.)
  
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
+
at the dorsal portion of the original cavity is actively obliterated by fusion
  
seen in Transverse Sections.
+
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.
  
Cervical Region.
+
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.
  
Transversely oval.
+
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.
  
Anterior Horn, short, broad, and blunt.
+
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.
  
Posterior Horn, long, narrow, and tapering.
 
  
Formatio Reticularis, well marked.
 
  
Lateral Horn, merged into anterior, except above fourth cervical vertebra.
+
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
  
White Matter, large in amount.
+
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.
  
Central Canal, nearer the ventral than the dorsal surface.
+
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.
  
Postero - intermediate Sulcus and Septum of pia mater, well marked.
+
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.
  
Thoracic Region.
+
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.
  
1. Circular.
+
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 remains 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.
  
2. Anterior and Posterior
+
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
  
Horns, both narrow, posterior more so than anterior.
 
  
3. Formatio Reticularis,
 
  
not very distinct.
+
Fig. 876. —Median Sagittal Sections of Ends of Cords in Sixth and Ninth Weeks.
  
4. Lateral Horn, con spicuous, especially in upper part
+
Ependymal tissue shown in black. Description in text.
  
5. White Matter, less in
 
  
amount, but large in proportion to Grey Matter.
+
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
  
6. Central Canal, nearer
+
le terminal ventricle, as in the younger stage.
  
the ventral than the dorsal surface.
+
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
  
7. Postero - intermediate
+
fetched nerve-tissue having disappeared.
  
Sulcus, absent, but Septum of pia mater recognizable.
+
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.
  
  
Lumbar Region.
+
Membranes of Spinal Cord. —The membranes—namely, pia mater, arachnoid and dura mater (theca)—are developed from the mesoderm which invests th neural tube.
  
1. Almost circular.
+
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.
  
2. Anterior and Posterior
+
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
  
Horns, both broad, anterior more so than posterior.
 
  
3. Formatio Reticularis,
 
  
absent.
+
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.
  
4. Lateral Horn, merged
+
The vertebral levels of the spinal nerves are indicated.
  
into anterior.
+
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.
  
  
5. White Matter, small
+
==The Encephalon==
  
in amount, and Grey Matter, large.
 
  
6. Central Canal, in the
+
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.
  
centre.
 
  
 +
Encephalon =
  
7. Postero - intermediate Sulcus and Septum of pia mater, absent.
+
Prosencephalon or Fore-brain. Mesencephalon or Mid-brain. Rhombencephalon or Hind-brain.
  
  
I 4 2 4
+
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.
  
  
A MANUAL OF ANATOMY
+
Fore-brain or Prosencephalon = { ^alamencephalon or Diencephalon. Mid-brain or Mesencephalon = Mesencephalon.
  
 +
Hind-brain or Rhombencephalon = { Mydenceplulon.
  
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
+
rhe various parts of the encephalon which are developed from these ubdivisions will be made evident from the following table:
  
Central Canal
 
  
 +
Telencephalon
  
  
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).
+
Thalamencephalon
  
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.
+
or
  
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
+
Diencephalon
  
 +
Mesencephalon
  
 +
Metencephalon
  
 +
Myelencephalon
  
  
  
  
 +
Cerebral Hemispheres.
  
 +
Lateral Ventricles.
  
 +
Anterior Part of Third Ventricle. Interventricular Foramina.
  
 +
Olfactory Lobes.
  
THE NERVOUS SYSTEM
+
Posterior Part of Third Ventricle.
  
 +
Optic Thalami and Corpora Geniculata. Pineal Body.
  
 +
Interpeduncular Structures.
  
4 25
+
Pituitary Body.
  
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.
+
Optic Nerve and Retina.
  
The intermedio-lateral cell-column is situated, as stated, in the teral horn of grey matter, and the cells constitute a column known
+
Corpora Quadrigemina.
  
the intermedio-lateral nucleus.
+
Crura Cerebri.
The thoracic cell-column is situated in the medial part of the
 
  
rvix of the dorsal cornu of grey matter. This extends throughout
+
Aqueduct (of Sylvius).
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.
+
Cerebellum.
  
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.
+
Pons (Varolii).
  
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.
+
Pontine Part of the Fourth Ventricle.
  
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
+
( Medulla Oblongata (or Bulb).
  
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.
+
\ Bulbar Part of Fourth Ventricle.
  
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
+
General Description of the Base and Superior Surface of the
  
 +
Encephalon.
  
1426
+
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
  
A MANUAL OF ANATOMY
+
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 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.
  
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.
+
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
  
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 :
+
Fig. 878. —The Base of the Encephalon, and the Cranial Nerves. 1, frontal lobe (orbital surface); 2, temporal lobe; 3, cerebellum.
  
(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.
+
occupied by the basilar artery. On either side the pons becomes th<
  
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.
+
middle peduncle of the cerebellum, passing outwards and backwards into the cerebellar hemisphere.
  
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 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 NERVOUS SYSTEM
+
The stem of the lateral sulcus lies immediately in front of the temporal 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.
  
1427
+
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 ventral horn of grey matter, after which each fibre acquires its imitive sheath or neurilemma.
+
The optic commissure, or chiasma, connects the two optic tracts fter their convergence.
  
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.
+
The optic nerves, right and left, leave the front of the commissure, nd pass forwards and outwards to the optic foramina.
  
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
+
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.
  
' IG . 869.— Course of Nerve-Fibres in the Spinal Cord (from Halliburton’s
+
The tuber cinereum extends from the mammillary bodies to the ptic commissure, and is composed of grey matter. The infundibulum
  
Handbook of Physiology ’ (after Schafer).
+
connected with the tuber cinereum close behind the optic comlissure, and passes downwards to the posterior part of the pituitary
 +
ody.
  
P. Cerebro-spinal Tract 1, 2, 3, 4. Anterior Cornual Cells K, A, A, A. Axons of Anterior Cornual Cells M. Muscular Fibre
+
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 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.
  
G. Unipolar Cell of a Spinal Ganglion, giving Origin to a Fibre of a Posterior NerveRoot
+
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.
  
B. Peripheral Branch of Fibre S. Skin . .
+
Superficial Origins of the Cranial Nerves.
  
(lowerC) Central Branch of Fibre, passing into the Spinal Cord
+
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.
  
. Descending Branch of Fibre in the Spinal Cord
+
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.
  
D. Ascending Branch of Fibre in the Spinal Cord Pi, P2. Posterior Cornual Cells
+
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.
  
C (upper C). Cell of Clarke’s Column or Thoracic
+
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.
  
Nucleus .
+
The sixth or abducent nerve appears at the lower border of the pons ust lateral to the pyramid of the medulla oblongata.
  
5. Collateral, passing directly to arborize around an
+
The seventh or facial nerve emerges at the lower border of the pons n front of the restiform body of the medulla oblongata.
  
Anterior Cornual Cell (2). >
+
The eighth or auditory nerve likewise appears at the lower border of
  
6. Collateral, with an Intermediate Cell-Station in a
+
he pons in front of the restiform body of the medulla oblongata,
 +
[t lies on the outer side of the facial nerve.
  
Posterior Cornual Cell (P 2 ).
+
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.
  
7. Collateral, arborizing around a Cell ot Clarke s
+
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.
  
Column (upper C). ,
+
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.
  
8. Continuation of Main Ascending Branch of Fibre.
+
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 dorso-lateral sulcus between the dorsal and lateral coiumns of ilte matter. The other half of the horizontal limb is peripheral, and
+
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.
  
sses outwards in the course of the nerve. • ol K 11 n f
+
Arteries at the Base of the Encephalon. —The arteries which supply the brain are the two vertebral and the two internal carotid arteries.
  
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
+
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.
  
y matter. Within this column the fibres divide into ^ branches
+
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 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
lending and descending. The descending branches, after a short
 
  
  
  
 +
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
  
 +
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.
  
1428
+
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.
  
A MANUAL OF ANATOMY
+
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.
  
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.
+
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.
  
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
+
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
  
come into close relation with the corresponding nerve-cells_ e.g., the
+
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 circulation 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.
  
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.
 
  
 +
1. Internal Carotid
  
Sensory Fibres entering Fasciculus Cuneatus (Burdach) Cells of Posterior Column and Thoracic Nucleus ' Posterior Ro<
+
2. Middle Cerebral
  
 +
3. Anterior Cerebral
  
 +
4. Anterior Communicating
  
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.
+
5. Posterior Communicating
  
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.
+
6 . Posterior Cerebral
  
 +
7. Basilar
  
 +
8 . Superior Cerebellar
  
 +
9. Transverse Pontine xo. Internal Auditory xr. Anterior Inferior Cerebellar
  
 +
12. Posterior Inferior Cerebellar
  
 +
13. Vertebral
  
THE NERVOUS SYSTEM
+
14. Anterior Spinal
  
 +
15. Posterior Spinal
  
1429
+
16. Anterior Choroid
  
 +
17. Posterior Choroid
  
Tracts of the Spinal Cord.
+
18. Cential or Ganglionic
  
Posterior Column. —The tracts of this column are ascending and xending, and they are as follows:
+
19. Central or Ganglionic
  
Ascending Tracts. Descending Tracts
+
20. Central or Ganglionic (Postero-mesial)
  
Fasciculus gracilis (Fig. 871, 1). Semilunar (comma) tract (Fig. 871, 9). Fasc. cuneatus (Fig. 871, 2). Septo-marginal bundle (Fig. 871, 10).
+
‘21. Central or Ganglionic (Postero-lateral)
  
Postero-lateral tract (of Lissauer)
+
Fig. 880. — The Arteries at the Base of the Brain, and the Circulus Arteriosus.
  
(Fig. 871, 3).
 
  
Lissauer’s tract also belongs to the lateral column.
 
  
  
 +
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 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.
  
19
 
  
Descending
 
  
  
Ascending.
+
Fig. 88i. — The Cerebral Hemispheres (Superior View). Fissure of Rolando—central fissure.
  
  
  
F IG . 871.— The Tracts of the Spinal Cord.
 
  
Ascending.
 
  
Fasciculus gracilis (Goll’s column).
+
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.
  
. Fasciculus cuneatus (Burdach’s column).
+
==Rhombencephalon==
Fasciculus postero-lateralis (Lissauer’s tract). _
 
  
|.. Posterior spino-cerebellar fasciculus (Flechsig’s tract).
+
===1. The Medulla Oblongata===
  
5. Anterior spino-cerebellar fasciculus (Gower’s tract).
+
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.
  
5 . Lateral spino-thalamic tract.
+
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.
  
7. Anterior spino-thalamic tract.
+
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 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.
  
1 . Intersegmental tract (fasciculus proprius: ground bundle).
+
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.
  
  
Descending.
 
  
9. Semilunar (or comma) tract.
 
  
10. Septo-marginal bundle.
+
Fig. 882. — The Medulla Oblongata, Pons, and Interpeduncular Region. C.C., crus cerebri; P., pyramid; O.B., olivary body.
  
n. Lateral cerebro-spinal fasciculus (crossed pyramidal tract).
 
  
12. Anterior cerebro-spinal fasciculus (direct pyra midal tract).
+
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.
  
13. Lateral intersegmental tract.
+
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.
  
14. Anterior intersegmental tract.
 
  
15. Sulco-marginal tract.
 
  
16. Rubro-spinal tract (Monakow’s bundle).
+
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.
  
17. Tecto-spinal tract.
+
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.
  
18. Olivo-spinal tract (Helweg’s 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.
  
19. Vestibulo-spinal tract.
+
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.
  
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
+
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 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.
  
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.
 
  
 +
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 closed and open part of the lower and upper.
  
  
  
 +
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.
  
  
  
  
 +
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 of 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.
  
1430
+
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.
  
A MANUAL OF ANATOMY
+
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 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
  
  
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.
+
Fig. 884. — Posterior View of Medulla. G, C, gracile and cuneate tubercles ; g, c, corresponding tracts; F, gelatinous tubercle; O, obex.
  
Antero-lateral Column,—1 he tracts of this column are arranged into descending and ascending, and are as follows:
 
  
  
Descending Tracts.
+
inferior cerebellar peduncle.
  
1. Lateral cerebro-spinal (crossed pyra midal) tract (Fig. 871, 11).
+
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
  
2. Anterior cerebro-spinal (direct pyra midal) tract (Fig. 871, 12).
 
  
3. Intersegmental tract (Fig. 871, 13 and
 
  
14).
+
Fig. 885. — Section through Medulla just above Decussation of Pyramids: Shows the Prominence of Spinal Tract of Fifth Nerve.
  
4. Rubrospinal tract (Fig. 871, 16).
+
F is the dorsal spino-cerebellar tract immediately ventral to this, and G is the ventral tract.
  
5. Vestibulo-spinal tract (Fig. 871, 19).
+
clear that it constitutes the great tract of connection between the cerebellar hemisphere, the bulb, and the spinal cord.
  
6. Olivo-spinal tract (Fig. 871, 18).
+
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.
  
7. Tecto-spinal tract (Fig. 871, 17).
+
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.
  
Ascending Tracts.
 
  
1. Dorsal spino-cerebellar tract
 
  
(Fig. 871, 4).
+
As compared with the grey matter of the spinal cord, it presents mportant modifications, and its component parts are as follows:
  
2. Ventral spino-cerebellar tract
+
1. Substantia or formatio reticularis.
  
(Fig. 871, 5).
+
2. A thick layer of grey matter around the central canal in the
  
3. Anterior spino-thalamic tract
+
lower or closed part of the bulb.
  
(Fig. 871, 7).
 
  
4. Lateral spino-thalamic tract
 
  
(Fig. 871, 6).
+
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 ’).
  
5. Spino-tectal tract (Fig. 871, 6).
 
  
 +
a. Pons
  
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
+
b. Medulla Oblongata (anterior aspect)
  
 +
c. Decussation of the Pyramids
  
THE NERVOUS SYSTEM
+
d. Section of the Cervical Spinal Cord
  
 +
1. Anterior Cerebro-spinal Tract
  
i43i
+
2. Lateral Cerebro-spinal Tract
  
 +
3. Sensory Tract
  
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
+
3'. Nucleus Gracilis et Nucleus Cuneatus
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.
+
4. Antero-lateral Intersegmental Tract
  
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.
+
5. Anterior Pyramid
  
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.
+
6. Fillet or Lemniscus
  
The vestibulo-spinal tract (fasciculus vestibulo-spinalis, Fig. 871,
+
7. Posterior Longitudinal Bundle
  
 +
8. Ventral Cerebellar Tract
  
1432
+
9. Dorsal Cerebellar Tract
  
  
A MANUAL OF ANATOMY
+
3. A thick layer of grey matter over the floor of the fourth
  
 +
ventricle in the upper or open part of the bulb.
  
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.
+
4. Substantia gelatinosa (nucleus of spinal tract, N. V.).
  
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.
+
5. Nuclei of 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 modifications undergone by the grey matter of the bulb in ts lower or closed part are brought about by the decussation of the 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 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.
+
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.
  
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
+
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.
  
THE NERVOUS SYSTEM
+
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
  
1433
+
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.
  
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
+
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.
  
sions of the bulb, involving both the fillet-fibres and the spino-thalamic
+
Substantia Gelatinosa (Fig. 885).—This caps the detached and splaced caput of the dorsal horn of grey matter. Having increased
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.
+
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.
  
The tracts of the antero-lateral column may be otherwise arranged s follows:
+
Nuclei of Grey Matter.— The nuclei, which will be considered in is place, are as follows:
  
 +
1. Nucleus gracilis.
  
Ventral Column.
+
2. Nucleus cuneatus.
  
nterior cerebro-spinal tract (descending). (12).
+
3. Olivary nuclei.
  
nterior intersegmental (descending)
+
4. Arcuate nucleus.
  
( 1 4 ) - . ;
+
5. Nucleus lateralis.
  
ulco-marginal tract (descending)
+
The nucleus gracilis is a collection of grey matter within the funiclis gracilis. For the most part it is connected with the grey matter 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).
  
( 15 ) estibulo-spinal tract (descending)
+
The nucleus cuneatus is a collection of grey matter within th< funiculus cuneatus. It is a direct extension from the basal part 0
  
( T 9);
 
  
nterior spino-thalamic tract (ascending) ( 7 ).
 
  
 +
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 ').
  
Lateral Column.
 
  
Lateral cerebro-spinal tract (descending) (11).
+
1. Anterior Median Fissure
  
Rubro-spinal tract (descending) (16). Tecto-spinal tract (descending) (17). Olivo-spinal tract (descending) (18). Dorsal cerebellar tract (ascending)
+
2. Fourth Ventricle
  
( 4 )-.
+
3. Formatio Reticularis 3'. Reticularis Alba
  
Anterior spino - cerebellar tract (Gowers, ascending) (5).
+
3". Reticularis Grisea
  
Posterior spino-thalamic and spinotectal tracts (ascending) (6). Lateral intersegmental (descending) ( 13 )
+
4. Raphe
  
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.
+
5. Anterior Pyramid
  
 +
6. Lemniscus
  
Association Fibres of Antero-lateral Column—Intersegmental Fasciculi.— The
+
7. Inferior Olive with the two
  
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.
+
Accessory Nuclei
  
The anterior intersegmental group is situated in the ventral column in front
 
  
the ventral cornu of grey matter, and has been already described.
+
7'. Peduncle of Olivary Body
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.
+
8. Hypoglossal Nerve 8'. Hypoglossal Nucleus
  
The fibres of the entire antero-lateral ground-bundle are association or ngitudinal commissural fibres, which serve to connect the grey matter of sue
+
9. Vagus Nerve
  
 +
g'. Terminal Nucleus of Vagus Nerve
  
 +
10. External Dorsal Vestibular Nucleus
  
I 434
+
xx. Nucleus Ambiguus
  
 +
12. Nucleus Gracilis
  
A MANUAL OF ANATOMY
 
  
 +
13. Nucleus Cuneatus
  
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.
+
14. Caput of Posterior Cornu 14'. Lower Sensory Root of Fift
  
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.
+
Nerve
  
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).
+
15. Fasciculus Solitarius
  
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.
+
16. External Anterior Arcuat
  
The anterior spinal branches of the vertebral arteries are seldom of equa size, and often only one is present.
+
Fibres
  
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.
+
16'. Arcuate Nucleus
  
It is only under very favourable conditions that all these arteries are injected equally.
+
17. Lateral Nucleus
  
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 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.
  
THE NERVOUS SYSTEM
 
  
 +
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.
  
1435
 
  
 +
The olivary nuclei are associated with the olive, and are three in imber—inferior, and two accessory (medial and dorsal).
  
) posterior extraspinal, (2) posterior intraspinal, (3) veins of the irtebral bodies, (4) anterior intraspinal, (5) anterior extraspinal.
+
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 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
+
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.
  
Fig. 872. —Schematic Sections to show Positions of Longitudinal
 
  
Arterial and Venous Channels.
+
Nucl. Cun.
  
vo foramina on the posterior surface of each vertebral body, open ito the transverse communicating branch between the two anterior •ngitudinal intraspinal veins.
+
— Fasc. Solitar.
  
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.
+
Sp. Nucleus of Trigeminal Sp. Root Fibres of Trigeminal Med. Longit. Fasc. (Post. Longit. Bundle) Nucl. Ambiguus Lateral Nucleus
  
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
+
Ant. Sp. Cerebellar Fasc. (Gowers)
  
 +
Dorsal Acc. 01 .
  
  
 +
Fig. 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.
  
  
A MANUAL OF ANATOMY
+
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..
  
!436
+
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.
  
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.
+
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).
  
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.
+
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.
  
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.
+
The structure of the two accessory olivary nuclei corresponds to that of the chief or inferior olivary nucleus.
  
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.
+
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.
  
Development of the Spinal Cord.
+
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.
  
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.
+
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).
  
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.
+
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:
  
The ependymal zone consists of a single layer of elongated cells, connected
+
1. Pyramidal tract (cerebro-spinal tract).
  
 +
2. Dorsal spino-cerebellar tract (direct cerebellar tract).
  
THE NERVOUS SYSTEM
+
3. Ventral spino-cerebellar tract (tract of Gowers).
  
 +
4. Restiform body (inferior cerebellar peduncle).
  
1437
+
5. Funiculus cuneatus.
  
 +
6. Funiculus gracilis.
  
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.
+
7. Medial or posterior longitudinal bundle.
  
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
+
8. Tecto-spinal tract.
  
 +
g. Rubro-spinal tract.
  
 +
10. Spino-tectal tract.
  
j. 873.—Three Sections from Different Levels of Cord in Embryo
+
11. Superficial arcuate tract.
  
of 4-9 Mm.
+
12. Deep arcuate tract.
  
ft lower figure, under higher power, shows nerve-fibres leaving ventro-lateral
+
13. Fillet (lemniscus).
  
wall.
+
14. Vestibulo-spinal tract.
  
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.
+
15. Olivo-cerebellar tract.
  
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.
+
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 cord.
  
As just said, the white matter of the cord is made by nerve-fibres growing
+
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
  
1438
+
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.
  
A MANUAL OF ANATOMY
+
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
  
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
+
following sources:
  
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.
+
1. The olivo-cerebellar fibres of the inferior olivary nucleus of the opposite side
  
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.
+
2. The posterior cerebellar tract of the lateral column of the spinal cord of the same side.
  
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.
+
3. The anterior superficial arcuate fibres from the nucleus gracilis and nucleus cuneatus of the opposite side.
  
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.
+
4. The posterior arcuate fibres from the nucleus gracilis and nucleus cuneatus of the same side.
  
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:
+
5. Vestibular fibres from the vestibular nuclei of the vestibular division of the auditory nerve.
  
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.
+
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.
  
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.
+
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).
  
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
+
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.
  
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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.
  
Fig. 874. —Section across Cord, Semi-diagrammatic, about End of First Month.
+
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.
  
(Explanation in text.)
+
Arcuate Tracts. —These tracts form two goups—superficial and deep.
  
  
 +
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.
  
  
  
THE NERVOUS SYSTEM
+
Fig. 890. — Front Aspect of Pons and Medulla, showing Oblique Fibres of Pons and Arcuate Fibres on Medulla.
  
  
J 439
+
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 the inferior sensory or spino-thalamic decussation, which takes place in the spinal cord.
  
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 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.
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.
+
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):
  
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.
+
Pyramid.
  
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.
+
Fillet.
  
 +
Tecto-spinal.
  
 +
Posterior longitudinal bundle.
  
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
 
  
 +
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.
  
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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,
  
  
 +
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.
  
1440
+
The upper part of the cuneate nucleus (C).
  
 +
The descending root of the vestibular nerve ^ 'po be afterwards
  
A MANUAL OF ANATOMY
+
The fasciculus solitarius (S) \ described.
  
 +
The spinal root of the fifth cranial nerve (V) )
  
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.
+
The substantia gelatinosa (G).
  
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.
+
Course of Chief Nerve Funiculi of Spinal Cord through Medulla Oblongata. Spinal Cord. Medulla Oblongata.
  
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.
+
Posterior Column.
  
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.
+
Column of Goll (fasciculus gracilis).
  
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.
+
Column of Burdach (fasciculus cuneatus).
  
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
+
Lateral Column.
  
 +
(Crossed) lateral cerebro-spinal tract.
  
THE NERVOUS SYSTEM
+
Anterior cerebro-spinal tract.
  
 +
Dorsal (or direct) spino - cerebellar tract.
  
1441
+
Ventral (or indirect) spino-cerebellar (tract of Gowers).
  
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.
+
Prepyramidal or rubro-spinal tract.
  
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
+
Lateral intersegmental.
  
 +
Anterior Column.
  
toirqinal jone / (ye rtf. comm.)
+
Anterior cerebro-spinal tract.
  
 +
Tecto-spinal tract.
  
 +
Ventral intersegmental.
  
16 mm
+
Anterior marginal bundle (of Lowenthal).
  
 +
Development of Medulla Oblongata.— The bulb is developed from the myelencephalon, which is the caudal division of the rhombencephalon.
  
!f- -\coccyK
+
===2. The Pons===
  
cyst/ v
+
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
  
Fig. 876. —Median Sagittal Sections of Ends of Cords in Sixth and
+
gracilis.
  
Ninth Weeks.
+
Funiculus cuneatus and nucleus
  
Ependymal tissue shown in black. Description in text.
+
cuneatus.
  
  
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
+
Inner three - quarters of opposite pyramid.
  
le terminal ventricle, as in the younger stage.
+
Outer one-quarter of pyramid of same side.
  
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
+
Lateral area below olive, and inferior peduncle.
  
fetched nerve-tissue having disappeared.
+
Lateral area below olive, and formatio reticularis.
  
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.
 
  
 +
Outer one-quarter of pyramid of same side.
  
9i
+
Posterior longitudinal bundle.
  
  
 +
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.
  
1442
 
  
  
A MANUAL OF ANATOMY
+
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
  
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.
+
grey matter. , , ,, £ •
  
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
+
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 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
  
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.
 
  
 +
Fig. 893.—Dissection of the Pons, showing the Course of the Pyramidal Tracts of the Medulla Oblongata (Hirschfeld and Leveille).
  
THE ENCEPHALON.
+
P., right pyramid; O.B., right olivary body.
  
  
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.
+
cells of the nucleus pontis, all of the same side.
  
  
Encephalon =
+
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.
  
Prosencephalon or Fore-brain. Mesencephalon or Mid-brain. Rhombencephalon or Hind-brain.
+
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 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
  
  
THE NERVOUS SYSTEM
+
Fig 8q4—Vertical Transverse Section through the Upper Part of the Pons and Fourth Ventricle (from L. Testut’s ‘ Anatomie Humaine,’ after Stilling).
  
  
1443
+
1. Fourth Ventricle
  
 +
2. Superior Velum
  
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.
+
3. Superior Root of Fifth Nerve
  
 +
4. Nerve-cells which accompany this Root
  
Fore-brain or Prosencephalon = { ^alamencephalon or Diencephalon. Mid-brain or Mesencephalon = Mesencephalon.
+
5. Posterior Longitudinal Bundle
  
Hind-brain or Rhombencephalon = { Mydenceplulon.
+
6. Formatio Reticularis
  
 +
7. Lateral Fissure of Isthmus
  
rhe various parts of the encephalon which are developed from these ubdivisions will be made evident from the following table:
 
  
 +
8. Section of Superior Cerebellar Peduncle 9,9. Medial and Lateral Portions of the Lemniscus
  
Telencephalon
+
10, 10. Transverse Fibres of the Pons
  
 +
11, 11. Longitudinal Fibres of the Pons
  
Thalamencephalon
+
12. Raph6
  
or
+
V. Fifth Nerve
  
Diencephalon
 
  
Mesencephalon
 
  
Metencephalon
+
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
  
Myelencephalon
+
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 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:
  
Cerebral Hemispheres.
+
1. Spinal sensory root of the fifth cranial nerve.
  
Lateral Ventricles.
+
2. Motor nucleus of the facial nerve.
  
Anterior Part of Third Ventricle. Interventricular Foramina.
+
3. Superior olivary nucleus.
  
Olfactory Lobes.
+
4. Nucleus of the sixth cranial nerve.
  
Posterior Part of Third Ventricle.
+
5. Posterior longitudinal bundle.
  
Optic Thalami and Corpora Geniculata. Pineal Body.
+
6. Rubro-spinal tract.
  
Interpeduncular Structures.
+
7. Tecto-spinal tract.
  
Pituitary Body.
+
8. Lemniscus.
  
Optic Nerve and Retina.
+
The funiculi of the spinal or descending sensory root of the fifth cranial nerve appear ventro-medial to the mass of the inferior cere
  
Corpora Quadrigemina.
 
  
Crura Cerebri.
 
  
Aqueduct (of Sylvius).
+
Fig. 895.— Nuclear Positions in Pons (Schematic).
  
Cerebellum.
+
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).
  
Pons (Varolii).
+
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
  
Pontine Part of the Fourth Ventricle.
 
  
  
( Medulla Oblongata (or Bulb).
+
Sensory Nucl. N.V. Motor Nucl. V.
  
\ Bulbar Part of Fourth Ventricle.
+
Superior Olive Fifth Nerve
  
  
General Description of the Base and Superior Surface of the
 
  
Encephalon.
+
•livary nucleus. The motor fibres of the facial nerve arise as the xons of the cells of this nucleus (see Fig. 896).
  
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 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.
  
The medulla oblongata (or bulb) lies on the under aspect of the
+
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.
  
erebellum in the median line, occupying the vallecula which separates
+
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
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
 
  
 +
supC- cerebellar peduncle.
  
  
 +
facial colliculus _ _
  
1444
+
nucleus of--., sixth H.
  
 +
post- longitudbundie.
  
A MANUAL OF ANATOMY
+
stalk of olive"'
  
 +
median lemniscus
  
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
 
  
 +
inferior! cerebellar middle j peduncle.
  
Olfactory Bulb Olfactory Tract
+
_\jai- vestibular
  
 +
> x \ nucleus.
  
Lateral Sulcus
+
TV 7- 5 pinai root of V*- h n. facial nucleus
  
  
Optic Nerve
+
-facial nerve
  
 +
^vestibular part ""'of 8th nerve.
  
Gyrus Rectus
+
. "Corpus trapezoides
  
 +
""Supr- olivary nucleus
  
 +
' oibducens nerve
  
Subst. Perfor Ant.
+
pyramidal tract.
  
 +
Fig. 896.— Diagrammatic Section through the Pons, to show Deep Origins of Sixth (Red) and Seventh (Black) Cranial Nerves.
  
Tuber Cinere and Infundibi Corpus Mami
 
  
 +
f the facial colliculus which lies on the pontine part of the floor of he fourth ventricle directly above the auditory striae.
  
Crus Cerebri
+
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. . , „
  
Area Perforal Post.
+
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 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:
  
Fifth Nerve Sixth Nerve
+
1. Superior peduncle of the cerebellum.
  
Facial Nerve Pars Intermedia
+
2. Nuclei of the fifth cranial nerve.
  
 +
3. Medial or posterior longitudinal bundle.
  
Auditory Nerve-' Glossopharyngeal Nerve Vagus Nerve
+
4. Medial fillet or lemniscus.
  
Accessory Nerve
+
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.
  
Oblongata
+
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.
  
Hypoglossal 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.
  
Fig. 878. —The Base of the Encephalon, and the Cranial Nerves. 1, frontal lobe (orbital surface); 2, temporal lobe; 3, cerebellum.
+
The mesencephalic root arises from groups of small cells which are placed in the grey matter of the mid-brain, beside the aqueduct, 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.
  
occupied by the basilar artery. On either side the pons becomes th<
+
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.
  
middle peduncle of the cerebellum, passing outwards and backwards into the cerebellar hemisphere.
+
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 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
+
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.
  
temporal pole.
+
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.
  
The stem of the lateral sulcus lies immediately in front of the
+
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.
  
  
 +
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
  
  
  
  
 +
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 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 NERVOUS SYSTEM
+
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.
  
1445
+
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 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.
  
mporal lobe, and is occupied by the middle cerebral artery. At the
+
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.
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 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 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 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 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 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.
  
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.
+
i locculus
  
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
+
Biventral Lobule
  
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.
+
Lobulus Gracilis 1 Great Horizontal
  
The tuber cinereum extends from the mammillary bodies to the ptic commissure, and is composed of grey matter. The infundibulum
+
Fissure , I
  
connected with the tuber cinereum close behind the optic comlissure, and passes downwards to the posterior part of the pituitary
+
t
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
+
Inferior Semilunar Lobule
  
  
I446
 
  
 +
Amygdala (Tonsil)
  
A MANUAL OF ANATOMY
 
  
 +
Pyramid Tuber Valvulae
  
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.
+
-Pregracile Fissure — Mid-gracile Fissure —Post gracile Fissure -•-Small Horizontal Fissure
  
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.
+
Fig. 899.— The Cerebellum (Inferior View).
  
The second or optic nerve is connected with the lateral extremity of the front part of the optic commissure.
+
1 he inferior semilunar lobule and the lobulus gracilis constitute the postero inferior lobule. Old terminology used.
  
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 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 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 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
 
  
  
1447
+
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.
  
The sixth or abducent nerve appears at the lower border of the pons ust lateral to the pyramid of the medulla oblongata.
+
Under Surface of the Cerebellum. —The under surface presents in
  
The seventh or facial nerve emerges at the lower border of the pons n front of the restiform body of the medulla oblongata.
+
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
  
The eighth or auditory nerve likewise appears at the lower border of
 
  
he pons in front of the restiform body of the medulla oblongata,
+
Fig. 900. — Lower Aspect of Cerebellum, showing the (Inferior) Ansiform Lobe and the Paramedian Lobule or Tonsil; also the Flocculus.
[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.
+
spheres are markedly convex, and are received into the inferior occipital or cerebellar fossae of the occipital bone.
  
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 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 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
+
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 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
  
ord as low as the level of the fifth cervical nerve. The first is the
+
nodule/ the terminal piece of the inferior vermis.
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 inferior vermis is composed of four lobules, named, in order from behind forwards, tuber valvulse, pyramid, uvula, and nodule.
  
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.
+
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.
  
Arteries at the Base of the Encephalon. —The arteries which supply the brain are the two vertebral and the two internal carotid arteries.
+
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 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 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 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
+
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.
  
1448 A MANUAL OF ANATOMY
+
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.
  
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
+
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 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.
  
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
+
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 NERVOUS SYSTEM
+
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 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.
  
1449
 
  
  
iach other by the third and fourth cranial nerves. The branches of
+
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.
  
he posterior cerebral artery are
+
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 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 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.
(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.
+
Fig. 901 b.— To illustrate Hypothetical Stages in the Evolution of Form of Human Cerebellum. (References as in previous figure.)
  
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.
+
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.
  
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
+
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.
  
arotid, (5) the anterior cerebral, and (6) the anterior communicating.
+
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.
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
+
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,
  
2. Middle Cerebral
+
he lower part of which includes the ‘ tuber vermis ' and ends at the
 +
prepyramidal fissure/ The central part of the posterior lobe includes
  
3. Anterior Cerebral
+
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
  
4. Anterior Communicating
+
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.
  
5. Posterior Communicating
+
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 hemispheres 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).
  
6 . Posterior Cerebral
+
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.
  
7. Basilar
+
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).
  
8 . Superior Cerebellar
+
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.
  
9. Transverse Pontine xo. Internal Auditory xr. Anterior Inferior Cerebellar
+
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 matter 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).
  
12. Posterior Inferior Cerebellar
+
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.
  
13. Vertebral
+
There are three other nuclei on each side as follows
 +
(1) the nucleus
 +
imboliformis, close to the inner
  
14. Anterior Spinal
+
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.
  
15. Posterior Spinal
+
Commissural and Association Fibres.—The commissural fibres pass
  
16. Anterior Choroid
+
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.
  
17. Posterior Choroid
+
Medullary Vela.—These are thin laminae or curtains of white matter, and are two in number, superior and inferior.
  
18. Cential or Ganglionic
+
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 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.
  
19. Central or Ganglionic
 
  
20. Central or Ganglionic (Postero-mesial)
 
  
‘21. Central or Ganglionic (Postero-lateral)
+
Fig. 902. — Anteroposterior Section through Lateral Lobe, showing Dentate Nucleus.
  
Fig. 880. —The Arteries at the Base of the Brain, and the Circulus Arteriosus.
 
  
  
  
13 15 14
 
  
 +
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.
  
  
1450
+
Layer of Purkinje's Cells
  
  
A MANUAL OF ANATOMY
 
  
 +
Fia Mater
  
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
+
_ Molecular Layer
  
  
Great Longitudinal Fissjarc
+
Granular Layer
  
  
 +
White Medullary Substance
  
Fig. 88i.—The Cerebral Hemispheres (Superior View). Fissure of Rolando—central fissure.
 
  
 +
Fig. 903. —Structure of a Lamina ■the Cerebellum (magnified).
  
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.
 
  
 +
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 NERVOUS SYSTEM
+
The inferior medullary velum forms a part of the lobns noduli, the other parts being the nodule and the two flocculi.
  
  
1451
+
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.
  
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.
+
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
  
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.
+
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.
  
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.
+
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.
  
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
+
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.
  
1452
+
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.
  
A MANUAL OF ANATOMY
+
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.
  
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 nerve-cells are of two kinds—namely, granule-cells and cells of Golgi.
  
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.
+
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
  
Optic Commissure
+
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
  
Optic Nerve
+
ells of Golgi; (5) some of the deep processes of the glia-cells; and (6) some fibres
Infundibulum, - ~
+
rom the white medullary substance.
  
Tuber Cinereum '■» * # „ .
+
White Matter.—The white matter of a cerebellai lamina is an >ffshoot of the principal white medullary substance, and composed 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.
  
C Optic Tract
+
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.
  
Mamillary Body . /
 
  
Third Nerve
+
===4. The Fourth Ventricle===
  
Mesial Root of Optic Tract '
 
  
Lateral Root of Optic Tract Lateral Geniculate Body
+
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
  
Post.Perforated Subst.
+
half of the bulb, and
  
  
Fourth Nerve
 
  
 +
Fig. 904.— Diagram to show the Composition of Floor and Roof of Fourth Ventricle.
  
Sixth Nerve
 
  
 +
SMV, IMV, upper and lower medullary vela.
  
Root of Fifth Nerve Root of Fifth Nerve
 
  
 +
(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).
  
Facial Nerve
+
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
  
Pars Intermedia Auditory Nerve - Glosso-pharyngeal Nerve -Vagus Nerve Superficial Arcuate Fibres—
 
  
Accessory Nerve-"
+
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.
  
First and Second Cervical Nerves;^
+
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 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.
  
Middle Peduncle of Cerebellum
+
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.
  
Inferior Peduncle - — Hypoglossal Nerve ..Anterior Median Fissure
+
The floor is traversed in the median line by a slight longitudinal roove, which divides it into two symmetrical longitudinal halves.
  
Decussation of the Pyramids
 
  
  
Fig. 882. —The Medulla Oblongata, Pons, and Interpeduncular Region. C.C., crus cerebri; P., pyramid; O.B., olivary body.
+
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 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.
+
The floor is divided into two parts, lower and upper, by the striae
  
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.
+
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 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.
  
  
 +
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.
  
THE NERVOUS SYSTEM
+
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
  
1453
+
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.
  
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 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
  
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.
+
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
  
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.
+
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 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 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 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.
  
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
 
  
 +
Fig. 907. —The Fourth Ventricle and Right Hemisphere of the Cerebellum (in Section) (Hirschfeld and Leveille).
  
T 454
 
  
 +
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.
  
A MANUAL OF ANATOMY
+
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 side 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
  
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.
+
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==
  
Fig. 883.—A Sketch to show the Disposition of Spinocerebellar Fibres in Lateral Region of Medulla.
+
Cerebral Hemispheres.
  
 +
The cerebral hemispheres are right and left. Each is semi-ovoid, md presents two extremities and three surfaces.
  
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).
+
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.
  
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.
+
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.
  
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 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, 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 ventricle. The difficulty is that, with this definition, the same depression is sometimes a fissure and sometimes a sulcus.
  
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.
 
  
 +
biG. 908. — Views of Brain, not showing Convolutions. A, from above; B, from left; C, from below; D, from behind.
  
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—
 
  
 +
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.
  
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 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.
  
  
 +
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 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 NERVOUS SYSTEM
 
  
  
  
455
+
Fig. 909. — Lateral View of Left Hemisphere.
  
>f the gracile and cuneate columns. It presents three longitudinal minences—namely, the funiculus gracilis, funiculus cuneatus, and uniculus gelatinosus.
+
In this specimen the horizontal (AH) and ascending (AV) anterior rami arise separately from lateral fissure. The pars triangularis lies between them.
  
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 central fissure (see Fig. 909), also known as the central sulcus and fissure of Rolando , begins at the supero-medial border of the hemisphere 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.
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.
+
Fig. 910.—Medial Aspect of Right Hemisphere. Approximate position of lateral ventricle marked in blue.
  
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
 
  
 +
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, 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).
  
Fig. 884.—Posterior View of Medulla.
+
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.
  
G, C, gracile and cuneate tubercles ; g, c, corresponding tracts; F, gelatinous tubercle; O, obex.
+
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 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).
  
A MANUAL OF ANATOMY
+
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
  
  
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
+
Fig. 911. — The Left Cerebral Hemisphere (Superior Surface).
  
inferior cerebellar peduncle.
+
Red=frontal lobe. Orange=parietal lobe. Blue=occipital lobe.
  
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
 
  
  
 +
—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.
  
Fig. 885.—Section through Medulla just above Decussation of Pyramids: Shows the Prominence of Spinal Tract of Fifth Nerve.
+
The gyri of the orbital surface are: gyrus rectus, medial orbital gyrus, anterior orbital gyrus, lateral orbital gyrus, and posterior orbital gyrus.
  
F is the dorsal spino-cerebellar tract immediately ventral to this, and G is the
+
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.
  
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.
+
Fig. 912.—The Right Cerebral and Cerebellar Hemispheres (Lateral Surface).
  
Internal Structure of the Medulla Oblongata. —Each half of the bulb is composed of grey nervous matter and tracts of white nervous matter.
+
Red=frontal lobe. Blue=occipital lobe.
  
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.
+
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.
  
  
THE NERVOUS SYSTEM
 
  
 +
The parietal lobe has two surfaces—lateral and medial.
  
1457
+
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).
  
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. 913. — The Encephalon (Right Lateral View) (Hirschfeld
  
 +
and Leveille).
  
  
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 ’).
+
1. Central Fissure
  
 +
2. Posterior Horizontal Limb of Lateral Fissure
  
a. Pons
+
3. Ascending Limb of Fissure
  
b. Medulla Oblongata (anterior aspect)
+
4. Anterior Horizontal Limb of Fissure 5,5. Intraparietal Sulcus
  
c. Decussation of the Pyramids
+
6. Ramus Horizontalis
  
d. Section of the Cervical Spinal Cord
 
  
1. Anterior Cerebro-spinal Tract
+
7. Ramus Occipitalis
  
2. Lateral Cerebro-spinal Tract
+
8. Transverse Occipital Sulcus
  
3. Sensory Tract
+
9. Lateral Occipital Sulcus
  
 +
10. External Occipito-parietal Fissure
  
3'. Nucleus Gracilis et Nucleus Cuneatus
+
11. Superior Temporal, or Parallel, Sulcus
  
4. Antero-lateral Intersegmental Tract
+
12. Inferior Temporal Sulcus
  
5. Anterior Pyramid
 
  
6. Fillet or Lemniscus
+
The intraparietal sulcus is often in two parts, horizontal and occipital (see Fig. 913).
  
7. Posterior Longitudinal Bundle
+
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.
  
8. Ventral Cerebellar Tract
+
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.
  
9. Dorsal Cerebellar Tract
 
  
  
3. A thick layer of grey matter over the floor of the fourth
+
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.
  
ventricle in the upper or open part of the bulb.
+
The gyri of the lateral surface are as follows: ascending parietal; superior parietal; and inferior parietal, with its supramarginal, angular, and postparietal gyri.
  
4. Substantia gelatinosa (nucleus of spinal tract, N. V.).
+
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.
  
5. Nuclei of grey matter.
+
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 modifications undergone by the grey matter of the bulb in ts lower or closed part are brought about by the decussation of the
+
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
  
92
 
  
 +
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 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
  
  
  
 +
Fig. 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 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
  
  
 +
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.
  
  
  
  
 +
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 lobe presents three surfaces—superior, lateral, and inferior.
  
A MANUAL OF ANATOMY
+
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.
  
1458
+
The sulci are called first and second temporal. The first temporal sulcus is parallel to the posterior limb of the lateral fissure, from which
  
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).
+
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 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.
+
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. 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.
+
Fig. 918. — The Inferior Surface of the Left Cerebral Hemisphere, showing the Gyri and Sulci.
  
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
 
  
  
 +
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 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 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.
  
  
  
  
 +
Fig. 920. — The Left Insula (Poirier, from Eberstaller). 1, 2, 3, gyri breves; 4, 5, gyri longi; X, limen insulae.
  
  
  
THE NERVOUS SYSTEM
 
  
 +
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 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.
  
1459
+
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 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.
  
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
+
Fig. 921. Medial Aspect of Hemisphere, with Approximate Position of Lateral Ventricle represented in Colour.
  
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.
+
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 dentatus above and the hippocampal gyrus below, and it terminates within the uncus of the hippocampal gyrus.
  
2. Nucleus cuneatus.
 
  
3. Olivary nuclei.
+
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.
  
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
 
  
 +
Fig. 922. — To show the Arrangement of Structures below the Level of the Splenium.
  
A MANUAL OF ANATOMY
+
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.
  
  
1460
+
Fig. 923. — Outline of Section across Hippocampal Region.
  
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).
+
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 nucleus cuneatus is a collection of grey matter within th< funiculus cuneatus. It is a direct extension from the basal part 0
 
  
 +
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). , ,
  
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 ').
+
Gyrus Dentatus Fascia Dentata).— The gyrus dentatus is situated
  
 +
above the hippocampal gyrus, and below the fimbria It is separated
  
1. Anterior Median Fissure
+
from the hippocampal gyrus by the hippocampal fissure, and from
  
2. Fourth Ventricle
+
the fimbria by a slight groove, called the fimbno-dentate sulcus - The dentate gyrus is narrow, and its free margin is indented or nhence the name dentatus. It begins behind the splenium of the corpus callosum, and is directed forwards above the hippocampal gyrus 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.
  
3. Formatio Reticularis 3'. Reticularis Alba
+
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.
  
3". Reticularis Grisea
+
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.
  
4. Raphe
+
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.
  
5. Anterior Pyramid
+
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.
  
6. Lemniscus
+
The sulci and gyri of the hemispheres first appear about the fifth month of intra-uterine life.
  
7. Inferior Olive with the two
+
Development of the Insula and Lateral Fissure. —The insula, or island of
  
Accessory Nuclei
+
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.
  
7'. Peduncle of Olivary Body
+
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.
  
8. Hypoglossal Nerve 8'. Hypoglossal Nucleus
+
(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.
  
9. Vagus Nerve
+
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 nucous membrane, and pass through the foramina of the cribriform date.
  
g'. Terminal Nucleus of Vagus Nerve
+
The olfactory tract is a white band which extends backwards from
  
10. External Dorsal Vestibular Nucleus
+
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.
  
xx. Nucleus Ambiguus
+
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.
  
12. Nucleus Gracilis
+
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.
  
13. Nucleus Cuneatus
+
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.
  
14. Caput of Posterior Cornu 14'. Lower Sensory Root of Fift
+
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.
  
Nerve
+
The grey matter of the anterior perforated substance is continuous superiorly with the grey matter of the lentiform and caudate nuclei.
  
15. Fasciculus Solitarius
+
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.
  
16. External Anterior Arcuat
+
===Development of the Olfactory Apparatus===
  
Fibres
+
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 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.
  
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.
+
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.
  
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 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
+
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.
  
1461
+
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.
  
  
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.
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FIG . 925.— Plan of the Structures constituting the Rhinencephalon.
  
  
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.
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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 ’
  
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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
  
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Lateral to each medial stmi and si uated under over of the callosal gyrus, there is another band, composed g V
  
Nucl. Grac.
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natter called the stria longitudinalis lateralis. . ,
  
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The strifof each side are lying in the grey layer alreadymentioned,
  
Nucl. Cun.
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.nd may be traced postenorly round ^ ' s dentatus.
  
— Fasc. Solitar.
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mown as fasciola cmerea, into the .f^rrespona b gy , it
  
Sp. Nucleus of Trigeminal Sp. Root Fibres of Trigeminal Med. Longit. Fasc. (Post. Longit. Bundle) Nucl. Ambiguus Lateral Nucleus
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interiorly each medial stria, along with the grey matter ^ ies, passes round the genu and backwards on t
  
Ant. Sp. Cerebellar Fasc. (Gowers)
 
  
Dorsal Acc. 01 .
 
  
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rostrum under the name of the geniculate gyrus. This enters the subcallosal gyrus, and finally passes to the temporal pole.
  
ig. 889.—Section through the Lower Half of Inferior Olive (shows also the Medial and Dorsal Accessory Olives).
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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
  
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the indusium griseum.
  
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.
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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.
  
  
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.
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Fig. 926. —The Corpus Callosum (Superior View) (Hirschfeld and Leveill£).
  
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.
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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 lamina terminalis. The gyrus then passes backwards and outwards
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ilong the posterior margin of the perforated area, forming now the liagonal band, and so reaches the temporal pole.
  
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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).
  
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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 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 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.
  
  
Line 2,437: Line 2,763:
  
  
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Fig. 927. — The Medial Surface of the Left Cerebral Hemisphere (Hirschfeld and Leveille;).
  
  
  
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Development. —The corpus callosum is developed fro m the lamina terminalis, but extends beyond this. It is the commissure of the neopallium.
  
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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).
  
  
  
  
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Fig. 928.—The Medial Surface of the Right Cerebral Hemisphere (Hirschfeld and Leveille).
  
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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.
  
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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,
  
  
  
  
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called the mamillo-thalamic tract, or bundle of Vicq d’Azyr, passes upwards and backwards into the thalamus.
  
A MANUAL OF ANATOMY
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The anterior pillars lie behind the anterior commissure, but give off a few precommissural fibres which, passing down in front of the
  
  
1462
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Fig. >29> _Part of Corpus Callosum cut away to expose Fornix and
  
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).
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Right Ventricle.
  
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.
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Inferior and posterior horns also opened from above.
  
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.
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Choroid Plexus Corp. Call. Fornix
  
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).
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Tail of Caudate Nucleus Hippocampus
  
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).
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Collateral Trigone Bulb
  
2. Dorsal spino-cerebellar tract (direct cerebellar tract).
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Calcar Avis
  
3. Ventral spino-cerebellar tract (tract of Gowers).
 
  
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commissure, reach the anterior perforated substance and subcallosal
  
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^ The posterior pillars are prolongations of the posterior part the body on either side. They are flattened bands, which at firs
  
  
  
THE NERVOUS SYSTEM
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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).
  
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Development. — The fornix is developed from the lamina terminalis.
  
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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.
  
4. Restiform body (inferior cerebellar peduncle).
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The anterior commissure, therefore, serves to connect the olfactory bulbs and the temporal lobes.
  
5. Funiculus cuneatus.
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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 surface of each lamina looks into the corresponding lateral ventricle, and is covered by the ventricular ependyma (epithelium). The medial
  
6. Funiculus gracilis.
 
  
7. Medial or posterior longitudinal bundle.
 
  
8. Tecto-spinal tract.
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Fig. 930.
  
g. Rubro-spinal tract.
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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.
  
10. Spino-tectal tract.
 
  
11. Superficial arcuate tract.
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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 is covered by grey matter on the surface looking towards the fifth ventricle.
  
12. Deep arcuate tract.
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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.
  
13. Fillet (lemniscus).
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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.
  
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
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Fig. 931. — The Tela Chorioidea and Internal Cerebral Veins.
  
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
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As regards development, it differs from the true ventricles in being originally a part of the great longitudinal fissure.
  
he lower part of the lateral area of the bulb nearly as high as the
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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
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.
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of 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
  
This peduncle is composed of fibres which are derived from the
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he structures which secrete
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the cerebro-spinal fluid.
  
following sources:
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The p