Book - Comparative Embryology of the Vertebrates 4-19

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Nelsen OE. Comparative embryology of the vertebrates (1953) Mcgraw-Hill Book Company, New York.

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Part IV - Histogenesis and Morphogenesis of the Organ Systems

Part IV - Histogenesis and Morphogenesis of the Organ Systems: 12. Structure and Development of the Integumentary System | 13. Structure and Development of the Digestive System | 14. Development of the Respiratory-buoyancy System | 15. The Skeletal System | 16. The Muscular System | 17. The Circulatory System | 18. The Excretory and Reproductive System | 19. The Nervous System | 20. The Development of Coelomic Cavities | 21. The Developing Endocrine Glands and Their Possible Relation to Definitive Body Formation and the Differentiation of Sex

The Nervous System

A. Introduction

1. Definition

The nervous system serves to integrate the various parts of the animal into a functional whole, and also to relate the animal with its environment. It consequently is specialized to detect changes in the environment (irritability) and to conduct (transmit) the impulses aroused by the environmental change to distant parts of the organism. The environmental change provides the stimulus, the protoplasmic property of irritability detects the stimulus, and transmission of impulses thus aroused makes it possible for the animal to respond once the impulse reaches the responding mechanism. This series of events is illustrated well in less complex animal forms such as an ameba. In this organism, the stimulus aroused by an irritating environmental change is transmitted directly to other parts of the cell, and the ameba responds by a contraction of its protoplasm away from the source of irritation. On the other hand, the complex structure of the vertebrate animal necessitates an association of untold numbers of cells, some of which are specialized in the detection of stimuli, and others transmit impulses to a coordinating center, from whence still other cells convey the impulses to specialized effector (responding) structures (fig. 352A).

2. Structural and Functional Features a. The Morphological and Functional Unit of the Nervous System

There are two opposing views regarding the morphological and functional unit of the nervous system. One view, widely championed, postulates that this unit is a specialized cell called the neuron. The neuron is a distinct cellular entity, having a cell body containing a nucleus and a central mass of cytoplasm from which extend cytoplasmic processes of various lengths (fig. 352B). The nervous system is made up of many neurons in physiological contact with each other at specialized functional junctions known as the synapses (fig. 352A). The synapse represents an area of functional contact specialized in the conduction of impulses from one neuron to another. However, it is not an area of morphological fusion between neurons. Each neuron, according to this view, originates from a separate embryonic cell or neuroblast of ectodermal origin, and each develops a definite polarity, i.e. impulses normally pass in one direction to the cell body and from thence distad to the area of synapse.

A contrary, older view is the reticular or nerve-net theory. This theory assumes that the nerve cells and their processes are a continuous mass of protoplasm or syncytium in which the “cell bodies” are local aggregations of a nucleus and a cytoplasmic mass. The entire controversy between this and the neuron theory revolves around the “synapse area.” The neuron doctrine as



sumes a distinct morphological separation at the synapse, but the reticular theory postulates a direct morphological continuity. We shall assume that the neuron doctrine is correct.

b. The Reflex Arc

While the neuron, in a strict sense, represents the functional unit of the nervous system, in reality, chains of physiologically related neurons form the functional reflex mechanism of the vertebrate nervous system. The functional

Fig. 352. Neuron structure and relationships. (A) Structural components of a simple reflex arc. (B) Diagrammatic representation of a motor neuron. (Redrawn from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders, after Barker.) (C) Developing nerve fiber (process) of young neuroblast. Observe growth or incremental eone at distal end of growing process. (Redrawn from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders, after Cajal, Prentiss-Arey. ) (D) Neuron from spinal ganglion of a dog showing ganglion cell body with its surrounding capsular cells and capsule. Observe that the capsular cells and capsule are continuous with sheath cell and neurilemma. (Redrawn, somewhat modified, from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders.) (E) Longitudinal section of myelinated nerve fiber. (Redrawn from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders, after Nemiloff, MaximowBloom.)



reflex mechanism is an arrangement of neurons known as the reflex arc. Theoretically, a simple type of reflex arc would possess (fig. 352A):

( 1 ) a sense receiving structure, the receptor;

(2) the sensory neuron, whose long afferent or sensory fiber contacts the sensory receptor, while its efferent fiber or axon continues from the body of the neuron to the central nervous system. Within the central nervous system the terminal fibers (telodendria) of the efferent fiber of the sensory neuron forms a synapse with

( 3 ) the dendrites of an efferent neuron. From the efferent or motor neuron a motor fiber (axon) leaves the central nervous system and continues to

(4) the effector organ.

Functionally, however, even the simplest type of reflex arc may not be as elementary as this. More probably, a system of one or more association neurons placed between the sensory and motor neurons exists in most instances.

c. Structural Divisions of the Vertebrate Nervous System

The nervous system of vertebrate animals consists of

( 1 ) the central nervous system, a tubular structure composed of a coordinated assembly of association neurons and their processes. The central nervous system is integrated with

(2) the peripheral nervous system constructed of a series of sensory and motor neurons which connect the central nervous system with distal parts of the body. Through the medium of various types of sense receptors the central nervous system is made aware of changes in the external and internal environment of the body.

d. The Supporting Tissue

In addition to the irritable cellular neurons, the nervous system contains connective or supporting tissue. However, unlike most of the other organ systems of the body, the supporting tissue of the nervous system is derived mainly from an ectodermal source. Small amounts of connective tissue of mesodermal origin parallel the various blood capillaries which ramify through nervous tissue, but the chief supporting tissue of the brain and spinal cord is the neuroglia of ectodermal origin. The neuroglia consists of two main cellular types, the ependymal cells and the cells of the neuroglia proper.

The ependymal cells (fig. 353 A) form a single layer of columnar epithelium which lines the lumen of the neural tube. From the inner aspect or base of each ependymal cell a process extends peripherad toward the external surface of the neural tube (fig. 353F-H). Later the peripheral process may be lost. During the earlier stages of their development the ependymal cells are ciliated on the aspect facing the neurocoel (fig. 353A).



The cells of the neuroglia proper lie within the substance of the nerve tube between the neuron-cell bodies of the gray matter and also between the nerve fibers of the white matter (fig. 353H). Conspicuous among the neuroglia cells are the protoplasmic astrocytes (fig. 353D) which reside mainly among the neurons of the gray matter and the fibrous astrocytes (fig. 353B) found in the white matter. The processes of the fibrous astrocytes are longer and finer than those of the protoplasmic astrocytes, and they may attach to blood vessels (fig. 353B). Two other cellular types of neuroglia, the oligodendroglia and the microglia cells, also are present (fig. 353C and E). The microglia cells presumably are of mesodermal origin (Ranson, ’39, p. 57).

B. Basic Developmental Features

1. The Embryonic Origin of Nervous Tissues

The ectoderm of the late gastrula is composed of two general organ-forming areas, namely, neural plate and epidermal areas (fig. 192A). Both of these primitive ectodermal areas are concerned with the development of the future nervous system and associated sensory structures. From the neural plate region arises the primitive neural tube (Chap. 10), the basic rudiment of the central nervous system, whereas the line of union between the neural plate and the epidermal areas gives origin to the ganglionic or neural crest cells which contribute much to the formation of the peripheral nervous system. As observed in Chapters 9 and 10, the determination of the neural plate material and the formation of the neural tube are phenomena dependent upon the inductive powers of the underlying notochord and somitic mesoderm in the Amphibia. Presumably the same basic conditions obtain in other vertebrate embryos.

Fig. 353. Structure of the developing neural tube. (A) Ciliated ependymal cells from ependymal layer of the fourth ventricle of a cat. (Redrawn from Maximow and Bloom, 1942, A Textbook of Histology, Philadelphia, Saunders, after Rubaschkin.) (B-E) Various types of neuroglia cells. (Redrawn from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders, after Rio Hortega.) (F) Transverse section of neural tube of three-day chick embryo. The spongioblasts are stained black after the method of Golgi. (Redrawn from Maximow and Bloom, 1942. See reference under A, after Cajal.) (G) Transverse section of part of spinal cord of 15 mm. pig embryo showing structural details. This section was constructed from several sections. The part of the section to the left reveals the neuroglial support of the developing neuroblasts. (Redrawn from Hardesty, 1904, Am. J. Anat., 3.) (H) Transverse sec tion, constructed from sections, of part of the spinal cord of 55 mm. pig embryo showing neuroglial support for developing neuron cells. (Redrawn from Hardesty, 1904, Am. J. Anat., 3.) (I) Transverse section of spinal cord of newborn mouse depicting

spongioblasts which are moving peripherally from the central canal. These spongioblasts are in the process of transforming into stellate neuroglia cells or astrocytes. (J) Transverse section of 9 mm. pig embryo portraying ependymal, mantle, and marginal layers, external and internal limiting membranes, and blood vessels growing into the nerve substance. (Redrawn from Hardesty, 1904, Am. J. Anat., 3.) (K) Transverse

section of spinal cord of 20 mm. opossum embryo indicating general structure of the spinal cord. Observe dorsal root of spinal nerve growing into nerve cord at the right of the section.

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2. The Structural Fundaments of the Nervous System

The early nervous system shortly after the neural tube is formed is composed of an elongated, hollow tube, aggregations of neural crest cells, and a series of sense placodes.

a. The Elongated Hollow Tube

The primitive neural tube, located dorsally in the median plane (fig. 217G and H), forms the basis for the central nervous system and potentially is composed of two major regions, namely, the future brain region at its anterior end and posteriorly the rudiment of the spinal cord. The future brain region quickly develops three regions, viz.:

( 1 ) the prosencephalon, or the rudiment of the forebrain;

(2) the mesencephalon, or future mid-brain region, and

(3) the rhombencephalon, or hindbrain region (fig. 354D and E).

The rhombencephalon passes imperceptibly into the developing spinal cord, or the primitive neural tube posterior to the brain region.

The cephalic end of the primitive neural tube from the time of its formation tends to present a primary neural flexure, the cephalic flexure (see Chap. 10). This flexure occurs in the region of the mesencephalon. It is slight in teleost fishes, more marked in amphibia, and pronounced in elasmobranch fishes, reptiles, birds and mammals (fig. 354E and F).

During the early stages of neural tube development, the anterior end of the tube tends to form primitive segments or neuromeres. These neuromeres fuse together as they contribute to the primitive brain regions as indicated in figure 354A-D (see Hill, 1900).

b. The Neural Crest Cells

As the neural tube is formed, the neural crest cells come to lie along the dorso-lateral aspect of the neural tube. The crest cells soon become aggregated together in clumps, each aggregation representing the initial stage in the formation of the various cranial and spinal ganglia (see figures 347A; 357B-F).

c. Special Sense Placodes

The special sense placodes are a series of epithelial thickenings of the lateral portions of the epidermal tube overlying the future head region. These placodes, which represent contributions of the epidermal tube to the forming nervous system, are as follows:

( 1 ) The nasal placodes, two in number, each arising on either side of the ventro-anterior region of the primitive head.

(2) The lens placodes, two in number, each arising in relation to the optic outpushing of the diencephalic portion of the forebrain.



Fig. 354. Early development of the brain in the chick and teleost fish showing the tendency to form neural segments or neuromeres. (All figures redrawn from Hill, 1900, Zool. Jahrblicher, abt. Anat. u. Ontogenie 13.) (A) Dorsal view of developing brain of

chick embryo of 4 pairs of somites. (B) Dorsal view of primitive brain or encephalon of chick embryo of 7 pairs of somites. (C) Dorsal view of brain of chick embryo with 11 jpairs of somites. (D) Dorsal view of developing brain of chick embryo with 14 pairs of somites. (E) Lateral view of brain of chick embryo about 75 to 80 hours of incubation. In the foregoing illustrations, observe that the neuromeres gradually fuse to form parts of primitive five-part brain shown in E. (F) Brain, lateral view, Salmo fario, 33 somites, 22 days old. Segments 1-3 represent the prosencephalon, 4 and 5 the mesencephalon, 6 the anterior part of the rhombencephalon, and 7-11 to the posterior region of the rhombencephalon. Observe that the cephalic flexure is present slightly at this time. A little later in the 36 day embryo it is more pronounced.

(3) The acoustic placodes, two in number, taking their origin from the dorso-lateral portion of the epidermal tube overlying the middle portion of the hindbrain.

In water-dwelling vertebrates, other placodes arise in the head region associated with the lateral-line system. The lateral line placodes probably represent an extension of the acoustic placodal system in lower vertebrates. Hence, the general term acoustico-lateral or neuromast system (see Goodrich, ’30, p. 732) may be applied to this general system of sensory structures.

(4) Taste-bud placodes. The taste buds are distributed variously in different vertebrate species. In man, cat and in other mammals they are located on the tongue, particularly its posterior part (fig. 285E) on the



soft palate, and in the pharyngeal area. In fishes, taste buds are found generally over the buccal cavity and pharynx, and also on the outer surface of the head and branchial region. In some teleosts they may be distributed generally over the external surface of the body (fig. 356C). The external distribution of taste buds over the head region occurs also in certain aquatic amphibia. Consequently, the distribution of the epithelial thickenings which give origin to the taste buds varies greatly in different vertebrates.

3. The Histogenesis of Nervous Tissue a. The Formation of Neurons

The neurons of the central nerve tube arise from primitive neuroblasts. The primitive neuroblasts in turn take their origin from the cells of the ependymal zone of the nerve tube. The ependymal zone is the layer, two to three cells in thickness, which lines the lumen or neurocoel of the developing tube. Cell proliferation occurs within this zone, and the primitive neuroblasts migrate outward into the more lateral areas. After leaving the immediate confines of the ependymal zone, the neuroblasts presumably begin to differentiate into the many peculiar forms of the neurons to be found within the central nervous system. The neurons of the peripheral nervous system arise from cells which migrate from the central nerve tube, and from cells of the neural crests and certain sense placodes.

1) General Cytoplasmic Changes. The basic physiological functions of irritability and conductivity found in living protoplasm is developed to a high degree in the neuron or essential cellular entity of the nervous system. In consequence, the morphological changes which the simple epithelial cell of the forming neural tube assumes during its differentiation into a neuron is in harmony with these basic functions. One of the morphological changes in the developing neuroblast is the formation of coagulated threads of cytoplasmic material embedded in a more liquid cytoplasm. These threads arc known as neurofibrils, while the more liquid, less-differentiated parts of the cytoplasm are called the neuroplasm. Accompanying the changes which produce the neurofibrils is the formation of another characteristic of neurons, namely, processes or cytoplasmic extensions from the body of the cell (fig. 352B). These processes are of two general types, the dendrites and the axon (neuraxis or axis cylinder). Several dendrites are generally present but only one axon is developed. The exact function of the dendrites has been questioned but the possibility is conceded that they function as “the chief receptive organelles of the neuron” (Maximow and Bloom, ’42, p. 190), whereas the axon is believed to convey the nerve impulse away from the cell body to the terminal arborizations or teledendria (fig. 352A). The teledendria make physiologic contact (i.e., they synapse) with the dendrites of other neurons or they form a specialized relationship with effector cells such as glandular cells or



muscle fibers (fig. 352A). The neurofibrils extend into the cell processes. The precise relationship of the neurofibrils to conduction and transmission of nervous impulses is unknown. {Note: The formation of the sheaths surrounding the nerve fiber is described on page 819.)

2) Nuclear Changes. Associated with the changes in the cytoplasm mentioned above are alterations of the nucleus. One of the striking features of nuclear change is that it enlarges, and becomes vesicular, though the basichromatin remains small in quantity. The nucleolus experiences profound changes, and is converted from a homogeneously staining body into a vacuolated structure in which the desoxyribose nucleic acid is irregularly localized along the edges. Contemporaneous with the nucleolar changes there is a ‘‘marked production of Nissl substance in the cytoplasm” (Lavelle, ’51, p. 466), Accompanying the changes in the nucleus is its loss of mitotic activity, although a centrosome is present in the cytoplasm. All neuroblasts, however, do not lose their power of division; only those which start to differentiate into neurons. During embryonic life many potential neurons remain in the neuroblast stage and these continue to proliferate and give origin to other neuroblasts. Shortly after birth or hatching this proliferative activity apparently ceases, and the undifferentiated ncuroblasts then proceed to differentiate into neurons.

3) Growth and Development of Nerve-cell Processes. The early neuroblasts of the central nerve tube are at first apolar, that is, that do not have distinct processes. These apolar cells presumably transform in unipolar and bipolar varieties of neuroblasts. The unipolar cells have one main process, the axon, and the bipolar cells have two processes, an axon and a dendrite. From these two primitive cell types multipolar neurons arise having several dendrites and one axon (fig. 352B).

As the nerve-cell process begins to develop, a small cytoplasmic extension from the cell body occurs. To quote directly from Harrison (’07), p. 118, who was the first to study growing nerve -cell processes in the living cell: “These observations show beyond question that the nerve fiber develops by the outflowing of protoplasm from the central cells. This protoplasm retains its amoeboid activity at its distal end, the result being that it is drawn out into a long thread which becomes the axis cylinder. No other cells or living structures take part in the process. The development of the nerve fiber is thus brought about by means of one of the very primitive properties of living protoplasm, amoeboid movement, which, though probably common to some extent to all cells of the embryo, is especially accentuated in the nerve cells at this period of development.” The distal end of a growing nerve fiber has a slight enlargement, the “growth cone” or “growth club” (fig. 352C). The conclusions of Harrison on growing nerve fibers in tissue culture were substantiated by Speidel (’33) in his observations of growing nerve fibers in the tadpole’s tail.

Many different shapes of cells are produced during the histogenesis of the



neural tube. However, two main morphological types of cells may be considered :

( 1 ) One type of neuron possesses a short axon or axis cylinder. This type of neuron lies entirely within the gray substance of the neural tube.

(2) In a second type of neuron a long fiber or axis cylinder is developed and this fiber leaves the gray substance and traverses along the white substance of the cord or within the fiber tracts of the forming brain. In many instances, the cell body of the second type of neuron lies within the gray matter of the spinal cord, but its axis cylinder passes out of the nerve tube as the efferent or motor fiber of a spinal or cranial nerve (fig. 355F and I).

b. The Development of the Supporting Tissue of the Neural Tube

The potential connective tissue cell of the neural tube is the spongioblast. Spongioblasts are of ectodermal origin and differentiate into two main types of cells: (1) Ependymal cells, and (2) neuroglia cells.

Spongioblasts together with primitive neuroblasts lie at first within the ependymal zone of the neural canal particularly close to the lumen. Cilia are developed on the free surface of each spongioblast lining the neurocoel. From the opposite end of the cell, that is, the end facing the periphery of the tube, an elongated process extends peripherad to the outer surface of the neural tube. In this way a slender framework of fibers extends radially across the neural tube, from the lumen to the periphery (fig. 353F~K). A spongioblast which retains a relationship with the lumen and at the same time possesses a fiber extending peripherad is known as an ependymal cell. The ependymal cells thus are those cells whose bodies and nuclei lie next to the lumen of the developing spinal cord and brain but possess processes which radiate outward toward the periphery of the cord (fig. 3 53 A and F). The peripheral fiber or extension may be lost in the later ependymal cell together with its cilia.

In fishes and amphibians the supporting elements of the central nerve tube retain the primitive arrangement outlined above (see Ariens-Kappers, ’36, p. 46). However, in reptiles, birds and mammals, the radial pattern of many of the primitive spongioblasts is lost, and these spongioblasts transform into neuroglia cells, losing their connection with the lumen and with the external limiting membrane of the tube (fig. 3531).

c. Early Histogenetic Zones of the Neural Tube

The neural plate of the late gastrula is a thickened area of cells of about 3 to 4 cells in thickness. As the neural plate is transformed into the neural tube the majority of the neural plate cells become aggregated within the lateral walls of the tube. The lateral walls of the developing neural tube in consequence are thicker than the dorsal and ventral regions. As already observed



in Chapter 10, this discrepancy in the thickness of the walls of the tube is due (in the amphibia) to the inductive influence of the somite which comes to lie along the lateral regions of the primitive tube. In the 9-mm. pig embryo, the neural tube in transverse section begins to present three general zones (fig. 353J), viz.:

(1) an ependymal layer of columnar cells lining the lumen,

(2) a relatively thick nucleated mantle layer occupying the middle zone of the neural tube, and

(3) a marginal layer without nuclei extending along the lateral margins of the tube.

The ependymal layer of cells lies against the internal limiting membrane of the tube, and consists of differentiating spongioblasts as indicated above. The mantle layer contains many neuroblasts and in consequence is referred to as the middle nucleated zone. It forms the future gray matter of the neural tube. The outer or marginal zone in its earlier phases of development is a meshwork of neuroglia and ependymal cell processes. Later, however, the processes of neurons come to lie among the fibrous processes of the neuroglia and ependymal cells as the nerve cell fibers extend along the spinal cord. The external limiting membrane lies around the outer edge of the marginal layer, and thus forms the outer boundary of the tube. In figure 353H is shown the relationships of the ependymal, mantle and marginal layers of the spinal cord of a 55-mm. pig embryo together with the ependymal and neuroglia cells. The arrangement of the ependymal, mantle and marginal layers in the spinal cord of a 22-mm. opossum embryo is shown in figure 35 3K.

d. Early Histogenesis of the Peripheral Nervous System

The formation of the cerebrospinal scries of nerves which comprise the peripheral nervous system involves cells located within the neural crest materials and also within the mantle layer (gray matter) of the neural tube. One feature of the development of the spinal nerves is their basic metamerism, for a pair of spinal nerves innervates the somites of each primitive segment or metamere.

The neuroblasts of each spinal nerve arise in two areas, viz.:

(1) the neural crest material which forms segmental masses along the lateral sides of the neural tube, and

(2) cells within the ventral portions of the gray matter of the tube.

In the development of a spinal nerve bipolar neuroblasts appear within the neural crest material. Each bipolar neuroblast sends a process distad toward the dorso-lateral portion of the neural tube and a second process lateroventrad toward the body wall tissues, or toward the viscera. Later these bipolar elements become unipolar and form the dorsal root ganglion cells.



Fig. 355. Development of general structural features of the spinal cord; the nuclei of origin and nuclei of termination of cranial nerves associated with the myelencephalon. (A-E) The formation of the central canal, dorsal median septum, dorsal median sulcus, and ventral median fissure in pig embryos. Arrows in the dorsal part of the developing nerve cord show obliteration of the dorsal part of the primary neurocoel by medial growth of the lateral walls of the spinal cord. By this expansive, medial growth, the dorsal median septum and the dorsal sulcus (fissure) are formed. Observe that the central canal is developed from the ventral remains of the primary neurocoel after the obliteration of the dorsal portion of the primary neurocoel has been effected. In diagrams C-E, the




Within the ventral gray matter of the spinal cord, fusiform bipolar cells arise which send processes at intervals out into the marginal layers and from thence outward through the external limiting membrane of the tube at the levels corresponding to the developing dorsal root ganglia. The groups of processes which thus emerge from the neural tube below a single dorsal root ganglion soon unite with the ventrolateral processes of the dorsal root ganglion cells to form the ventral root of the spinal nerve. Within the neural tube the cell bodies of the ventral root fibers soon form multipolar neuron cells.

As development proceeds, the cell bodies of the neurons within the dorsal root ganglia become encased by capsular cells which develop from some of the neural crest cells (fig. 352D). The capsular cells in consequence are of ectodermal origin and they are continuous with the neurilemma sheath. The cells of the neurilemma sheath also arise from certain neural crest cells and from cells within the neural tube. These cells migrate distad as sheath cells along with the growing nerve fiber. The neurilemma or sheath of Schwann arises as an outward growth from the cytoplasm of the sheath cells; the neurilemma sheath thus appears in the form of a delicate tube surrounding the nerve fiber (axis cylinder) of the neuron (352D). Later on, a secondary substance appears between the nerve fiber (axis cylinder) and the neurilemma in many nerve fibers. This substance is of a fatty nature and forms the myelin (medullary) sheath (fig. 352E). Myelin deposition by sheath cells depends primarily upon an axis cylinder stimulus and not upon the sheath cells, for it is only a particular type of nerve fiber, the myelin-emergent fiber, which possesses the ability to form myelin (Speidel, ’33). In the peripheral nerve fibers, the neurilemma at certain intervals dips inward toward the axis cylinder, forming the node of Ranvier. The area between two nodes is known as an internodal segment (fig. 352B). One sheath cell is present in each internodal segment. The nerve fibers of the peripheral nervous system with respect to

Fig. 355 — Continued

arrows drawn in the ventral portions of the nerve tube indicate the ventro-medial expansion of lateral portions of the developing nerve tube with the subsequent formation of the ventral median fissure. In E the dorsal, ventral, and lateral columns or funiculi of white matter are shown. (F) Diagram depicting some of the principal fiber tracts of the spinal cord of man. Ascending tracts on the right; descending tracts on the left. (Redrawn from Ranson, 1939. For reference see G.) (G) Ventral view of human

spinal cord, nerves removed, showing cervical and lumbar enlargements. (Redrawn from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders.) (H) Diagram revealing the relation of the meninges, i.e., the protective membranes of the central nervous system, to the spinal cord. (Redrawn from Ranson, 1939. For reference see G.) (I) Schematic diagram of transverse section through myelencephalon (medulla),

portraying dorso-ventral position of nuclei of origin in motor plate and the nuclei of termination in alar plate of cranial nerves associated with the myelencephalon.



their sheath-like coverings are of two kinds, viz., myelinated fibers with neurilemma and unmyelinated (Remak’s) fibers with a thin neurilemma. The latter are found especially among the sympathetic nerve fibers of the cerebrospinal series. (See Ranson, ’39, p. 51.)

It may be observed here, parenthetically, that the myelinated fibers of the brain and spinal cord differ from the myelinated fibers of the peripheral nervous system in that the sheaths are formed by an investment of neuroglia fibers and nuclei and not by a neurilemma sheath. Many naked axons also are present in the central nervous system.

C. Morphogenesis of the Central Nervous System

1. Development of the Spinal Cord a. Internal Changes in the Cord

During the early development of the spinal cord described above the following areas are evident:

( 1 ) the ependymal layer,

(2) the mantle layer, and

(3) the marginal layer.

The further development of these areas results in the formation of a thin dorsal roof plate and a ventral floor plate mainly from the ependymal layer (fig. 353J and K), Somewhat later the neural cavity of the cord is reduced by the apposition and fusion of the dorso-lateral walls of the lumen immediately under the dorsal plate, leaving a rounded central canal below located near the floor plate (fig. 355A-E). Synchronized with these events the lateral walls of the neural tube expand greatly as the mass of cells and fibers increases. During this expansion, the two dorsal parts of the lateral walls move dorsad and mediad and in this way come to lie apposed together in the median plane above the central canal. This apposition forms the dorsal median septum (fig. 355D and E). The dorsal roof plate becomes obliterated during this process. Ventrally, also, the lateral portions of the neural tube move toward the mid-ventral line below the central canal. However, the two sides do not become closely apposed, and as a result the ventral median Assure is formed (fig. 355D andE).

During the growth and expansion of the two lateral walls of the neural tube, the neuroblasts of the nucleated mantle layer in the dorsal or alar plate of the spinal cord increase greatly in number and form the dorsal (or posterior) gray column (fig. 355A-E). The developing neuroblasts of the dorsal gray column become associated with the dorsal root fibers of the spinal nerves. Ventrally, the neuroblasts of the mantle layer increase in number in the basal plate area of the spinal cord and form a ventral (anterior) gray column. The ventral root fibers of the spinal nerves emerge from the ventral



gray column. In the region of the central canal the mantle layer forms the dorsal and ventral gray commissures which extend across the nerve cord joining the gray columns in the lateral walls of the cord. Somewhat later, a lateral gray column on either side may be formed between the dorsal and ventral gray columns.

As the above growth and development of the mantle layer is achieved, the marginal zone of the spinal cord also increases in size as nerve fibers from the developing neurons in the gray columns and in the spinal ganglia of the dorsal roots grow into the marginal layer between the neuroglia elements. Moreover, nerve fibers from developing neuroblasts in the brain grow posteriad in the marginal layer of the cord. As the growth and expansion of the dorsal and ventral gray columns toward the periphery of the spinal cord occurs, the marginal layer becomes divided into definite regions or columns known as funiculi. The dorsal funiculus, for example, lies between the dorsal median septum and the dorsal gray column while the ventral funiculus is bounded by the ventral median fissure and the ventral gray column. The lateral funiculus lies laterally between the dorsal and ventral gray columns (fig. 355F). Below the ventral gray commissure, fibers cross from one side of the cord to the other, forming the ventral white commissure.

Eventually the nerve fibers of each funiculus become segregated into fiber tracts. As a result, the dorsal funiculus becomes subdivided into the two fibertract bundles, the fasciculus gracilis near the dorsal medial septum and the fasciculus cuneatus near the dorsal gray column. Other fiber tracts are shown in figure 355F. (Consult Ranson, ’39, p. 110.)

b. Enlargements of the Spinal Cord

The spinal cord in many tetrapoda tends to show two enlarged areas, viz. (fig. 355G):

( 1 ) The brachial (cervical) enlargement in the area of origin of the brachial nerves;

(2) The lumbar (sacral) enlargement in the area of origin of the lumbosacral plexus.

Posteriorly the cord tapers toward a point, and anteriorly, in the region of the first spinal nerve, it swells to become continuous with the myelencephalon.

c. Enveloping Membranes of the Cord

Immediately surrounding the spinal cord is a delicate membrane, the pia mater, presumably developed from neural crest cells. More lateral is the arachnoid layer, developed probably from neural crest cells and mesenchyme. Between the pia mater and the arachnoid is the subarachnoid space containing blood vessels, connective tissue fibers, and a lymph-like fluid. Outside



of the arachnoid layer is a cavity, the subdural cavity. The external boundary of the subdural cavity is formed by the dura mater. The latter is a tough connective tissue membrane of mesenchymal origin (fig. 355H).

2. Development of the Brain

a. The Development of Specialized Areas and Outgrowths of the Brain

1) The Formation of the Five-part Brain. The primitive vertebrate brain from its earliest stages of development begins to show certain enlargements, sacculations and outpushings. Furthermore, it possesses two main areas which are non-nervous and membranous in character, namely, the thin roof plate of the rhombencephalon and the thin roof plate of the posterior portion (diencephalon) of the prosencephalon (figs. 354E; 356A). These thin roof plates ultimately form a part of the tela chorioidea. Vascular tufts, the chorioid plexi, also project from these roof plates into the third and fourth ventricles.

The anterior region of the primitive brain known as the prosencephalon or forebrain soon divides into the anterior t elenc ephalon and a more posterior diencephalon (fig. 354C-E). The telencephalon gives origin to two lateral outgrowths or pouches, the telencephalic vesicles (figs. 354E; 357E). The telencephalic vesicles represent the rudiments of the cerebral lobes. From the diencephalon, four or five evaginations occur, namely, a mid-dorsal evagination, the epiphysis or rudiment of the pineal body (fig. 356A) , and in front of the epiphysis a second mid-dorsal evagination occurs normally in most vertebrates, namely, the paraphysis (see Chapter 21); two ventro-lateral outgrowths, the optic vesicles (fig. 354B-D) from which later arise the optic nerves, retina, etc., and a mid-ventral evagination, the infundibulum. The infundibulum unites with Rathke’s pouch (figs. 354E; 356A), a structure which arises from the stomodaeum. Rathke’s pouch ultimately differentiates into the anterior lobe of the pituitary body (see Chapter 21 ).

The mesencephalon, unlike the fore- and hind-brain regions, does not divide. However, from the mesencephalic roof or tectum dorsal swellings occur which appear to be associated with visual and auditory reflexes. In fishes and amphibia, two swellings occur, the so-called optic lobes or corpora bigemina. In reptiles, birds and mammals four swellings arise in the tectum, the corpora quadrigemina. (fig. 357H-0).

The rhombencephalon divides into an anterior metencephalon and posterior medulla or myelencephalon (fig, 354E and G). Two cerebellar outpushings arise from the roof of the metencephalon.

The primitive five-part brain forms the basic embryonic condition for later brain development in all vertebrates.

2) The Cavities of the Primitive Five-part Brain and Spinal Cord. As, previously observed, the brain and spinal cord are hollow structures, and its generalized cavity is called the neural cavity or neurocoel (fig. 357A). From



the primitive neurocoel, special cavities in the brain arise, as follows (see figure 357A):

(1) The telencephalon is made up of the anterior part of the prosencephalon and two telencephalic vesicles. Each vesicle ultimately gives origin to a cerebral lobe. The cavities of the telencephalic vesicles are known as the first and second ventricles.

(2) The cavity of the posterior, median portion of the telencephalon and that of the diencephalon form the third ventricle.

(3) The roof of the original mesencephalon may give origin to hollow, shallow outpushings, but the cavity of the mesencephalon itself becomes a narrow passageway and is known as the cerebral aqueduct or the aqueduct of Sylvius.

(4) The cavity of the rhombencephalon is called the fourth ventricle.

b. The Formation of Cervical and Pontine Flexures

In addition to the primary or cephalic flexure previously described (p. 812) other flexures may appear in the developing vertebrate brain, especially in higher vertebrates. The cervical flexure develops at the anterior portion of the spinal cord, as it joins the myelencephalon. It involves the caudal portion of the myelencephalon, and the anterior part of the cord. It bends the entire brain region ventrally (see figure 357D and E). The latter flexure is absent in fishes, is present to a slight degree in the early neural tube of the amphibia, and is pronounced in reptiles, birds and mammals. The third or pontine flexure of the brain bends the brain dorsally. It arises in the mid-region of the rhombencephalon, in the area between the myelencephalon and the metencephalon. It appears later in development than the cephalic and cervical flexures, and is found only in higher vertebrates.

c. Later Development of the Five-part Brain

The various fundamental regions of the five-part brain develop differently in different vertebrates. Figure 357B-G and H-O illustrates the changes of the regions of the primitive five-part brain in the shark, frog, bird, dog, and human. For detailed discussion of the function of the various parts of the brain of the vertebrate, see Ranson, ’39.

D. Development of the Peripheral Nervous System

1. Structural Divisions of the Peripheral Nervous System

The peripheral nervous system integrates the peripheral areas of the body with the central nervous system. It is composed of two main parts,

( 1 ) the cerebrospinal system of nerves and

(2) the autonomic system. The latter is associated intimately with the cerebrospinal system.

telencephalon diencfphalon mesencephalon metencephalon


PREMUSCLE MASS OF sternomastoid

AND â–















Fio. 356. The cranial nerves; nuclei of origin and termination; functional components. {Note: The accompanying figures illustrate the nuclei of origin and nuclei of termination of the various cranial nerves. They are generalized figures and should be regarded only as approximate representations. This must be true, for the position of the respective nuclei within the brain “varies greatly in different orders of vertebrates” [Ranson]. This variation presumably is the result of a developmental principle known as neurohiotaxis. This principle postulates that the dendrites of a neuron together with the cell body move




toward the source from whence the neuron receives its stimulation. That is, the dendrites grow, and the neuron cell body as a whole moves, toward the particular nerve fiber tract from which the impulses are received. As these impulses and fiber tracts vary slightly with the particular environmental conditions under which the different animal groups live, the location of the nuclei within the brain correspondingly will vary to a degree within the respective vertebrate groups. It is- to be observed, also, that the nuclei of origin of the afferent fibers of the cranial nerves, and of the cerebrospinal nerves in general, are located outside of the central nerve tube, with the exception of the neuron cell bodies of the second or optic nerve which are located in the retina, an extension of the forebrain, and the mesencephalic nucleus of the fifth nerve. The nuclei of origin of the efferent fibers are placed within the latero-basal areas of the nerve tube (fig. 3551).)

(A) The nuclei of origin of the various motor components of the cranial nerves here are shown to be located within fairly definite regions along the antero-posterior axis of the vertebrate brain. Reference may be made to Fig. 3551, for the dorso-ventral distribution of these nuclei.

The following symbols are used:

1. Somatic motor fibers are shown in solid black.

2. Special visceral motor fibers are indicated in black with white circles.

3. General visceral motor fibers are black with white markings.

Nuclei of origin within the brain are as follows:

111 — black = Edinger-Westphal nucleus, origin of general visceral efferent fibers of Oculomotor Nerve

III — cross lines — nucleus of origin of somatic motor fibers of Oculomotor Nerve

IV — cross lines = nucleus of origin of somatic motor fibers of Trochlear Nerve

V — cross hatched = special visceral motor nucleus, origin of special visceral motor fibers of Mandibular division of Trigeminal Nerve

VI — cross lines = nucleus of origin of somatic motor fibers of Abducent Nerve

VII — cross hatched = special visceral motor nucleus of Facial Nerve

VII — black = superior salivatory nucleus (?), origin of general visceral motor fibers of Facial Nerve

IX — cross hatched = origin of special visceral motor fibers of Glossopharyngeal Nerve (this nucleus represents the anterior portion of nucleus ambiguus of Vagus Nerve)

IX — solid black = inferior salivatory nucleus (?), origin of general visceral motor fibers of Glossopharyngeal Nerve

X — cross hatched = nucleus ambiguus or origin of special visceral motor fibers of Vagus Nerve

X — solid black = dorsal motor nucleus, origin of general visceral motor fibers of Vagus Nerve

XI — cross hatched = probable nucleus of origin of special visceral motor fibers of Spinal Accessory Nerve

XII — cross lines = nucleus of origin of somatic motor fibers of Hypoglossal Nerve

(B) Sensory nuclei or nuclei of termination of fifth, seventh, ninth, and tenth cranial nerves, shown along the antero-posterior axis of the vertebrate brain. (The dorso-ventral distribution of these nuclei is presented in Fig. 3551.) The nuclei of termination of the eighth cranial nerve has been omitted. (Figs. A and B are schematized from data supplied by Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders.)

(C) Cutaneous taste-bud branches of the right Facial Nerve in the fish, Ameiurus. (Redrawn from Johnston, 1906, The Nervous System of Vertebrata, Philadelphia, Blakiston, after Herrick.)

(D) Head of the pollack, Pollachius virens, revealing seventh and tenth cranial nerve distribution to lateral line system of the head. (Redrawn from Kingsley, 1912, Comparative Anatomy of Vertebrates, Philadelphia, Blakiston, after Cole.)



2. The Cerebrospinal System

The cerebrospinal system of nerves is composed of the cranial and spinal nerves. Two sets of neurons enter into the composition of the cranial and spinal nerves, viz.:

( 1 ) afferent neurons, whose fibers receive stimuli from certain receptor organs and convey the impulses to the central nervous system, and

(2) efferent neurons, with fibers which convey the impulses from the central nervous system to the peripheral areas. The central nervous system with its multitudes of association neurons thus acts to correlate the incoming impulses from afferent neurons and to shunt them into the correct outgoing pathways through the fibers of the efferent neurons (see figure 358A).

Most of the afferent or sensory neurons are located in ganglia outside of the central nerve tube, within the dorsal root ganglia of the spinal nerves and in the ganglia of the cranial nerves in close association with the brain (fig. 356B). On the other hand, the cell bodies of the somatic efferent or motor fibers are found within the gray matter of the central nerve tube, and the cell bodies of the visceral efferent or motor fibers are located within the gray matter of the central nerve tube and also in peripheral (autonomic) ganglia.

3. General Structure and Function of the Spinal Nerves

In each of the spinal nerves the nerve fibers are of four functional varieties, namely, visceral sensory (afferent); visceral motor (efferent); somatic sensory (afferent); and somatic motor (efferent). The visceral components are distributed to the glands, smooth muscles, etc., of the viscera located within the thoracic and abdominal cavities, together with the blood vessels of the general body areas. The somatic components innervate the body wall tissues including the skin and its appendages. A spinal nerve and its component fibers in the trunk region is shown in figure 358 A, and figure 358B shows this distribution in the region of the brachial plexus.

A typical spinal nerve is composed of the following general parts:

( 1 ) The dorsal or sensory root with its ganglion, and

(2) the ventral or motor root.

(3) Each spinal nerve divides into

(4) a dorsal ramus, and

(5) a ventral ramus. The ventral ramus may divide into

(6) a lateral branch and

(7) a ventral branch. Connecting with the spinal nerve also are

(8) the gray and white rami of the autonomic nervous system.

As the peripheral nerve fibers grow distad they become grouped together to form peripheral nerves. Each nerve in consequence is an association of



bundles or fasicles of fibers surrounded and held together by connective tissue. Most of the peripheral nerve fibers are myelinated. The connective tissue which surrounds a nerve is called the perineurium and that which penetrates inward between the fibers is the endoneurium (fig. 358C).

4. The Origin, Development and Functions of the Cranial Nerves

Consult diagrams, figures 356A and B, also 3551.

O. Terminal

The nervus terminalis is a little understood nerve closely associated with the olfactory nerve. It was discovered by F. Pinkus in 1894, in the dipnoan fish, Protopterus, after the other cranial nerves were described. In consequence it does not have a numerical designation. (Consult Larsell, ’18, for references and discussion.)

I. Olfactory

Arises from bipolar cells located in olfactory epithelium. These cells give origin to fibers which grow into the olfactory bulb to synapse with olfactorybulb neurons (fig. 356B).

Summary of functional components: Special visceral afferent fibers.

II. Optic

The optic nerve arises from neurons located in the retina of the eye. They grow niediad along the lumen of the optic stalk to form the optic nerve. In mammals part of the fibers from the median half of each retina decussate, i.e., cross over, and follow the fibers from the lateral half of the retina of the other eye into the brain (fig. 356B). In birds, however, decussation of the optic nerve fibers is complete, as it is in reptiles and fishes, and probably also in amphibians.

Summary of functional components: Special somatic afferent fibers, cell bodies in the retina. In fishes, there are efferent fibers in the optic nerve controlling, possibly, movements of retinal elements (Arey, ’16, and Arey and Smith, ’37).

III. Oculomotor

The third cranial nerve is composed mainly of somatic motor fibers which originate from neuroblasts in the anterior basal area of the mesencephalon. These fibers grow latero-ventrad from the mesencephalic wall to innervate the premuscle masses of the inferior oblique, inferior, medial and superior rectus muscles of the eyeball (fig. 356A).

Summary of functional components: ( 1 ) Somatic motor fibers controlling eye muscles indicated, (2) general somatic afferent (sensory) fibers, i.e. pro



prioceptive fibers for eye muscle tissue, (3) general visceral efferent fibers. The neuron bodies of the visceral efferent fibers are located in the EdingerWestphal nucleus of mesencephalon. The fibers from these neurons form the preganglionic fibers which terminate in the ciliary ganglion. The postganglionic fibers from cell bodies in ciliary ganglion innervate the intrinsic (smooth) muscles of the ciliary body and iris.

IV. Trochlear

The fourth cranial nerve arises from neuroblasts in the posterior ventral floor of the mesencephalon near the ventral commissure. The fibers grow dorsad and somewhat posteriad within the wall of the mesencephalon to the mid-dorsal line where they emerge to the outside and decussate (i.e. cross), the nerve from one side passing laterad toward the eye of the opposite side where it innervates the developing premuscle mass of the superior oblique muscle (fig. 356A).

Summary of functional components: ( 1 ) Somatic motor fibers controlling superior oblique muscle, (2) general somatic afferent (sensory) fibers, i.e. proprioceptive fibers from eye muscle tissue.

V. Trigeminal

The trigeminal nerve is a complex association of sensory and motor fibers (fig. 356A, B). It has the following divisions:

A. Ophthalmicus or Deep Profundus

Composed of somatic sensory fibers to the snout region. Fibers originate from neuroblasts in the dorso-anterior part of the neural crest cells which give origin to the Gasserian (semilunar) ganglion. This portion of the semilunar ganglion probably should be regarded as a separate and distinct ganglion. One fiber from each bipolar neuroblast grows anteriad toward the snout while the other fiber enters the wall of the metencephalon. These neurons later become unipolar.

Summary of functional components: General somatic afferent (sensory) fibers.

B. Maxillaris

The maxillary ramus of the fifth cranial nerve is composed of somatic sensory fibers from the upper jaw and snout and mucous membranes in these areas. The fibers arise from neuroblasts within the neural crest material which forms the central mass of the semilunar ganglion. One fiber from each bipolar neuroblast grows anteriad toward the snout while the other fiber grows mediad to enter the wall of the metencephalon along with fibers from the ophthalmic and mandibular divisions. These neurons later become unipolar.

Summary of functional components: General somatic afferent (sensory) fibers.



C. Mandibular is

The mandibular ramus is composed of general sensory (afferent) fibers with cell bodies lying in the mesencephalic nucleus of the fifth nerve (see figure 356A). Associated with these sensory fibers are motor fibers (generally spoken of as special visceral motor fibers) distributed to the muscles of mastication. The latter muscles arise from mesoderm associated with the first or mandibular visceral arch. During development the motor fibers arise from a localized mass of neuroblasts lying in the pons of the mctencephalon (see figure 356A), and they emerge from the ventro-lateral aspect of the pons and grow out toward the mandibular arch. Later they become associated with the sensory fibers observed above.

Summary of functional components: ( 1 ) General somatic afferent (sensory) fibers, of the proprioceptive variety, originating in mesencephalic nucleus of the fifth nerve (fig. 356A, B), (2) special visceral efferent (motor) fibers to muscles of mastication from motor nucleus noted above.

VI, Abducens

The word abducens means to lead away, or draw aside. It is applied to the sixth cranial nerve because it innervates the lateral rectus muscle of the eyeball whose function is to pull the eye away or outward from the median line. It is composed almost entirely of somatic efferent (motor) fibers whose origin is within a nucleus lying in the caudo-ventral area of the pons (fig. 356A). In the embryo, ncuroblasts in this area grow outward from the ventro-lateral wall of the pons and forward into the developing premuscle mass of the external (lateral) rectus muscle.

Summary of functional components: (1) Somatic efferent fibers, (2) general somatic afferent fibers, i.e. proprioceptive fibers from the external rectus muscle.

VII. Facial

In higher vertebrates this nerve is composed largely of motor fibers of the special visceral variety innervating the musculature derived from the hyoid visceral arch. As indicated previously (Chap. 16) the muscle tissue of this arch forms the facial (mimetic) and platysma musculature of mammals and the posterior belly of digastric and stylohyoid muscles. In fishes muscle tissue is restricted to the region of the hyoid arch and is concerned with movements of this arch. The motor fibers distributed to the hyoid arch of fishes are located in the hyomandibular branch of the facial nerve (see figure 3571). Aside from these special visceral motor fibers, sensory fibers are present whose cell bodies lie within the geniculate ganglion of the facial nerve. The sensory fibers which innervate some of the taste buds on the anterior two-thirds of the tongue in mammals are special visceral afferent fibers coursing in the chorda tympani nerve, whereas those along the pathway of the facial nerve are



general visceral sensory fibers providing deep sensibility to the general area of distribution of the facial nerve. The special visceral afferent fibers to the taste bud system are prominent elements in the seventh cranial nerve of many fishes (fig. 356C). In fishes also, the seventh cranial nerve contains lateralline components distributed to the lateral-line organs of the head (fig. 356D).

The special motor fibers of the facial nerve arise from neuroblasts located in the pons as indicated in figure 356A, and the general visceral motor fibers take origin from cell bodies in the nucleus salvatorius superior.

Summary of components: (1) Special visceral efferent (motor) fibers to musculature arising in area of hyoid arch, (2) in mammals, preganglionic general visceral efferent fibers by way of chorda tympani nerve to submaxillary ganglion; and from thence, postganglionic fibers to submaxillary and sublingual salivary glands. (3) Special visceral afferent fibers to taste buds on anterior portion of tongue by way of chorda tympani nerve; in fishes, special visceral afferent fibers are extensive. (4) General visceral afferent fibers. (5) In fishes, lateral-line components to head region are present.

VIII. Acoustic

The acoustic nerve contains special somatic sensory components which receive sensations from the special sense organs derived from the otic vesicle. The otic vesicle differentiates into two major structures, viz.: (1) one related to balance or equilibration, and (2) the other concerned with hearing or the detection of wave motions aroused in the external medium. This differentiation is obscure in fishes. However, in those vertebrates which dwell in water other hearing devices may be used aside from those which may involve the developing ear vesicle. One aspect of the mechanism which enables waterdwelling vertebrates to detect pressure or wave motions of low frequency in the surrounding watery medium is the lateral line system associated with the fifth, seventh, ninth and tenth cranial nerves.

In accordance with the differentiation of the otic vesicle into two senseperceiving organs, the sensory neurons of the acoustic ganglion of the eighth cranial nerve become segregated into two ganglia, namely, ( 1 ) the vestibular ganglion containing bipolar neurons which transmit proprioceptive stimuli through the vestibular nerve from the organ of equilibration composed of the utricle, saccule and semicircular canals, and (2) the spiral ganglion containing bipolar neurons which transmit somatic sensations from the spiral or hearing organ (fig. 361H).

Summary of functional components: (1) Special somatic afferent fibers of proprioceptive variety associated with equilibration, (2) special somatic afferent fibers of exteroceptive variety, associated with hearing.

IX. Glossopharyngeal

The glossopharyngeal nerve is associated with the third visceral arch and nearby areas of the pharynx. It has two major components; one of these



components is motor, innervating the musculature derived from the embryonic third visceral arch, while the other component is sensory. The sensory components are derived from neuron bodies within the superior and petrosal ganglia (fig. 356B). Aside from receiving general sense impulses from the pharyngeal area, many of these sensory components are associated with the taste buds on the caudal portion of the tongue. The latter components thus are special sensory components.

The visceral motor (efferent) components to the musculature derived from the third visceral arch arise from neuroblasts located in the ventro-lateral floor of the anterior part of the myelencephalon (fig. 356A). The sensory components take origin from neural crest cells located in the region of the third visceral arch. Fibers from these neuroblasts grow mediad into the nerve tube, and latero-ventrad toward the third visceral arch region.

Summary of functional components: ( 1 ) General visceral afferent fibers with cell bodies in petrosal ganglion whose peripheral fibers terminate in the posterior tongue region and in the pharyngeal area, (2) special visceral afferent fibers with cell bodies in petrosal ganglion whose peripheral fibers contact the taste buds in the posterior third of the tongue, (3) special visceral efferent fibers to musculature derived from the third visceral arch. In mammals, this musculature is the stylopharyngeus muscle, (4) in mammals: general visceral efferent fibers, composed of preganglionic fibers from neurons in inferior salivatory nucleus located probably in the region between the pons and medulla pass to the otic ganglion. Postganglionic fibers from otic ganglion innervate the parotid gland. (5) In fishes: lateral-line components are present and distributed to posterior head region. In mammals, some general somatic afferent fibers from cell bodies in the superior ganglion appear to innervate cutaneous areas in the ear region.

X. Vagus

The tenth cranial or vagus nerve is composed of several functional components. It is a prominent nerve associated with the autonomic nervous system as indicated below. In addition to these autonomic components, the functional components of the tenth cranial nerve are related to the visceral arches caudal to the third visceral arch. The tenth cranial nerve thus supplies several visceral arches. In consequence, it must be regarded as a composite nerve, arising from extensive motor nuclei, the dorsal motor nucleus and the nucleus ambiguus in the ventro-lateral area of the myelencephalon (fig. 356A). The tenth nerve has two main ganglia, the jugular and nodose ganglia. The motor fibers arise from neuroblasts in the nuclei mentioned above and grow out laterally to the visceral arch area, and the sensory components take origin from neuroblasts of neural crest origin which become aggregated in the jugular and nodose ganglia.

Summary of functional components: (1) Special visceral afferent fibers


Fig. 357. External morphological development of various vertebrate brains. (A) Diagram showing the fundamental regional cavities of the primitive five-part vertebrate brain. (B-G) External morphological changes of the developing human brain and cranial nerves. (Redrawti, somewhat modified, from Patten, 1946, Human Embryology, Philadelphia, Blakiston, adapted primarily from Streeter and reconstructions in Carnegie Collection.) (B) 20 somite embryo, probably 3V2 weeks. (C) 4 mm. embryo, about 4 weeks. (D) 8 mm. embryo, about 51/3 weeks. (E) 17 mm. embryo, about 7 weeks. (F) 50-60 mm. embryo, about 11 weeks. The brain now begins to assume the configuration shown by the chick at hatching (see Fig. 347L and M). Roman numerals III, IV, V, VI, VII, IX, X, XI and XII indicate cranial nerves. See Fig. 356A and B for functional components of the cranial nerves at this time. (G) Lateral view of brain at about the ninth month. (H, I, and I') Adult form of the brain of Squalus acanthias. It is to be observed that the brain of Squalus acanthias loses the marked cephalic flexure (see Fig. 347A) present in the early embryo, and assumes a straightened form during the later stages of its development. (H and I ventral and dorsal views, respectively, drawn from dissected specimens; T redrawn and slightly modified from Norris and Hughes, 1919, J. Comp. Neurol., 31.) (J and K) Ventral and dorsal




whose cell bodies lie in nodose ganglion with peripheral terminations in taste buds of pharyngeal area, (2) general visceral afferent fibers whose cell bodies lie in nodose ganglion, with peripheral distribution to pharynx, esophagus, trachea, thoracic and abdominal viscera, (3) general somatic afferent fibers with cell bodies in jugular ganglion and peripheral distribution to external ear region, (4) special visceral efferent fibers to striated musculature of pharyngeal area; cell bodies lie in nucleus ambiguus, (5) general visceral efferent fibers. Preganglionic cell bodies in dorsal motor nucleus; terminate in sympathetic ganglia associated with thoracic and abdominal viscera, (6) in fishes: a prominent lateral line component is present which is distributed along the lateral body wall.

The special visceral motor fibers of the vagus are associated with musculature arising from the caudal visceral arches.

XI. Spinal Accessory

The spinal accessory nerve arises in close association with the vagus. It is composed mainly of motor fibers and distributed to musculature derived from premuscle masses in the caudal branchial area (fig. 356A). They may be regarded as special visceral motor fibers.

Summary of functional components: ( 1 ) Special visceral efferent fibers whose cell bodies lie in nucleus ambiguus and in anterior part of spinal cord and distributed to trapezius, and sternocleidomastoid, muscles and striated muscles of pharynx and larynx, (2) general visceral efferent fibers associated with vagus nerve, with cell bodies in dorsal motor nucleus of vagus.

XII. Hypoglossal Nerve

The twelfth cranial nerve is a somatic motor nerve composed mainly of efferent fibers distributed to the hypobranchial or tongue region. These fibers arise from neuroblasts in an extensive nuclear region from the anterior cervical area along the floor of the myelencephalon near the midventral line (fig. 356A). In lower vertebrates these fibers innervate certain of the anterior trunk myotomes whose muscle fibers travel ventrad into the hypobranchial area. In higher vertebrates the hypoglossal nerve fibers innervate the tongue and associated muscles.

Fig. 357 — Continued

views, respectively, of the adult form of the brain in the frog, Rana cateshiana. Like the developing brain in Squalus, the brain of the developing frog loses its pronounced cephalic flexure as development proceeds. (L and M) Ventral and dorsal views, respectively, of the adult form of brain in the chick shortly before hatching. The cervical, pontine, and cephalic flexures are partly retained in developing brain of chick, and in this respect it resembles the developing mammalian brain. Compare these diagrams with Figs. 354E, 259. (N and O) Ventral and dorsal views, respectively, of the adult brain of the dog. (Redrawn from models.)







posttrematic Rami of the


Fig. 357 — Continued

For legend see p. 832.

Summary of functional components: (1) Somatic motor fibers; (2) somatic sensory, i.e., proprioceptive fibers, from tongue musculature,

5. The Origin and Development of the Autonomic System a. Definition of the Autonomic Nervous System The autonomic nervous system is that part of the peripheral nervous system which supplies the various glands of the body together with the musculature









Fig. 357 — Continued For legend see p. 832.

of the heart, blood vessels, digestive, urinary and reproductive organs, and other involuntary musculature. It differs from the cerebrospinal nerve series in its efferent system of neurons, and not in the afferent system. The latter is composed of ordinary afferent neurons located in the ganglia of the cerebro



Spinal series and these differ from the somatic sensory neurons of the dorsal root ganglia only in that they convey sensations from the viscera instead of the body wall and cutaneous surfaces* On the other hand, the efferent system of neurons is unlike that of the cerebrospinal series in that two neurons are involved in conveying the efferent nerve impulse instead of one as in the cerebrospinal series. The body of one of these two neurons, the preganglionic neuron, lies within the brain or spinal cord, whereas the cell body of the other, the postganglionic neuron, is associated with similar cell bodies within certain aggregations called sympathetic ganglia (fig. 358A). The axons of the postganglionic neurons run to and end in the cardiac and blood vessel musculature, gland tissue and smooth musculature in general throughout the body. According to Ranson, T8, p. 308, “The autonomic nervous system is that functional division of the nervous system which supplies the glands, the heart, and all smooth muscle, with their efferent innervation and includes all general visceral efferent neurones both pre- and postganglionic.”

b. Divisions of the Autonomic Nervous System

There are two main divisions of the autonomic system, viz.:

(1) The thoracicolumbar autonomic system, also called the sympathetic division of the autonomic system, and

(2) The craniosacral autonomic system, also called the parasympathetic division of the autonomic system (see figure 358D).

The thoracicolumbar outflow of efferent fibers has preganglionic fibers which pass from the spinal cord along with the thoracic and upper (anterior) lumbar spinal nerves, whereas the preganglionic fibers of the craniosacral outflow depart from the central nervous system via cranial nerves III, VII, IX, X and XI, and in the II, III and IV sacral nerves.

c. Dual Innervation by Thoracicolumbar and Craniosacral Autonomic


Most structures innervated by the autonomic nervous system receive a double innervation, one from the sympathetic and the other from the parasympathetic division, both, in many instances, having opposite functional effects upon the organ tissue.

Examples of this dual innervation are:

1) Autonomic Efferent Innervation of the Eye. Preganglionic cell bodies in

oculomotor nucleus, fibers passing with nerve III to ciliary ganglion. Postganglionic cell bodies in ciliary ganglion; postganglionic fibers by way of short ciliary nerves to ciliary muscle and circular muscle fibers of iris. Function: Accommodation of eye and decrease in diameter of pupil. The foregoing innervation is a part of the cranio-sacral autonomic outflow. A parallel inner



vation to the iris of the eye occurs through the thoracicolumbar autonomic system as follows:

Cell bodies of preganglionic neurons in intermedio-lateral column of spinal cord, from which preganglionic fibers pass to superior cervical ganglion of autonomic nervous system. Cell bodies of postganglionic fibers lie in the superior cervical ganglion and fibers pass from this ganglion along the internal carotid plexus to the ophthalmic division of the fifth nerve, and from thence along the long ciliary and nasociliary nerves to iris. Function: dilation of the pupil.

2) Autonomic Efferent Innervation of the Heart. Preganglionic cell bodies

in dorsal motor nucleus of vagus in myelencephalon. Fibers pass by way of vagus nerve to terminal (intrinsic) ganglia of the heart. Postganglionic cell bodies in terminal ganglia of heart; postganglionic fibers pass to heart muscle. Function: slows the heart beat. The foregoing represents the craniosacral autonomic or parasympathetic innervation. The corresponding sympathetic innervation is as follows:

Preganglionic cell bodies in intermedio-lateral column of spinal cord; preganglionic fibers pass to superior, middle and inferior cervical ganglia of sympathetic ganglion series. Postganglionic cell bodies in cervical ganglia from which postganglionic fibers pass via cardiac nerves to cardiac musculature.

Function: acceleration of heart beat.

d. Ganglia of the Autonomic System and Their Origin

The ganglia of the autonomic nervous system represent aggregations of the cell bodies of postganglionic neurons; the cell bodies of the preganglionic neurons lie always within the central nervous system. These autonomic ganglia arise from two sources; viz.:

1 ) The neural crest material of the dorsal root ganglion of the spinal nerves and the neural crest material associated with certain cranial nerves, and

2) from cells of the neural tube which migrate from the tube along the forming ventral or efferent nerve roots of the spinal nerves (Kuntz and Batson, ’20).

These migrating neural cells become aggregated to form three sets of ganglia as follows:

1) The sympathetic chain ganglia lying on either side of the vertebral column.

2) The collateral or subvertebral ganglia located between the chain ganglia and the viscera. Examples of collateral ganglia are the coeliac, superior mesenteric and inferior mesenteric ganglia.

3 ) The terminal or intrinsic ganglia lie near or within the organ tissue such

as the ciliary and submaxillary ganglia.



Fig. 358. General structural features of spinal nerves, and of nerve fibers terminating in muscle tissue. (A) Diagrammatic representation of a spinal nerve in the region of the* mammalian diaphragm showing functional components. Three facts are evident relative to the components of a typical spinal nerve, viz., (1) The somatic efferent motor neuron lies within the central nerve tube; its fiber extends peripherad to the effector organ. One neuron therefore is involved in the somatic efferent system (see Fig. 352A). (2) Unlike the somatic efferent system, the visceral efferent (motor) system is composed of a chain of two neurons, a preganglionic neuron whose cell body lies within the central nerve tube, and a postganglionic neuron whose cell body lies in one of the peripheral ganglia. (3) The somatic afferent (sensory) and visceral afferent (sensory) fibers both possess but one neuron whose cell body lies within the dorsal root ganglion. The somatic afferent fiber connects with a sense or receptor organ lying somewhere between the viscera and the external surface (i.e., cutaneous surface) of the body, whereas the visceral afferent fiber contacts the structural makeup of the visceral structures. (B) A spinal nerve in the region of the brachial plexus. The main difference between this type of nerve and the typical spinal nerve resides in the fact that the ventral ramus proceeds into the limb and not into the body wall. Before proceeding into the limb it inosculates with the ventral rami of other nerves to form the brachial plexus. (C) Portion of a transverse section of the sciatic nerve of a newborn showing groups of nerve fibers joined together into bundles. Each nerve-fiber bundle is surrounded by connective tissue, the perineurium, and is partly divided by septa of connective tissue, the endoneuriiim. External to the perineurium is the epineurium, or the connective tissue which holds the entire nerve together (Redrawn from Maximow and Bloom, 1942, A Textbook of Histology, W. B. Saunders Co., Philadelphia, after Schaffer.) (D) Diagram of the autonomic efferent system of neurons and ganglia. The parasympathetic (craniosacral) outflow is shown in heavy black lines with white spaces; the sympathetic (thoracicolumbar) outflow is represented by ordinary black lines. (Adapted from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders, after Meyer and Gottlieb.)

G. cerv. sup. = superior cervical ganglion G. stellatum = inferior cervical or stellate ganglion G. mes. sup. = superior mesenteric ganglion G. mes. inf. = inferior mesenteric ganglion G. pelv. = pelvic ganglion

Neurohumoral substances are produced at the terminal (effector) tips of the various autonomic nerve fibers. A substance similar to adrenalin appears to be produced at the tips of the sympathetic nerves proper, whereas in the case of the parasympathetic fibers the substance is acetylcholine. These humoral substances stimulate the effector structures. (E, F, and G) Nerve endings associated with muscle tissue. (E) Effector (motor) nerve endings associated with cardiac or smooth muscle. Sympathetic motor endings terminate in small swellings. This figure portrays sympathetic motor endings on a smooth muscle cell of an artery of the rabbit’s eye. (Redrawn from Maximow and Bloom, 1942, A Textbook of Histology, Philadelphia, Saunders, after Retzius.) (F) Another example of the termination of sympathetic nerve fiber endings on smooth muscle fibers. In this instance the bronchial musculature is the effector organ. (Redrawn from Maximow and Bloom, 1942, A Textbook of Histology, Philadelphia, Saunders, after Larsell & Dow.) (G and G') Nerve endings in striated muscle. (G redrawn from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders, after Huber & De Witt; G' redrawn from Maximow and Bloom, 1942, A Textbook of Histology, Philadelphia, Saunders, after Boeke.) (G) Represents a neuromuscular end organ of a sensory nerve fiber terminating within a muscle spindle in striated muscle from a dog. These muscle spindles are in the form of a connective tissue capsule which invests spindle-shaped bundles of muscle fibers. Within this capsule, large myelinated nerve fibers terminate in non-myelinated branches which spiral around the muscle fibers or end in flattened discs. (G') Represents a somatic motor (efferent) nerve fiber terminating in a motor plate within a striated muscle fiber. The motor plate is composed of an irregular mass of sarcoplasm below the sarcolemma of the muscle fiber. This motor plate receives the naked terminal ramifications of the nerve fiber.

Fig. 358. (See facing page for legend,) 839

Fig. 359. Types of peripheral sense receptors (see also Fig. 358G). (A) Meissner's

tactile corpuscle. Consists of a thin connective tissue capsule. One or more myelinated nerve fibers enter the . capsule, where the myelin sheaths are lost. These terminating non-myelinated fibers break up into branches which form a complex mass of twisting coils. The coils show varicose enlargements. Found in the dermis of feet, hands, lips, forearms. (B) End-hulb of Krause. Small rounded bodies somewhat resembling Meissner’s corpuscles. Found in lips, conjunctiva, and edge of cornea. (C) Pacinian corpuscle. This type of nerve ending is in the form of a large, oval corpuscle composed of concentric layers of connective tissue. The central axis of the corpuscle receives the




The general arrangement of these ganglia and the autonomic nerve fibers to the spinal nerve series is shown in figure 358A. It is to be observed that only two neurons, a preganglionic and a postganglionic, are involved in the efferent chain regardless of the number of ganglia traversed.

E. The Sense or Receptor Organs

1. Definition

The sense organs are the sentinels of the nervous system. Endowed particularly with that property of living matter known as irritability, they are able to detect changes in the environment and to transmit the stimulus thus aroused to afferent nerve fibers. However, the perceptive ability of all sense organs is not the same, for specific types of sense receptors are developed specialized in the detection of particular environmental changes.

There are two general areas of sensory reception, viz.; (1) The somatic sensory area, and (2) the visceral sensory area. The location of somatic and visceral areas in the myelencephalon are shown in figure 3551.

The somatic sensory organs are associated with the general cutaneous surface of the body and also in tissues within the body wall. Consequently, this area may be divided for convenience into two general fields, namely, (1)

Fig. 359 — Continued

terminal ends of one or more unmyelinated fibers, and also, in addition, the terminal end of a myelinated fiber which loses its myelin as it enters the axial core of the corpuscle. Side branches arise from the central core of nerve fibers. Found in deeper parts of dermis, and also in association with tendons, joints, intermuscular areas as well as in the mesenteries of the peritoneal cavity, and the linings of the pleural and pericardial cavities. (D) Nerve endings in skin and hair follicles. As the myelinated fibers enter the skin they break up into smaller myelinated fibers. After many divisions the myelin sheaths are lost, and finally the neurilemma also disappears. The free nerve endings enter the epidermis and after other divisions form a network of terminal fibers among the epidermal cells. Below the stratum germinativum of the skin, some of the fibers terminate in small, leaf-like enlargements around the hair-follicles below the level of the sebaceous glands. (A-D, redrawn and somewhat modified from Ranson, 1939, The Anatomy of the Nervous System, Philadelphia, Saunders.) (E) Part of longitudinal section of the lateral line canal of a Mustelus “pup” at the level of the first dorsal fin. Observe termination of nerve fibers among groups of sensory hair cells. The lateral line canal communicates with the surface at intervals by means of small tubules. (Redrawn and modified from Johnson, 1917, J. Comp. Neurol., 28.) (F) Transverse section of

lateral line canal, higher magnification, showing termination of nerve endings among the secondary sense (hair) cells. (Redrawn from Johnson, 1917, J. Comp. Neurol., 28.) (G) The lateral line sensory cord is shown growing posteriad within the epidermal pocket of a 21 mm. embryo of Squaliis. (Redrawn from Johnson, 1917, J. Comp. Neurol., 28.) (H) Taste bud of human. (Redrawn from Neal and Rand, 1939, Chordate Anat omy, Philadelphia, Blakiston.) (I) Sagittal section through human nasal cavity depicting nasal conchae (turbinates) and various openings leading off from the lateral wall of the nasal cavity. The olfactory area of the mucous membrane extends over the superior concha and medially over the upper part of the nasal septum. Observe opening of eustachian tube (tuba auditiva).



The exteroceptive or general cutaneous field, having sense organs detecting stimuli at or near the surface of the body, and (2) the proprioceptive field, with sense organs located in the body-wall tissues, such as striated muscles, tendons, joints and the equilibration structures of the internal ear.

The visceral sensory organs receive stimuli from the interoceptive field, that is, the visceral structures of the body.

2. Somatic Sense Organs a. Special Somatic Sense Organs

The visual organs, the ear, and in water-living vertebrates the lateral-line system, are sense organs of the special variety.

/?, General Somatic Sense Organs

These structures are in the form of free nerve endings, terminating among cells and around the roots of hairs, or they are present as encapsulated nerve endings such as the corpuscles of Meissner, end bulbs of Krause, and Pacinian corpuscles (fig. 359A-D).

3. Visceral Sense Organs a. Special Visceral Sense Organs

The taste buds of various sorts, located generally on the tongue, mucous surface of the buccal cavity and pharynx and in some fishes on the external body surface are specialized visceral sense organs (fig. 285E).

In most craniates the paired olfactory organs are exteroceptive in function, although, possibly, olfactory organs may be regarded as primitively interoceptive. The olfactory organ is regarded generally as a special visceral sense organ.

b. General Visceral Sense Organs

General visceral sense organs are located among the viscera of the body. They represent free-nerve endings lying in the walls of the digestive tract and other viscera. They respond to mechanical stimuli.

4. The Lateral-line System

The lateral-line organs are a specialized series of organs located in the cutaneous areas of the body. They are found in fishes and water-living amphibia. A sense organ of the lateral-line system is composed of a patch of hair cells or neuromasts, columnar in shape, possessing cilia-like extensions at the free end (fig. 359E). Basally the hair cells are associated with the terminal fibrillae of sensory nerves. The hair cells are supported by elongated, sustentacular elements. In cyclostomous fishes the neuromasts are exposed to the surface, but in Gnathostomes they lie embedded within a canal system

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