McMurrich1914 Chapter 15
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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Chapter XvV The Development of the Nervous System
The Histogenesis of the Nervous System
The entire central nervous system is derived from the cells lining the medullary groove, whose formation and conversion into the medullary canal has already been described (p. 72). When the groove is first formed, the cells lining it are somewhat more columnar in shape than those on either side of it, though like them they are arranged in a single layer; later they increase by mitotic division and arrange themselves in several layers, so that the ectoderm of the groove becomes very much thicker than that of the general surface of the body. At the same time the cell boundaries, which were originally quite distinct, gradually disappear, the tissue becoming a syncytium. While its tissue is in this condition the lips of the medullary groove unite, and the subsequent differentiation of the canal so formed differs somewhat in different regions, although a fundamental plan may be recognized. This plan is most readily perceived in the region which becomes the spinal cord, and may be described as seen in that region.
Throughout the earlier stages, the cells lining the inner wall of the medullary tube are found in active proliferation, some of the cells so produced arranging themselves with their long axes at right angles to the central canal (Fig. 227), while others, whose destiny is for the most part not yet determinable, and which therefore may be termed indifferent cells are scattered throughout the syncytium. At this stage a transverse section of the medullary tube shows it to be composed of two well-defined zones, an inner one immediately surrounding the central canal and composed of the indifferent cells and the bodies of the inner or ependymal cells, and an outer one consisting of branched prolongations of the syncytial cytoplasm. This outer layer is termed the marginal velum (Randschleier) (Fig. 227, m). The indifferent cells now begin to wander outward to form a definite layer, termed the mantle layer, lying between the marginal velum and the bodies of the ependymal cells (Fig. 228), and when this layer has become well established the cells composing it begin to divide and to differentiate into (1) cells termed neuroblasts, destined to become nerve-cells, and (2) others which appear to be supportive in character and are termed neuroglia cells (Fig. 228, B).
Fig. 227. - Transverse Section through the Spinal Cord of a Pig Embryo of 30 mm., the Upper Part showing the Appearance produced by the Silver Method of Demonstrating the Neuroglia Fibers.
a, Ependyma of floor plate; b, boundary between mantle layer and marginal zone; cs, mesenchymal connective- tissue syncytium; ep, ependymal cells; i, ingrowth of connective tissue; m, marginal velum; mn, mantle layer; mv, mantle layer of floor plate; p, pia mater; r, neuroglia fibers. - (Hardesty.) The latter are for the" most part small and are scattered among the neuroblasts, these, on the other hand, being larger and each early developing a single strong process which grows out into the marginal velum and is known as an axis-cylinder. At a later period the neuroblasts also give rise to other processes, termed dendrites, more slender and shorter than the axis-cylinders, branching repeatedly, and, as a rule, not extending beyond the limits of the mantle layer. In connection with the neuroglia cells peculiar neuroglia fibrils develop very much in the same way as the fibers are formed in mesenchymal connective tissue. That is to say, they are formed from the peripheral portions of the cytoplasm of the neuroglial and ependymal cells. But since these cells are connected i together to form a syncytium the fibrils are not confined to the territories of the individual cells, but may extend far beyond these, passing in the syncytium from the territory of one neuroglial cell to another, many of those, indeed, arising in connection with the ependymal cells extending throughout the entire thickness of the medullary wall (Fig. 227). The fibrils branch abundantly and form a supportive network extending through all portions of the central nervous system. The axis-cylinder processes of the majority of the neuroblasts on reaching the marginal velum bend upward or downward and, after traversing a greater or less length of the cord, re-enter the mantle layer and terminate by dividing into numerous short branches which come into relation with the dendrites of adjacent neuroblasts. The processes of certain cells situated in the ventral region of the mantle zone pass, however, directly through the marginal velum out into the surrounding tissues and constitute the ventral nerveroots (Fig. 231).
Fig. 228. - Diagrams showing the Development of the Mantle Layer in the Spinal Cord. The circles, indifferent cells; circles with dots, neuroglia cells; shaded cells, germinal cells; circles with cross, germinal cells in mitosis; black cells, nerve-cells. - (Schaper.)
The dorsal nerve-roots have a very different origin. In embryos of about 2.5 mm., in which the medullary canal is only partly closed (Fig. 53), the cells which lie along the line of transition between the lips of the groove and the general ectoderm form a distinct ridge readily recognized in sections and termed the neural crest (Fig. 229, A). When the lips of the groove fuse together the cells of the crest unite to form a wedge-shaped mass, completing the closure of the canal (Fig. 229, B), and later proliferate so as to extend outward over the surface of the canal (Fig. 229, C). Since this proliferation is most active in the regions of the crest which correspond to the mesodermic somites there is formed a series of cell masses, arranged segmentally and situated in the mesenchyme at the sides of the medullary canal (Fig. 214). These cell masses represent the dorsal root ganglia, and certain of their constituent cells, which may also be termed neuroblasts, early assume a fusiform shape and send out a process from each extremity. One of these processes, the axiscylinder, grows inward toward the medullary canal and penetrates its
Fig. 229. - Three Sections through the Medullary Canal of an Embryo of 2.5 mm. - (vonLenhossek.)
marginal velum, and, after a longer or shorter course in this zone, enters the mantle layer and comes into contact with the dendrites of some of the central neuroblasts. The other process extends peripherally and is to be regarded as an extremely elongated dendrite. The processes from the cells of each ganglion aggregate to form a nerve, that formed by the axis-cylinders being the posterior root of a spinal nerve, while that formed by the dendrites soon unites with the ventral nerve-root of the corresponding segment to form the main stem of a spinal nerve.
There is thus a very important difference in the mode of development of the two nerve-roots, the axis-cylinders of the ventral roots arising from cells situated in the wall of the medullary canal and growing outward (centrif ugally) , while those of the dorsal root spring from cells situated peripherally and grow inward (centripetally) toward the medullary canal. In the majority of the dorsal root ganglia the points of origin of the two processes of each bi-polar cell gradually approach one another (Fig. 230, b) and eventually come to rise from a common stem, a process of the cell-body, which thus assumes a characteristic T form (Fig. 230, d).
Fig. 230. - Cells from the Gasserian Ganglion of a Guinea-pig Embryo. a, Bipolar cell; b and c, transitional stages to d, T-shaped cells. - (von Gehuchten.)
From what has been said it will be seen that each axis-cylinder is an outgrowth from a single neuroblast and is part of its cell-body, as are also the dendrites. Another view has, however, been advanced to the effect that the nerve fibers first appear as chains of cells and that the axiscylinders, being differentiated from the cytoplasm of the chains, are really multicellular products. Many difficulties stand in the way of the acceptance of this view and recent observations, both histogenetic (Cajal) and experimental (Harrison), tend to confirm the unicellular origin of the axis-cylinders. The embryological evidence therefore goes to support the neurone theory, which regards the entire nervous system as composed of definite units, each of which corresponds to a single cell and is termed a neurone.
By the development of the axis-cylinders which occupy the meshes of the marginal velum, that zone increases in thickness and comes to consist principally of nerve-fibers, while the cell-bodies of the neurones of the cord are situated in the mantle zone. No such definite distinction of color in the two zones as exists in the adult is, however, noticeable until a late period of development, the medullary sheaths, which give to the nerve-fibers their white appearance not beginning to appear until the fifth month and continuing to form from that time onward until after birth. The origin of the myelin which composes the medullary sheaths is as yet uncertain, although the more recent observations tend to show that it is picked out from the blood and deposited around the axis-cylinders in some manner not yet understood. Its appearance is of importance as being associated with the beginning of the functional activity of the nerve-fibers.
In addition to the medullary sheaths the majority of the fibers of the peripheral nervous system are provided with primitive sheaths, which are lacking, however, to the fibers of the central system. They are formed by cells which wander out from the dorsal root-ganglia and are therefore of ectodermal origin. Frog larvae deprived of their neural crests at an early stage of development produce ventral nerve-fibers altogether destitute of primitive sheaths (Harrison).
Various theories have been advanced to account for the formation of the medullary sheaths. It has been held that the myelin is formed at the expense of the outermost portions of the axis-cylinders themselves (von Kolliker), and on the other hand, it has been regarded as an excretion of the cells which compose the primitive sheaths surrounding the fibers (Ranvier) , a theory which is, however, invalidated by the fact that myelin is formed around the fibers of the central nervous system which possess no primitive sheaths. As stated above, the more recent observations (Wlassak) indicate its exogenous origin.
It has been seen that the central canal is closed in the mid-dorsal line by a mass of cells derived from the neural crest. These cells do not take part in the formation of the mantle layer, but become completely converted into ependymal tissue, and the same is true of the cells situated in the mid-ventral line of the canal. In these two regions, known as the roof -plate and floor -plate respectively, the wall of the canal has a characteristic structure and does not share to any great extent in the increase of thickness which distinguishes the other regions (Fig. 231). In the lateral walls of the canal there is also noticeable a differentiation into two regions, a dorsal one standing in relation to the ingrowing fibers from the dorsal root ganglia and known as the dorsal zone, and a ventral one, the ventral zone, similarly related to the ventral nerve-roots. In different regions of the medullary tube these zones, as well as the roof- and floor-plates, undergo different degrees of development, producing peculiarities which may now be considered.
Trie Development of the Spinal Cord
Even before the lips of the medullary groove have met a marked enlargement of the anterior portion of the canal is noticeable, the region which will become the brain being thus distinguished from the more posterior portion which will be converted into the spinal cord. When the formation of the mesodermic somites is completed, the spinal cord terminates at the level of the last somite, and in this region still retains its connection with the ectoderm of the dorsal surface of the body; but in that portion of the cord which is posterior to the first coccygeal segment the histological differentiation does not proceed beyond the stage when the walls consist of several layers of similar cells, the formation of neuroblasts and nerve-roots ceasing with the segment named. After the fourth month the more differentiated portion elongates at a much slower rate than the surrounding tissues and so appears to recede up the spinal canal, until its termination is opposite the second lumber vertebra. The less differentiated portion, which retains its connection with the ectoderm until about the fifth month, is, on the other hand, drawn out into a slender filament whose cells degenerate during the sixth month, except in its uppermost part, so that it comes to be represented throughout the greater part of its extent by a thin cord composed of pia mater. This cord is the structure known in the adult as the filum terminate, and lies in the center of a leash of nerves occupying the lower part of the spinal canal and termed the cauda equina. The existence of the cauda is due to the recession of the cord which necessitates for the lower lumbar, sacral and coccygeal nerves, a descent through the spinal canal for a greater or less distance, before they can reach the intervertebral foramina through which they make their exit.
In the early stages of development the central canal of the cord is quite large and of an elongated oval form, but later it becomes somewhat rhomboidal in shape (Fig. 231, A), the lateral angles marking the boundaries between the dorsal and ventral zones. As development proceeds the sides of the canal in the dorsal region gradually approach one another and eventually fuse, so that this portion of the canal becomes obliterated (Fig. 231, B) and is indicated by the dorsal longitudinal fissure in the adult cord, the central canal of which corresponds to the ventral portion only of the embryonic cavity. While this process has been going on both the roofand the floor-plate have become depressed below the level of the general surface of the cord, and by a continuance of the depression of the floor-plate - a process really due to the enlargement and consequent bulging of the ventral zone - the anterior median fissure is produced, the difference between its shape and that of the dorsal fissure being due to the difference in its development.
The development of the mantle layer proceeds at first more rapidly in the ventral zone than in the dorsal, so that at an early stage (Fig. 231, A) the anterior column of gray matter is much more pronounced, but on the development of the dorsal nerve-roots the formation of neuroblasts in the dorsal zone proceeds apace, resulting in the formation of a dorsal column. A small portion of the zone, situated between the point of entrance of the dorsal nerve-roots and the roof-plate, fails, however, to give rise to neuroblasts and is entirely converted into ependyma. This represents the future funiculus gracilis (fasciculus of Goll) (Fig. 231, A, cG), and at the point of entrance of the dorsal roots into the cord a well-marked oval bundle of fibers is formed (Fig. 231, A, ob) which, as develop
Fig. 231. - Transverse Sections through the Spinal Cords of Embryos .of (A) about Four and a Half Weeks and (B) about Three Months'. cB, Fasciculus of Burdach; cG, fasciculus of Goll; dh, dorsal column; dz, dorsal zone; fp, floor-plate; ob, oval bundle; rp, roof-plate; vh, ventral column; vz, ventral zone. - (His.) ment proceeds, creeps dorsally over the surface of the dorsal horn until it meets the lateral surface of the fasciculus of Goll, and, its further progress toward the median line being thus impeded, it insinuates itself between that fasciculus and the posterior horn to form the funiculus cuneatus (fasciculus of Burdach) (Fig. 231, B, cB).
Little definite is as yet known concerning the development of the other fasciculi which are recognizable in the adult cord, but it seems certain that the lateral and anterior cerebro-spinal (pyramidal) fasciculi are composed of fibers which grow downward in the meshes of the marginal velum from neuroblasts situated in the cerebral cortex, while the cerebellospinal (direct cerebellar) fasciculi and the fibers of the ground-bundles have their origin from cells of the mantle layer of the cord.
The myelination of the fibers of the spinal cord begins between the fifth and sixth months and appears first in the funiculi cuneati, and about a month later in the funiculi graciles. The myelination of the great motor paths, the lateral and anterior cerebro-spinal fasciculi, is the last to develop, appearing toward the end of the ninth month of fetal life.
The Development of the Brain
The enlargement of the anterior portion of the medullary canal does not take place quite uniformly, but is less along two transverse lines than else where, so that the brain region early becomes divided into three primary vesicles which undergo further differentiation as follows. Upon each side of the anterior vesicle an evagination appears and becomes converted into a club-shaped structure attached to the ventral portion of the vesicle by a pedicle. These evaginations (Fig. 232, op) are known as the optic evaginations, and being concerned in the formation of the eye will be considered in the succeeding chapter. After their formation the antero-lateral portions of the vesicle become bulged out into two protuberances (h) which rapidly increase in size and give rise, eventually to the two cerebral hemispheres, which form, together with the portion of the vesicle which lies between them, what is termed the telencephalon or forebrain, the remainder of the vesicle giving rise to what is known as the diencephalon or Hween-brain (Fig. 232, /). The middle vesicle is bodily converted into the mesencephalon or mid-brain (m), but the posterior vesicle differentiates so that three parts may be recognized : (1) a rather narrow portion which immediately succeeds the midbrain and is termed the isthmus (i); (2) a portion whose roof and floor give rise to the cerebellum and pons respectively, and which is termed the metencephalon or hind-brain (mi) ; and (3) a terminal portion which is known as the medulla oblongata, or, to retain a consistent nomenclature, the myelencephalon or after-brain (my). From each of these six divisions definite structures arise whose relations to the secondary divisions and to the primary vesicles may be understood from the following table and from the annexed figure (Fig. 233), which represents a median longitudinal section of the brain of a fetus of three months.
Fig. 232. - Reconstruction of the Brain of an Embryo of 2.15 mm
h, Hemisphere; i, isthmus; m, mesencephalon; mf, mid-brain flexure; mt, metencephalon ; myl, myelencephalon; nf, nape flexure; ot, otic capsule; op, optic evagination; t, diencephalon. - (His.)
Medulla oblongata (I) .
/ Pons (II 1).
^ Cerebellum (II 2).
SBrachia conjunctiva (III). Cerebral peduncles (posterior portion) .
2nd Vesicle Mesencephalon
Cerebral peduncles (anterior portion) (IV 1). Corpora quadrigemina (IV 2).
1st Vesicle <
Pars mammillaris (V 1). Thalamus (V 2). Epiphysis (V 3).
Infundibulum (VI 1). Corpus striatum (VI 2). Olfactory bulb (VI 3). Hemispheres (VI 4).
But while the walls of the primary vesicles undergo this complex differentiation, their cavities retain much more perfectly their original relations, only that of the first vesicle sharing to any great extent the modifications of the walls.
The cavity of the third vesicle persists in the adult as the fourth ventricle, traversing all the subdivisions of the vesicle; that of the second, increasing but little in height and breadth, constitutes the aquaductus cerebri; while that of the first vesicle is continued into the cerebral hemispheres to form the lateral ventricles, the remainder of it constituting the third ventricle, which includes the cavity of the median portion of the telencephalon as well as the entire cavity of the diencephalon.
Fig. 233. - Median Longitudinal Section of the Brain of an Embryo of the Third Month. - (His.)
During the differentiation of the various divisions of the brain certain flexures appear in the roof and floor, and to a certain extent correspond with those already described as occurring in the embryo. The first of these flexures to appear occurs in the region of the midbrain, the first vesicle being bent ventrally until it comes to lie at practically a right angle with the axis of the mid-brain. This may be termed the mid-brain flexure (Fig. 232, mf) and corresponds with the head-bend of the embryo. The second flexure occurs in the region of the medulla oblongata and is known as the nape flexure (Fig. 232, nf); it corresponds with the similarly named bend of the embryo and is produced by a bending ventrally of the entire head, so that the axis of the mid-brain comes to lie almost at right angles with that of the medulla and that of the first vesicle parallel with it. Finally, a third flexure occurs in the region of the metencephalon and is entirely peculiar to the nervous system; it consists of a bending ventrally of the floor of the hind-brain, the roof of this portion of the brain not being affected by it, and it may consequently be known as the pons flexure (Fig. 233).
In the later development the pons flexure practically disappears, owing to the development in this region of the transverse fibers and nuclei of the pons, but the mid-brain and nape flexures persist, though greatly reduced in acuteness, the axis of the anterior portion of the adult brain being inclined to that of the medulla at an angle of about 134 degrees.
The Development of the Myelencephalon
In its posterior portion the myelencephalon closely resembles the spinal cord and has a very similar development. More anteriorly, however, the roof-plate (Fig. 234, rp) widens to form an exceedingly thin membrane, the posterior velum; with the broadening of the roof-plate there is associated a broadening of the dorsal portion of the brain cavity, the dorsal and ventral zones bending outward, until, in the anterior portion of the after-brain, the margins of the dorsal zone have a lateral position, and are, indeed, bent ventrally to form a reflected lip (Fig. 234, I). The portion of the fourth ventricle contained in this division of the brain becomes thus converted into a broad shallow cavity, whose floor is formed by the ventral zones separated in the median line by a deep groove, the floor of which is the somewhat thickened floor-plate. About the fourth month there appears in the roof-plate a transverse groove into which the surrounding mesenchyme dips, and, as the groove deepens in later stages, the mesenchyme contained within it becomes converted into blood-vessels, forming the chorioid plexus of the fourth ventricle, a structure which, as may be seen from its development, does not lie within the cavity of the ventricle, but is separated from it by the portion of the roofplate which forms the floor of the groove.
In embryos of about 9 mm. the differentiation of the dorsal and ventral zones into ependymal and mantle layers is clearly visible (Fig. 234), and in the ventral zone the marginal velum is also well developed. Where the fibers from the sensory ganglion of the vagus nerve enter the dorsal zone an oval area (Fig. 234, fs) is to be seen which is evidently comparable to the oval bundle of the cord and consequently with the fasciculus of Burdach. It gives rise to the solitary fasciculus of adult anatomy, and in embryos of 11 to 13 mm. it becomes covered in by the fusion of the reflected lip of the dorsal zone with the sides of the myelencephalon, this fusion, at the same time, drawing the margins of the roof-plate ventrally to form a secondary lip (Fig. 235). Soon after this a remarkable migration ventrally of neuroblasts of the dorsal zone begins. Increasing rapidly in number the migrating cells pass on either side of the solitary fasciculus toward the territory of the ventral zone, and, passing ventrally to the ventral portion of the mantle layer, into which fibers have penetrated and which becomes the formatio reticularis (Fig. 235, fr), they differentiate to form the olivary body (ol).
Fig. 234. - Transverse Section through the Medulla Oblongata of an Embryo of 9.1 mm.
dz, Dorsal zone; fp, floor-plate; /s, fasciculus solitarius; I, lip; rp, roof-plate; vz, ventral zone; X and XII, tenth and twelfth nerves. - (His.)
The thickening of the floor-plate gives opportunity for fibers to pass across the median line from one side to the other, and this opportunity is taken advantage of at an early stage by the axis-cylinders of the neuroblasts of the ventral zone, and later, on the establishment of the olivary bodies, other fibers, descending from the cerebellum, decussate in this region to pass to the olivary body of the opposite side. In the lower part of the medulla fibers from the neuroblasts of the nuclei gracilis and cuneatus, which seem to be developments from the mantle layer of the dorsal zone, also decussate in the substance of the floor-plate; these fibers, known as the arcuate fibers, pass in part to the cerebellum, associating themselves with fibers ascending from the spinal cord and with the olivary fibers to form a round bundle situated in the dorsal portion of the marginal velum and known as the restiform body (Fig. 235, tr).
Fig. 235. - Transverse Section through the Medulla Oblongata of an Embryo of about Eight Weeks.
av, Ascending root of the trigeminus ;fr, reticular formation; ol, olivary body; sf, solitary fasciculus; tr, restiform body; XII, hypoglossal nerve. - (His.)
The principal differentiations of the zones of the myelencephalon may be stated in tabular form as follows:
Roof-plate Posterior velum.
(Nuclei of termination of sensory roots of cranial nerves. Nuclei gracilis and cuneatus. The olivary bodies.
. ( Nuclei of origin of the motor roots of cranial nerves.
The reticular formation.
Foor-plate The median raphe.
The Development of the Metencephalon and Isthmus
Our knowledge of the development of the metencephalon, isthmus, and mesencephalon is by no means as complete as is that of the myelencephalon. The pons develops as a thickening of the portion of the brain floor which forms the anterior wall of the pons flexure, and its transverse fibers are well developed by the fourth month (Mihalkovicz), but all details regarding the origin of the pons nuclei are as yet wanting. If one may argue from what occurs in the myelencephalon, it seems probable that the reticular formation of the metencephalon is derived from the ventral zone, and that the median raphe represents the floor-plate. Furthermore, the relations of the pons nuclei to the reticular formation on the one hand, and its connection by means of the transverse pons fibers with the cerebellum on the other, suggest the possibility that they may be the metencephalic representatives of the olivary bodies and are formed by a migration ventrally of neuroblasts from the dorsal zones, such a migration having been observed to occur (Essick).
Fig. 236. - A, Dorsal View of the Brain or a Rabbit Embryo of 16 mm.; B, Median Longitudinal Section of a Calf Embryo of 3 cm. c, Cerebellum; m, mid-brain. - (Mihalkovicz?)
The cerebellum is formed from the dorsal zones and roof-plate of the metencephalon and is a thickening of the tissue immediately anterior to the front edge of the posterior velum. This latter structure has in early stages a rhomboidal shape (Fig. 236, A) which causes the cerebellar thickening to appear at first as if composed of two lateral portions inclined obliquely toward one another. In reality, however, the thickening extends entirely across the roof of the brain (Fig. 236, B), the roof-plate probably being invaded by cells from the dorsal zones and so giving rise to the vermis, while the lobes are formed directly from the dorsal zones. During the second month a groove appears on the ventral surface of each lobe, marking out an area which becomes the flocculus, and later, during the third month, transverse furrows appear upon the vermis dividing it into five lobes, and later still extend out upon the lobes and increase in number to produce the lamellate structure characteristic of the cerebellum.
The histogenetic development of the cerebellum at first proceeds along the lines which have already been described as typical, but after the development of the mantle layer the cells lining the greater portion of the cavity of the ventricle
rease to rrmltinlv onlv those
FlG - 237- - Diagram Representing the cease to multiply, oniy tnose DifferenT iation of the Cerebellar Cells.
which are situated in the roof- The circles, indifferent cells; circles with plate of the metencephalon d Â°f ' n< r ur . g lia c f s > shaded c ? lls : gâ„¢. al 1 r cells; circles with cross, germinal cells in and along the line of junction mitosis; black cells, nerve-cells. L, Lateral
recess; M, median furrow, and R, floor of IV, of the cerebellar thickening fourth ven tricle. - (Schaper.) with the roof-plate continuing to divide. The indifferent cells formed in these regions migrate outward from the median line and forward in the marginal velum to form a superficial layer, known as the epithelioid layer, and cover the entire surface of the cerebellum (Fig. 237). The cells of this layer, like those of the mantle, differentiate into neuroglia cells and neuroblasts, the latter for the most part migrating centrally at a later stage to mingle with the cells of the mantle layer and to become transformed into the granular cells of the cerebellar cortex. The neuroglia cells remain at the surface, however, forming the principal constituent of the outer or, as it is now termed, the molecular layer of the cortex, and into this the dendrites of the Purkinje cells, probably derived from the mantle layer, project. The migration of the neuroblasts of the epithelial layer is probably completed before birth, at which time but few remain in the molecular layer to form the stellate cells of the adult. The origin of the dentate and other nuclei of the cerebellum is at present unknown, but it seems probable that they arise from cells of the mantle layer.
The nerve-fibers which form the medullary substance of the cerebellum do not make their appearance until about the sixth month, when they are to be found in the ependymal tissue on the inner surface of the layer of granular cells. Those which are not commissural or associative in function converge to the line of junction of the cerebellum with the pons, and there pass into the marginal velum of the pons, myelencephalon, or isthmus as the case may be.
The dorsal surface of the isthmus is at first barely distinguishable from the cerebellum, but as development proceeds its roof-plate undergoes changes similar to those occurring in the medulla oblongata and becomes converted into the anterior velum. In the dorsal portion of its marginal velum fibers passing to and from the cerebellum appear and form the brachia conjunctiva, while ventrally fibers, descending from the more anterior portions of the brain, form the cerebral peduncles. Nothing is at present known as to the history of the gray matter of this division of the brain, although it may be presumed that its ventral zones take part in the formation of the tegmentum, while from its dorsal zones the nuclei of the brachia conjunctiva are possibly derived.
The following table gives the origin of the principal structures of the metencephalon and isthmus: Metencephalon. Isthmus.
/ Posterior velum. Anterior velum.
^ Vermis of cerebellum.
Lobes of cerebellum. Brachia conjunctiva.
Nuclei of termination of sensory roots of cranial nerves. Pons nuclei.
f Nuclei of origin of motor Posterior part of cerebral Ventral zones -j roots of cranial nerves. peduncles.
[ Reticular formation. Posterior part of tegmentum.
Floor-plate Median raphe. Median raphe.
The Development of the Mesencephalon
Our knowledge of the development of this portion of the brain is again very imperfect. During the stages when the flexures of the brain are well marked (Figs. 232 and 233) it forms a very prominent structure and possesses for a time a capacious cavity. Later, however, it increases in size less rapidly than adjacent parts and its walls thicken, the roofand floor-plates as well as the zones, and, as a result, the cavity becomes the relatively smaller canal-like cerebral aquaeduct. In the marginal velum of its ventral zone fibers appear at about the third month, forming the anterior portion of the cerebral peduncles, and, at the same time, a median longitudinal furrow appears upon the dorsal surface, dividing it into two lateral elevations which, in the fifth month, are divided transversely by a second furrow and are thus converted from corpora bigemina (in which form they are found in the lower vertebrates) into corpora quadrigemina.
Nothing is known as to the differentiation of the gray matter of the dorsal and ventral zones of the mid-brain. From the relation of the parts in the adult it seems probable that in addition to the nuclei of origin of the oculomotor and trochlear nerves, the ventral zones give origin to the gray matter of the tegmentum, which is the forward continuation of the reticular formation. Similarly it may be supposed that the corpora quadrigemina are developments of the dorsal zones, as may also be the red nuclei, whose relations to the brachia conjunctiva suggest a comparison with the olivary bodies and the nuclei of the pons.
A tentative scheme representing the origin of the mid-brain structures may be stated thus: Roof -plate (?) J Corpora quadrigemina. .LJorsal zones. ...... \ .
^ Red nuclei.
[ Nuclei of origin of the third and fourth nerves. Ventral zones \ Anterior part of tegmentum.
[ Anterior part of cerebral peduncles. Floor-plate Median raphe.
The Development of the Diencephalon
A transverse section through the diencephalon of an embryo of about five weeks (Fig. 238) shows clearly the differentiation of this portion of the brain into the typical zones, the roof-plate (rp) being represented by a thinwalled, somewhat folded area, the floor-plate (fp) by the tissue forming the floor of a well-marked ventral groove, while each lateral wall is divided into a dorsal and ventral zone by a groove known as the sulcus Monroi (Sm), which extends forward and ventrally toward the point of origin of the optic evagination (Fig. 240). At the posterior end of the ridge-like elevation which represents the roof-plate is a rounded elevation (Fig. 239, p) which, in later stages, elongates until it almost reaches the dermis, forming a hollow evagination of the brain roof known as the pineal process. The distal extremity of this process enlarges to a sac-like structure which later beFig. 238. - Transverse Section comes lobed, and, by an active pro of the Diencephalon of an Em- Hferation of the cells lining the cavibryo of Five Weeks. ^ dz, Dorsal zone; fp, floor-plate; tieS Â° f the various lobes, finally be
rp, roof-plate; Sm, sulcus Monroi vz, ventral zone. - (His.)
comes a solid structure, the pineal body.
The more proximal portion of the evagination, remaining hollow, forms the pineal stalk, and the entire structure, body and stalk, constitutes what is known as the epiphysis.
The significance of this organ in the Mammalia is doubtful. In the Reptilia and other lower forms the outgrowth is double, a secondary outgrowth arising from the base or from the anterior wall of the primary one. This anterior evagination elongates until it reaches the dorsal epidermis of the head, and, there expanding, develops into an unpaired eye, the epidermis which overlies it becoming converted into a transparent cornea. In the Mammalia this anterior process does not develop and the epiphysis in these forms is comparable only to the posterior process of the Reptilia.
In addition to the epiphysial evaginations, another evagination arises from the roof-plate of the first brain vesicle, further forward, in the region which becomes the median portion of the telencephalon. This paraphysis as it has been called, has been observed in the lower vertebrates and in the Marsupials (Selenka), but up to the present has not been found in other groups of the Mammalia. It seems to be comparable to a chorioid plexus which is evaginated from the brain surface instead of being invaginated as is usually the case. There is no evidence that a paraphysis is developed in the human brain.
The portion of the roof-plate which lies in front of the epiphysis represents the velum interpositum of the adult brain, and it forms at first a distinct ridge (Fig. 239, rp). At an early stage, however, it becomes reduced to a thin membrane upon the surface of which bloodvessels, developing in the surrounding mesenchyme, arrange themselves at about the third month in two longitudinal plexuses, which, with the subjacent portions of the velum, become invaginated into the â€ž â€ž, riG. 239. - Dorsal View of the cavity of the third ventricle to form Brain, the Roof of the Lateral its chorioid Mexu* Ventricles being Removed, of an us cnomoia plexus. Embryo of 13.6 mm.
The dorsal zones thicken in b, Superior brachiuui; eg, lateral their more dorsal and anterior S eniculate . bod y; C P> chorioid plexus; tneir more aorsai ana anterior cqa> anterior corpu3 quadrigeminum; portions to form massive Structures, h > hippocampus; hf, hippocampal fis, 7 7 . , sure; ot, thalamus; p, pineal body; rp, the thatami [rigs. 233, V2, and roof-plate. - (Aw.) 239, ot), which, encroaching upon the cavity of the ventricle, transform it into a narrow slit-like space, so narrow, indeed, that at about the fifth month the inner surfaces of the two thalami come in contact in the median line, forming what is known as the intermediate mass. More ventrally and posteriorly another thickening of the dorsal zone occurs, giving rise on each side to the pulvinar of the thalamus and to a lateral geniculate body, and two ridges extending backward and dorsally from the latter structures to the thickenings in the roof of the midbrain which represent the anterior corpora quadrigemina, give a path along which the nerve-fibers which constitute the superior quadrigeminal brachia pass.
From the ventral zones what is known as the hypothalamic region develops, a mass of fibers and cells whose relations and development are not yet clearly understood, but which may be regarded as the forward continuation of the tegmentum and reticular formation. In the median line of the floor of the ventricle an unpaired thickening appears, representing the corpora mamillaria, which during the third month becomes divided by a median furrow into two rounded eminences; but whether these structures and the posterior portion of the tuber cinereum, which also develops from this region of the brain, are. derivatives of the ventral zones or of the floor-plate is as yet uncertain.
Assuming that the mamillaria and the tuber cinereum are derived from the ventral zones, the origins of the structures formed from the walls of the diencephalon may be tabulated as follows:
Roof-plate (_ Epiphysis.
(Thalami. Pulvinares. Lateral geniculate bodies. (Hypothalamic region. Corpora mamillaria. Tuber cinereum (in part) . Floor-plate Tissue of mid-ventral line.
The Development of the Telencephalon
For convenience of description the telencephalon may be regarded as consisting of a median portion, which contains the anterior part of the third ventricle, and two lateral outgrowths which constitute the cerebral hemispheres. The roof of the median portion undergoes the same transformation as does the greater portion of that of the diencephalon and is converted into the anterior part of the velum interpositum (Fig. 240, vi). Anteriorly this passes into the anterior wall of the third ventricle, the lamina terminalis (It), a structure which is to be regarded as formed by the union of the dorsal zones of opposite sides, since it lies entirely dorsal to the anterior end of the sulcus Monroi. From the ventral part of the dorsal zones the optic evaginations are formed, a depression, the optic recess (or), marking their point of origin.
The ventral zones are but feebly developed, and form the anterior part of the hypothalamic region, while at the anterior extremity of the floor-plate an evagination occurs, the infundibular recess (ir), which elongates to form a funnel-shaped structure known as the hypophysis. At its extremity the hypophysis comes in contact during the fifth week with the enlarged extremity of Rathke's pouch formed by an invagination of the roof of the oral sinus (see p. 285), and applies itself closely to the posterior surface of this (Fig. 233) to form with it the pituitary body. The anterior lobe at an early stage separates from the mucous membrane of the oral sinus, the stalk by which it was attached completely disappearing, and toward the end of the second month it begins to send out processes from its walls into the surrounding mesenchyme and so becomes converted into a mass of solid epithelial cords embedded in a mesenchyme rich in blood and lymphatic vessels. The cords later on divide transversely to a greater or less extent to form alveoli, the entire structure coming to resemble somewhat the parathyreoid bodies (see p. 297), and, like these, having the function of producing an internal secretion. The posterior lobe, derived from the brain, retains its connection with that structure, its stalk being the injundibidum, but its terminal portion does not undergo such extensive modifications as does the anterior lobe, although it is claimed that it gives rise to a glandular epithelium which may become arranged so as to form alveoli.
The cerebral hemispheres are formed from the lateral portions of the dorsal zones, each possessing also a prolongation of the roofplate. From the more ventral portion of each dorsal zone there is formed a thickening, the corpus striatum (Figs. 240, cs, and 233, VI 2), a structure which is for the telencephalon what the optic thalamus is for the diencephalon, while from the more dorsal portion there is formed the remaining or mantle (pallial) portions of the hemispheres (Figs. 240, h, and 233, VI 4). When first formed, the hemispheres are slight evaginations from the median portion of the telencephalon, the openings by which their cavities communicate with the third ventricle, the interventricular foramina, being relatively very large (Fig. 240), but, in later stages (Fig. 233), the hemispheres increase more markedly and eventually surpass all the other portions of the brain in magnitude, overlapping and completely concealing the roof and sides of the diencephalon and mesencephalon and also the anterior surface of the cerebellum. In this enlargement, however, the interventricular foramina share only to a slight extent, and consequently become relatively smaller (Fig. 233), forming in the adult merely slit-like openings lying between the lamina terminalis and the thalami and having for their roof the anterior portion of the velum interpositum.
Fig. 240. - Median Longitudinal Section of the Brain of an Embryo of 16.3 mm. br, Anterior brachium; eg, corpus geniculatum laterale; cs, corpus striatum; h, cerebral hemisphere; ir, infundibular recess; It, lamina terminalis; or, optic recess; ot, thalamus; p, pineal process; sm, sulcus Monroi; st, hypothalamic region; vi, velum interpositum. - (His.)
The velum Interpositum - that is to say, the roof-plate - where it forms the roof of the interventricular foramen, is prolonged out upon the dorsal surface of each hemisphere, and, becoming invaginated, forms upon it a groove.' As the hemispheres, increasing in height, develop a mesial wall, the groove, which is the so-called chorioidal fissure, comes to lie along the ventral edge of this wall, and as the growth of the hemispheres continues it becomes more and more elongated, being carried at first backward (Fig. 241), then ventrally, and finally forward to end at the tip of the temporal lobe. After the establishment of the grooves the mesenchyme in their vicinity dips into them, and, developing blood-vessels, becomes the chorioid plexuses of the lateral ventricles, and at first these plexuses grow much more rapidly than the ventricles, and so fill them almost completely. Later, however, the walls of the hemispheres gain the ascendancy in rapidity of growth and the plexuses become relatively much smaller. Since the portions of the roof-plate which form the chorioidal fissures are continuous with the velum interpositum in the roofs of the interventricular foramina, the chorioid plexuses of the lateral and third ventricles become continuous also at that point.
Fig. 241. Median Longitudinal Section of the Brain of an Embryo Calf of 5 cm.
cb, Cerebellum; cp, chorioid plexus; cs, corpus striatum; JM, interventricular foramen; in, The mode of growth of the chorioid hypophysis; m, mid-brain; oc, , optic commissure; t, posterior fissures seems to indicate the mode of par t of the diencephalon - growth of the hemispheres. At first the Wihalkovicz.) growth is more or less equal in all directions, but later it becomes more extensive posteriorly, there being more room for expansion in that direction, and when further extension backward becomes difficult the posterior extremities of the hemispheres bend ventrally toward the base of the cranium, and reaching this, turn forward to form the temporal lobes. As a result the cavities of the hemispheres, the lateral ventricles, in addition to being carried forward to form an anterior horn, are also carried backward and ventrally to form the lateral or descending horn, and the corpus striatum likewise extends backward to the tip of each temporal lobe as a slender process known as" the tail of the caudate nucleus. In addition to the anterior and lateral horns, the ventricles of the human brain also possess posterior horns extending backward into the occipital portions of the hemispheres, these portions, on account of the greater persistence of the mid-brain flexure (see p. 388), being enabled to develop to a greater extent than in the lower mammals.
The scheme of the origin of parts in the telencephalon may be stated as follows: Median Part. Hemispheres.
â€ž , , f Anterior part of velum inter-
Roof-plate < . < Moor of chonoidai nssure.
(^ positum. [ r , . ... Pallium.
-r. , Lamina terminahs. _ Dorsal zones â– (_... < Corpus striatum.
Optic evaginations. _,, , , . ..
> . , . Olfactory lobes (see p. 406) Anterior part of hypothalamic [ Ventral zones < region.
[ Anterior part of tuber cinereum.
The Convolutions of the Hemispheres
The growth of the hemispheres to form the voluminous structures found in the adult depends mainly upon an increase of size of the pallium. The corpus striatum, although it takes part in the elongation of each hemisphere, nevertheless does not increase in other directions as rapidly and extensively as the pallium, and hence, even in very early stages, a depression appears upon the surface of the hemispheres where the corpus is situated (Fig. 242). This depression is the lateral cerebral fossa, and for a considerable period it is the only sign of inequality of growth on the outer surfaces of the hemispheres. Upon the mesial surfaces, however, at about the time that the choroid fissure appears, another linear depression is formed dorsal to the chorioid, and when fully formed extends from in front of the interventricular foramen to the tip of the temporal lobe (Fig. 244, h). It affects the entire thickness of the pallial wall and consequently produces an elevation upon the inner surface, a projection into the cavity of the ventricle which is known as the hippocampus, whence the fissure may be termed the hippocampal fissure. The portion of the pallium which intervenes between this fissure and the chorioidal forms what is known as the dentate gyrus.
Toward the end of the third or the beginning of the fourth month two prolongations arise from the fissure just where it turns to be continued into the temporal lobe, and these, extending posteriorly, give rise to the parieto-occipital and calcarine fissures. Like the hippocampal, these fissures produce elevations upon the inner surface of the pallium, that formed by the parieto-occipital early disappearing, while that produced by the calcarine persists to form the calcar (hippocampus minor) of adult anatomy.
The three fissures just described, together with the chorioidal and the lateral cerebral fossa, are all formed by the beginning of the fourth month and all the fissures affect the entire thickness of the wall of the hemisphere, and hence have been termed the primary or total fissures. Until the beginning of the fifth month they are the only fissures present, but at that time secondary fissures, which, with one exception, are merely furrows of the surface of the pallium, make their appearance and continue to form until birth and possibly later. Before considering these, however, certain changes which occur in the neighborhood of the lateral cerebral fossa may be described.
The fossa is at first a triangular depression situated above the temporal lobe on the surface of the hemisphere. During the fourth month it deepens considerably, so that its upper and lower margins become more pronounced and form projecting folds, and, during the fifth month, these two folds approach one another and eventually cover in the floor of the fossa completely, the groove which marks the line of their contact forming the lateral cerebral fissure, while the floor of the fossa becomes known as the insula.
Fig. 242. - Brain of an Embryo of the Fourth Month. c, Cerebellum; p, pons; s, lateral cerebral fossa.
The first of the secondary fissures to appear is the sulcus cinguli, which is formed about the middle of the fifth month on the mesial surface of the hemispheres, lying parallel to the anterior portion of the hippocampal fissure and dividing the mesial surface into the gyri marginalis and fornicatus. A little later, at the beginning of the sixth month, several other fissures make their appearance upon the outer surface of the pallium, the chief of these being the central sulcus, the inter-parietal, the pre- and post-central, and the temporal sulci, the most ventral of these last running parallel with the lower portion of the hippocampal fissure and differing from the others in forming a ridge on the wall of the ventricle termed the collateral eminence, whence the fissure is known as the collateral. The position of most of these fissures may be seen from Fig. 243, and for a more complete description of them reference may be had to text-books of descriptive anatomy.
Fig. 243. - Cerebral Hemisphere oe an Embryo of about the Seventh Month. fs, Superior frontal sulcus; ip, interparietal; IR, insula; pet, inferior pre-central; pes, superior pre-central; ptc, post-central; R, central; S, lateral; t 1 , first temporal. - (Cunningham )
In later stages numerous tertiary fissures make their appearance and mask more or less extensively the secondaries, than which they are, as a rule, much more inconstant in position and shallower. The Corpus Callosum and Fornix. - While these fissures have been forming, important structures have developed in connection with the lamina terminalis. Up to about the fourth month the lamina is thin and of nearly uniform thickness throughout, but at this time it begins to thicken near its dorsal edge and fibers appear in the thickening. These fibers belong to three sets. In the first place, certain of them arise in connection with the olfactory tracts (see p. 407) and from the region of the hippocampal gyrus, which is also associated with the olfactory sense, and, passing through tbe substance of the lamina terminalis, they extend across the median line to the corresponding regions of the opposite cerebral hemisphere. They are therefore commissural fibers and form what is termed the anterior commissure (Figs. 244, ca and 245, ac). Secondly, fibers, which have their origin from the cells of the hippocampus, develop along the chorioidal edge of that structure, forming what is termed the fimbria. They follow along the edge of the chorioidal fissure and, when this reaches the interventricular foramen, they enter as the pillars of the fornix (Figs. 244, cf; Fig. 245,/) the substance of the lamina terminalis and, passing ventrally in it, eventually reach the hypothalamic region, where they terminate in the corpora mammillaria.
Thirdly, as the mantle develops fibers radiate from all parts of it toward the dorsal portion of the lamina terminalis and traversing it are distributed to the corresponding portions of the mantle of the opposite side. There fibers are also commissural in character and form the corpus callosum (Figs. 244 and 245, cc). With the development of these three sets of fibers and especially those forming the corpus callosum, the dorsal portion of the lamina terminalis becomes enlarged so as to form a triangular area extending between the two cerebral hemispheres (Fig. 245), the corpus callosum forming its dorsal portion and base, which is directed anteriorly, the pillars of the fornix its ventral portion, while the anterior commissure occupies its ventral anterior angle.
The portion of the triangle included between the callosum and the fornix remains thin and forms the septum pellucidum, and a split occurring in the center of this gives rise to the so-called^///* ventricle, which, from its mode of formation, is a completely closed cavity and is not lined with ependymal tissue of the same nature as that found in the other ventricles.
Owing to the very considerable size reached by the triangular area whose history has just been described, important changes are wrought in the adjoining portions of the mesial surface of the hemispheres. Before the development of the area the gyrus dentatus and the hippocampus extend forward into the anterior portion of the hemispheres (Fig. 244), but on account of their position they become encroached upon by the enlargement of the corpus callosum, with the result that the hippocampus becomes practically obliterated in that portion of its course which lies in the region occupied by the corpus callosum, its fissure in this region becoming known as the callosal fissure, while the corresponding portions of the dentate gyrus become reduced to narrow and insignificant bands of nerve tissue which rest upon the upper surface of the corpus callosum and are known as the lateral longitudinal stria.
Fig. 244. - Median Longitudinal Section or the Brain of an Embryo of Four Months.
c, Calcarine fissure; ca, anterior commissure; cc, corpus callosum; cf. chorioidal fissure; dg, dentate gyrus; fm, interventricular foramen; h, hippocampal fissure; po, parieto-o c c i p i t a 1 fissure. - (Mihalkovicz.)
The Olfactory Lobes
At the time when the cerebral hemispheres begin to enlarge - that it to say, at about the fourth week - a slight furrow, which appears on the ventral surface of each anteriorly, marks off an area which, continuing to enlarge with the hemispheres, gradually becomes constricted off from them to form a distinct lobelike structure, the olfactory lobe (Fig. 233, VI 3). In most of the lower mammalia these lobes reach a very considerable size, and consequently have been regarded as constituting an additional division of the brain, known as the rhinencephalon, but in man they remain smaller, and although they are at first hollow, containing prolongations from the lateral ventricles, the cavities later on disappear and the lobes become solid. Each lobe becomes differentiated into two portions, its terminal portion becoming converted into the club-shaped structure, the olfactory bulb and stalk, while its proximal portion gives rise to the olfactory tracts, the trigone, and the anterior perforated substance.
Histogenesis of the Cerebral Cortex
A satisfactory study of the histogenesis of the cortex has not yet been made. In embryos of three months a marginal velum is present and probably gives rise to the stratum zonale of the adult brain; beneath this is a cellular layer, perhaps representing the mantle layer; beneath this, again, a layer of nerve-fibers is beginning to appear, representing the white substance of the pallium; and, finally, lining the ventricle is an ependymal layer. In embryos of the fifth month, toward the innermost part of the second layer, cells are beginning to differentiate into the large pyramidal cells, but almost nothing is known as to the origin of the other layers recognizable in the adult cortex, nor is it known whether any migration, similar to what occurs in the cerebellar cortex, takes place. The fibers of the white substance do not begin to acquire their myelin sheaths until toward the end of the ninth month, and the process is not completed until some time after birth (Flechsig), while the fibers of the cortex continue to undergo myelination until comparatively late in life (Kaes).
Fig. 245. - Median Longitudinal Section of the Brain oe an Embryo of the Fifth Month.
ac, Anterior commissure; cc, corpus callosum; dg, dentate gyrus;/, fornix; i, infundibulum; mc, intermediate mass; si, septum pellucidum; vi, velum interpositum. - (Mihal kovicz.)
The Development of the Spinal Nerves
It has already been seen that there is a fundamental difference in the mode of development of the two roots of which the typical spinal nerves are composed, the ventral root being formed by axis-cylinders which arise from neuroblasts situated within the substance of the spinal cord, while the dorsal roots arise from the cells of the neural crests, their axiscylinders growing into the substance of the cord while their dendrites become prolonged peripherally to form the sensory fibers of the nerves. Throughout the thoracic, lumbar and sacral regions of the cord the fibers which grow out from the anterior horn cells converge to form a single nerve-root in each segment, but in the cervical region fibers which arise from the more laterally situated neuroblasts make their exit from the cord independently of the more ventral neuroblasts and form the roots of the spinal accessory nerve (see p. 416). In the cervical region there are accordingly three sets of nerve-roots, the dorsal, lateral, and ventral sets.
In a typical spinal nerve, such as one of the thoracic series, the dorsal roots as they grow peripherally pass ventrally as well as outward, so that they quickly come into contact with the ventral roots with whose fibers they mingle, and the mixed nerve so formed soon after divides into two trunks, a dorsal one, which is distributed to the dorsal musculature and integument, and a larger ventral one. The ventral division as it continues its outward growth soon reaches the dorsal angle of the pleuro-peritoneal cavity, where it divides, one branch passing into the tissue of the body- wall while the other passes into the splanchnic mesoderm. The former branch, continuing its onward course in the body- wall, again divides, one branch becoming the lateral cutaneous nerve, while the other continues inward to terminate in the median ventral portion of the body as the anterior cutaneous nerve. The splanchnic branch forms a ramus communicans to the sympathetic system and will be considered more fully later on.
The conditions just described are those which obtain throughout the greater part of the thoracic region. Elsewhere the fibers of the ventral divisions of the nerves as they grow outward tend to separate from one another and to become associated with the fibers of adjacent nerves, giving rise to plexuses. In the regions where the limbs occur the formation of the plexuses is also associated with a shifting of the parts to which the nerves are supplied, a factor in plexus formation which is, however, much more evident from comparative anatomical than from embryological studies.
The Development of the Cranial Nerves
During the last thirty years the cranial nerves have received a great deal of attention in connection with the idea that an accurate knowledge of their development would afford a clue to a most vexed problem of vertebrate morphology, the metamerism of the head. That the metamerism which was so pronounced in the trunk should extend into the head was a natural supposition, strengthened by the discovery of head-cavities in the lower vertebrates and by the indications of metamerism seen in the branchial arches, and the problem which presented itself was the correlation of the various structures belonging to each metamere and the determination of the modifications which they had undergone during the evolution of the head.
In the trunk region a nerve forms a conspicuous element of each metamere and is composed, according to what is known as Bell's law, of a ventral or efferent and a dorsal or afferent root. Until comparatively recently the study of the cranial nerves has been dominated by the idea that it was possible to extend the application of Bell's law to them and to recognize in the cranial region a number of nerve pairs serially homologous with the spinal nerves, some of them, however, having lost their afferent roots, while in others a dislocation, as it were, of the two roots had occurred.
The results obtained from investigation along this line have not, however, proved entirely satisfactory, and facts have been elucidated which seem to show that it is not possible to extend Bell's law, in its usual form at least, to the cranial nerves. It has been found that it is not sufficient to recognize simply afferent and efferent roots, but these must be analyzed into further components, and when this is done it is found that in the series of cranial nerves certain components occur which are not represented in the nerves of the spinal series.
Before proceeding to a description of these components it will be well to call attention to a matter already alluded to in a previous chapter (p, 84) in connection with the segmentation of the mesoderm of the head. It has been pointed out that while there exist "head-cavities" which are serially homologous with the mesodermal somites of the trunk, there has been imposed upon this primary cranial metamerism a secondary metamerism represented by the branchiomeres associated with the branchial arches, and, it may be added, this secondary metamerism has become the more prominent of the two, the primary one, as it developed, gradually slipping into the background until, in the higher vertebrates, it has become to a very considerable extent rudimentary. In accordance with this double metamerism it is necessary to recognize two sets of cranial muscles, one derived from the cranial myotomes and represented by the muscles of the eyeball, and one derived from the branchiomeric mesoderm, and it is necessary also to recognize for these two sets of muscles two sets of motor nerves, so that, with the dorsal or sensory nerve-roots, there are altogether three sets of nerve-roots in the cranial region instead of only two, as in the spinal region.
These three sets of roots are readily recognizable both in the embryonic and in the adult brain, especially if attention be directed to the cell groups or nuclei with which they are associated (Fig. 246). Thus there can be recognized: (1) a series of nuclei from which nerve-fibers arise, situated in the floor of the fourth ventricle and aquaeduct close to the median line and termed the ventral motor nuclei; (2) a second series of nuclei of origin, situated more laterally and in the substance of the formatio reticularis, and known as the lateral motor nuclei; and (3) a series of nuclei in which afferent nervefibers terminate, situated still more laterally in the floor of the ventricle and forming the dorsal or sensory nuclei. None of the twelve cranial nerves usually recognized in the text-books contains fibers associated with all three of these nuclei; the fibers from the lateral motor nuclei almost invariably unite with sensory fibers to form a mixed nerve, but those from all the ventral motor nuclei form independent roots, while the olfactory and auditory nerves alone, of all the sensory roots (omitting for the present the optic nerve), do not contain fibers from either of the series of motor nuclei. The relations of the various cranial nerves to the nuclei may be seen from the following table, in which the + sign indicates the presence and the - sign the absence of fibers from the nuclear series under which it stands':
Fig. 246. - Transverse Section through the Medulla Oblongata of an Embryo of 10 mm., showing the Nuclei of Origin of the Vagus (X) and Hypoglossal (XII) Nerves. - (His.)
THE CRANIAL NERVES
Vagus. 1 Spinal Accessory. J
Two nerves - namely, the second and twelfth - have been omitted from the above table. Of these, the second or optic nerve undoubtedly belongs to ah entirely different category from the other peripheral nerves, and will be considered in the following chapter in connection with the sense-organ with which it is associated (see especially p. 460). The twelfth or hypoglossal nerve, on the other hand, really belongs to the spinal series and has only secondarily been taken up into the cranial region in the higher vertebrates. It has already been seen (p. 170) that the bodies of four vertebrae are included in the basioccipital bone, and that three of the nerves corresponding to these vertebrae are represented in the adult by the hypoglossal and the fourth by the first cervical or suboccipital nerve. The dorsal roots of the hypoglossal nerves seem to have almost disappeared, although a ganglion has been observed in embryos of 7 and 10 mm. in the posterior part of the hypoglossal region (His), and probably represents the dorsal root of the most posterior portion of the hypoglossal nerve. This ganglion disappears, as a rule, in later stages, and it is interesting to note that the ganglion of the suboccipital nerve is also occasionally wanting in the adult condition. The hypoglossal roots are to be regarded, then, as equivalent to the ventral roots of the cervical spinal nerves, and the nuclei from which they arise lie in series with the cranial ventral motor roots, a fact which indicates the equivalency of these latter with the fibers which arise from the neuroblasts of the anterior horns of the spinal cord.
The equivalents of the lateral motor roots may more conveniently be considered later on, but it may be pointed out here that these are the fibers which are distributed to the muscles of the branchiomeres. In the case of the sensory nerves a further analysis is necessary before their equivalents in the spinal series can be determined. For this the studies which have been made in recent years of the components entering into the cranial nerves of the amphibia (Strong) and fishes (Herrick) must supply a basis, since as yet a direct analysis of the mammalian nerves has not been made. In the forms named it has been found that three different components enter into the formation of the dorsal roots of the cranial nerves: (i) fibers belonging to a general cutaneous or somatic sensory system, distributed to the skin without being connected with any special sense-organs; (2) fibers belonging to what is termed the communis or viscerosensory system, distributed to the walls of the mouth and pharyngeal region and to special organs found in the skin of the same character as those occurring in the mouth; and (3) fibers belonging to a special set of cutaneous sense-organs largely developed in the fishes and known as the organs of the lateral line.
The fibers of the somatic sensory system converge to a group of cells, situated in the lateral part of the floor of the fourth ventricle and forming what is termed the trigeminal lobe, and also extend posteriorly in the substance of the medulla (Fig. 247), forming what has been termed the spinal root of the trigeminus and terminating in a column of cells which represents the forward continuation of the posterior horn of the cord. In the fishes and amphibia fibers belonging to this system are to be found in the fifth, seventh, and tenth nerves, but in the mammalia their distribution has apparently become more limited, being confined almost exclusively to the trigeminus, of whose sensory divisions they form a very considerable part. Since the cells around which the fibers of the spinal root of the trigeminus terminate are the forward continuations of the posterior horn of the cord, it seems probable that the fibers of this system are the cranial representatives of the posterior roots of the spinal nerves, which, it may be noted, are also somatic in their distribution. The fibers of the viscero-sensory system are found in the lower forms principally in the ninth and tenth nerves (see Fig. 247), although groups of them are also incorporated in the seventh and fifth. They converge to a mass of cells, known as the lobus vagi, and like the first set are also continued down the medulla to form a tract known as the fasciculus solitarius or: fasciculus communis. In the mammalia the system is represented by the sensory fibers of the glosso-pharyngeo-vagus set of nerves, of which it represents practically the entire mass; by the sensory fibers of the facial arising from the geniculate ganglion and included in the chorda tympani and probably also the great superficial petrosal; and also, probably, by the lingual branch of the trigeminus. Furthermore, since the mucous membrane of the palate is supplied by branches from the trigeminus which pass by way of the spheno-palatine (Meckel's) ganglion, and the same region is supplied in lower forms by a palatine branch from the facial, it seems probable that the palatine nerves of the mammalia are also to be assigned to this system.* If this be the case, a very evident clue is afforded to the homologies of the system in the spinal nerves, for since the spheno-palatine ganglion is to be regarded as part of the sympathetic system, the sensory fibers which pass from the viscera to the spinal cord by way of the sympathetic system (p. 420) present relations practically identical with those of the palatine nerves.
Fig. 247. - Diagrams showing the Sensory Components of the Cranial Nerves of a Fish (Menidia) . The somatic sensory system is unshaded, the viscero-sensory is cross-hatched, and the lateral line system is black, asc.v, Spinal root of trigeminus; brx, branchial branches of vagus; Ix, lobus vagi; ol, olfactory bulb; op, optic nerve; rc.x, cutaneous branch of the vagus; rix, intestinal branch of vagus; rl, lateral line nerve; rl.acc, accessory lateral line nerve; ros, superficial ophthalmic; rp, ramus palatinus of the facial; thy, hyomandibular branch of the facial; t.inf, infraorbital nerve. - (Herrick.)
Finally, with regard to the system of the lateral line, there seems but little doubt that it has no representation whatsoever in the spinal nerves. It is associated with a peculiar system of cutaneous senseorgans found only in aquatic or marine animals, and also with the auditory and possibly the olfactory organs, the former of which are certainly and the latter possibly primarily parts of the lateral line system of organs. The organs are principally confined to the head, although they also extend upon the trunk, where they are followed by a branch from the vagus nerve, the entire system being accordingly supplied by cranial nerves. In the fishes, in which the development of the organs is at a maximum, fibers belonging to the system are found in all the branchiomeric nerves and all converge to a portion of the medulla known as the tuberculum acusticum. In the Mammalia, with the disappearance of the lateral line organs there has been a disappearance of the associated nerves, and the only certain representative of the system which persists is the auditory nerve.
The table given on page 412 may now be expanded as follows, though it must be recognized that such an analysis of the mammalian nerves is merely a deduction from what has been observed in lower
- The fact that the palatine branches are associated with the trigeminus in the Mammalia and with the facial in the Amphibia is readily explained by the fact that in the latter the Gasserian and geniculate ganglia are not always separated, so that it is possible for fibers originating from the compound ganglion to pass into either forms, and may require some modifications when the components have been subjected to actual observation:
An additional word is necessary concerning the spinal accessory nerve, for it presents certain interesting relations which possibly furnish a clue to the spinal equivalents of the lateral motor roots. In the first place, the neuroblasts which give rise to those fibers of the nerve which come from the spinal cord are situated in the dorsal part of the ventral zones. As the nuclei of origin are traced anteriorly they will be found to change their position somewhat as the medulla is reached and eventually come to lie in the reticular formation, the most anterior of them being practically continuous with the motor nucleus of the vagus. Indeed, it seems that the spinal accessory nerve is properly to be regarded as an extension of the vagus downward into the cervical region (Furbringer, Streeter), a process which reaches its greatest development in the mammalia and seems to-stand in relation to the development of those portions of the trapezius and sterno-mastoid muscles which are supplied by the spinal accessory nerve.
It is believed that the white rami communicantes which pass from the spinal cord to the thoracic and upper lumbar sympathetic ganglia arise from cells situated in the dorso-lateral portions of the ventral horns, and it is noteworthy that white rami are wanting in the region in which the spinal accessory nerve occurs. Since this nerve represents a cranial lateral motor root the temptation is great to regard the cranial lateral motor roots as equivalent to the white rami of the cord, and this temptation is intensified when it is recalled that there are both embryological and topographical reasons for regarding the branchiomeric muscles, to which the cranial lateral motor nerves are supplied, as equivalent to the visceral muscles of the trunk. But in view of the fact that a sympathetic neurone is always interposed between a white ramus fiber and the visceral musculature (see Fig. 249), while the lateral motor fibers connect directly with the branchiomeric musculature, it seems advisable to await further studies before yielding to the temptation.
As regards the actual development of the cranial nerves, they follow the general law which obtains for the spinal nerves, the motor fibers being outgrowths from neuroblasts situated in the walls of the neural tube, while the sensory nerves are outgrowths from the cells of ganglia situated without the tube. In the lower vertebrates a series of thickenings, known as the suprabranchial placodes, are developed from the ectoderm along a line corresponding with the level of the auditory invagination, while on a line corresponding with the upper extremities of the branchial clefts another series occurs which has been termed that of the epibranchial placodes, and with both of these sets of placodes the cranial nerves are in connection. In the human embryo epibranchial placodes have been found in connection with the fifth, seventh, ninth and tenth nerves, to whose ganglia they contribute cells. The suprabranchial placodes, which in the lower vertebrates are associated with the lateral line nerves, are unrepresented in man, unless, as has been maintained, the sense-organs of the internal ear are their representatives.
From what has been said above it is clear that the usual arrangement of the cranial nerves in twelve pairs does not represent their true relationships with one another. The various pairs are serially homologous neither with one another nor with the typical spinal nerves, nor can they be regarded as representing twelve cranial segments. Indeed, it would seem that comparatively little information with regard to the number of myotomic segments which have fused together to form the head is to be derived from the cranial nerves, for while there are only four of these nerves which are associated with structures equivalent to the mesodermic somites of the trunk, a much greater number of head cavities or mesodermic somites has been observed in the cranial region of the embryos of the lower vertebrates, Dohrn, for instance, having found nineteen and Killian eighteen in the cranial region of Torpedo. Furthermore, it is not possible to say at present whether the branchiomeres and their associated nerves correspond with one or several of the cranial mesodermic somites, or whether, indeed, any correspondence whatever exists.
In early stages of development a series of constrictions have been observed in the cranial portion of the neural tube and have been regarded as indicating a primitive segmentation of that structure. The neuromeres, as the intervals between successive constrictions have been termed, seem to correspond with the cranial nerves as usually recognized and hence cannot be regarded as primitive segmental structures. They are more probably secondary and due to the arrangement of the neuroblasts corresponding to the various nerves.
The Development of the Sympathetic Nervous System
From the embryological standpoint the distinction which has been generally recognized between the sympathetic and central nervous systems does not exist, the former having been found to be an outgrowth from the latter. This mode of origin has been observed with especial clearness in the embryos of some of the lower vertebrates, in which masses of cells have been seen to separate from the posterior root ganglia to form the ganglia of the ganglionated cord (Fig. 248). In the mammalia, including man, the relations of the two sets of ganglia to one another is by no means so apparent, since the sympathetic cells, instead of being separated from the posterior root ganglion en masse, migrate from it singly or in groups, and are therefore less readily distinguishable from the surrounding mesodermal tissues.
To understand the development of the sympathetic system it must be remembered that it consists typically of three sets of ganglia. One of these is constituted by the ganglia of the ganglionated cord (Fig. 249, GC), the second is represented by the ganglia of the prevertebral plexuses (PVG), such as the cardiac, solar, hypogastric, and pelvic, while the third or peripheral set (PG) is formed by the cells which occur throughout the tissues of probably most of the visceral organs, either in small groups or scattered through plexuses such as the Auerbach and Meissner plexuses of the intestine. Each cell in these various ganglia stands in direct contact with the axiscylinder of a cell situated in the central nervous system, probably in the lateral horn of the spinal cord or the corresponding region of the brain, so that each cell forms the terminal link of a chain whose first link is a neurone belonging to the central system (Huber) . Through
Fig. 248. - Transverse Section through an Embryo Shark (Scyllium) of ii mm., SHOWING THE ORIGIN OF A SYMPATHETIC GANGLION.
Ch, Notochord; E, ectoderm; G, posterior root ganglion; Gs, sympathetic ganglion; .1/, spinal cord. - (Onodi.)
Fig. 249. - Diagram showing the Arrangement of the Neurones of the Sympathetic System. The fibers from the posterior root ganglia are represented by the broken black lines; those from the anterior horn cells by the solid black; the white rami by red; and the sympathetic neurones by blue. DR, Dorsal ramus of spinal nerve; GC, ganglionated cord; GR, gray ramus communicans; PG, peripheral ganglion; PVG, prsevertebral ganglion; VR, ventral ramus of spinal nerve; WR, white ramus communicans. - (Adapted from Huber.) out the thoracic and upper lumbar regions of the body the central system neurones form distinct cords known as the white rami communicantes (Fig. 249, WR), which pass from the spinal nerves to the adjacent ganglia of the ganglionated cord, some of them terminating around the cells of these ganglia, others passing on to the cells of the prsevertebral ganglia, and others to those of the peripheral plexuses. In the cervical, lower lumbar and sacral regions white rami are wanting, the central neurones in the first-named region probably making their way to the sympathetic cells by way of the upper thoracic nerves, while in the lower regions they may pass down the ganglionated cord from higher regions or may join the prevertebral and peripheral ganglia directly without passing through the proximal ganglia. In addition to these white rami, what are known as gray rami also extend between the proximal ganglia and the spinal nerves; these are composed of fibers, arising from sympathetic cells, which join the spinal nerves in order to pass with them to their ultimate distribution.
Fig. 250. - Transverse Section through the Spinal Cord of an Embryo of 7 mm.
c, Notochord; g, posterior root ganglion; m, spinal cord; s, sympathetic cell migrating from the posterior root ganglion; wr, white ramus.- - (His.)
The brief description here given applies especially to the sympathetic system of the neck and trunk. Representatives of the system are also found in the head, in the form of a series of ganglia connected with the trigeminal and facial nerves and known as the ciliary, spheno-palatine, otic, and submaxillary ganglia; and, as will be seen later, there are probably some sympathetic cells which owe their origin to the root ganglia of the vagus and glossopharyngeal nerves. There is nothing, however, in the head region corresponding to the longitudinal bundles of fibers which unite the various proximal ganglia of the trunk to form the ganglionated cord.
The first distinct indications of the sympathetic system are to be seen in a human embryo of about 7 mm. As the spinal nerves reach the level of the dorsal edge of the body-cavity, they branch, one of the branches continuing ventrally in the body-wall, while the other (Fig 250, wr) passes mesially toward the aorta, some of its fibers reaching that structure, while others bend so as to assume a longitudinal direction. These mesial branches represent the white rami communicantes, but as yet no ganglion cells can be seen in their course. The cells of the posterior root ganglia have already, for the most part, assumed their bipolar form, but among them there may still be found a number of cells in the neuroblast condition, and these (Fig. 250, s), wandering out from the ganglia, give rise to a column of cells standing in relation to the white rami. At first there is no indication of a segmental arrangement of the cells of the column (Fig. 251), but at about the seventh week such an arrangement makes its appearance in the cervical region, and later, extends posteriorly, until the column assumes the form of the ganglionated cord.
This origin of the ganglionated cord from cells migrating out from the posterior root ganglia has been described by various authors, but recently the origin of the cells has been carried a step further back, to the mantle layer of the central nervous system (Kuntz). Indifferent cells and neuroblasts are said to wander out from the walls of the medullary canal by way of both the posterior and anterior nerve roots and it is claimed that these are the cells that give rise to the ganglionated cord in the manner just described.
Before, however, the segmentation of the ganglionated cord becomes marked, thickenings appear at certain regions of the cell column, and from these, bundles of fibers may be seen extending ventrally toward the viscera. The thickenings represent certain of the prevertebral ganglia, and later cells wander out from them and take a position in front of the aorta. In an embryo of 10.2 mm. two ganglionic masses (Fig. 251, pc) occur in the vicinity of the origin of the vitelline artery (am), one lying above and the other below that vessel; these masses represent the ganglia of the cceliac plexus and have separated somewhat from the ganglionated cord, the fiber bundles which unite the upper mass with the cord representing the greater and lesser splanchnic nerves (sp), while that connected with the lower mass represents the connection of the cord with the superior mesenteric ganglion. Lower down, in the neighborhood of the umbilical arteries, is another enlargement of the cord (bg), which probably represents the inferior mesenteric and hypogastric ganglia which have not yet separated from the cell column.
Fig. 251. - Reconstruction of the Sympathetic System of an Embryo of 10.2 mm. am, Vitelline artery; ao, aorta; au, umbilical artery; bg, ganglionic mass representing the pelvic plexus; d, intestine; oe, oesophagus; pc, ganglia of the cceliac plexus; ph, pharynx; rv, right vagus nerve; sp, splanchnic nerves; sy, ganglionated cord; t, trachea; *, peripheral sympathetic ganglia in the walls of the stomach. - (His, Jr.)
With the peripheral ganglia the conditions are slightly different, in that they are formed very largely, if not exclusively, from cells that migrate from the walls of the hind-brain by way of the vagus nerves (Fig. 251). In this way the ganglia of the myenteric, pulmonary and cardiac plexuses are formed, though in the case of the last named it is probable that contributions are also received from the ganglionated cord.
The elongated courses of the cardiac sympathetic and splanchnic nerves in the adult receive an explanation from the recession of the heart arid diaphragm (see pp. 239 and 322), the latter process forcing downward the coeliac plexus, which originally occupied a position opposite the region of the ganglionated cord from which the splanchnic nerves arise.
As regards the cephalic sympathetic ganglia, the observations of Remak on the chick and Kolliker on the rabbit show that the ciliary, sphenopalatine, and otic ganglia arise by the separation of cells from the semilunar (Gasserian) ganglion, and from their adult relations it may be supposed that the cells of the submaxillary and sublingual ganglia have similarly arisen from the geniculate ganglion of the facial nerve. Evidence has also been obtained from human embryos that sympathetic cells are derived from the ganglia of the vagus and glossopharyngeal nerves, but, instead of forming distinct ganglia in the adult, these, in all probability, associate themselves with the first cervical ganglia of the ganglionated cord.
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