Book - The development of the chick (1919) 8
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Part II The Forth Day to Hatching, Organogeny, Development of the Organs
Chapter VIII The Nervous System
I. The Neuroblasts
The account given in Chapters V and VI outlines the origin of the larger divisions of the central nervous system and ganglia. The subsequent growth and differentiation is due to multiplication of cells, aggregation of embryonic nerve-cells, or neuroblasts, in particular regions or centers, the formation and growth of nerve-fibers which combine to form nerves and tracts, and the origin and differentiation of nerve-sheaths, and the supporting cells, neuroglia, of the central system. The most important factors are the origin of the neuroblasts and of nerve-fibers in connection with them; these fibers form the various nerve-tracts and commissures within the central nervous system and the system of peripheral nerves. The origin of neuroblasts and the development of fibers is the clue to differentiation in all parts of the nervous system.
Neuroblasts are found in two primary locations in the embryo; (1) in the neural tube, and (2) in the series of ganglia derived from the neural crest; these are known as medullary and ganglionic neuroblasts respectively.
The Medullary Neuroblasts
In the neural tube of the chick, up to about the third day, there are present only two kinds of ceils, the epithelial cells and the germinal cells (Fig. 138).
The epithelial cells constitute the main bulk of the walls, and extend from the central canal to the exterior; their inner ends unite to form an internal limiting membrane lining the central canal, and their outer ends to form an external limiting membrane. Each cell in the lateral walls of the tube is much elongated and usually shows three enlargements, viz., at each end and in the region of the nucleus, the cell being somewhat constricted between the nucleus and each end. In different cells the nuclei are at different levels; thus in a section several layers of nuclei appear. These cells are not closely packed together, except at their outer ends, but are more or less separated by intercellular spaces that form a communicating system of narrow channels.
1 Neuroblasts arise also in the olfactory epithelium. (See Chap. IX.)
Fig. 138. — Section of the neural tube, 29 s embryo.
c. C, Central canal, ep. C, Epithelial cells, g. C, Germinal cells. 1. m. ex., External limiting membrane. 1. m. in., Internal limiting membrane. Ms'ch., Mesenchyme, m. v., Marginal velum.
The germinal cells are rounded cells situated next the central canal between the inner ends of the epithelial cells; karyokinetic figures are very common in them. According to His the germinal cells are the parent cells of the neuroblasts alone; it is probable, however, that they are not so limited in function, and that they represent primitive cells from which proceed other epithelial cells and embryonic neuroglia cells as well as neuroblasts.
A narrow non-nucleated margin known as the marginal velum, appears in the lateral walls of the neural tube external to the nuclei (Fig, 138). This is occupied by the outer ends of the epithelial cells. At this time, therefore, three zones may be distinctly recognized in the walls of the neural tube, viz., (1) the zone of the germinal cells, including also the inner ends of the epithelial cells, (2) the zone of the nuclei of the epithelial cells, (3) the marginal velum. No chstinctly nervous elements are yet differentiated.
Such elements, however, soon begin to appear: Fig. 139 represents a section through the cord of a chick embryo of about the end of the third day; it is from a Golgi preparation in which the distinctly nervous elements are stained black, and the epithelial and germinal cells are seen only very indistinctly. The stained elements are the neuroblasts, and it will be observed that they form a layer roughly intermediate in position between the marginal velum and the nuclei of the epithelial cells. They are usually regarded as derived from germinal cells that have migrated from their central position outwards; but it is possible that some of them may have been derived from epithelial cells. However this may be in such an early stage, it is certain that the neuroblasts formed later are derived from germinal cells. It will be observed that each neuroblast consists of a cellbody and a process ending in an enlargement. The process arises as an outgrowth of the cell-body, and forms the axis cylinder or axone of a nerve-fiber; the terminal enlargement is known as the cone of growth, because the growth processes by which the axone increases in length are presumably located here. It may be stated as an invariable rule that each axone process of a medullary neuroblast arises as an outgrowth, and grows to its final termination without addition on the part of other cells. The body of the neuroblast forms the nerve-cell, from which, later on, secondary processes arise constituting the dendrites.
Fig. 139. — Transverse section through the spinal cord and ganglion of a chick about the end of the third day; prepared by the method of Golgi. (After Ramon y Cajal.)
C, Cones of growth. Nbl. 1, 2, 3, 4, Neuroblasts of the lateral wall (1 and 2); of the spinal ganglion (3); of the ventral horn (motor neuroblasts) (4).
The view that each nerve-cell with its axone process and dendrites is an original cellular individual, is known as the neurone theory. For the central nervous system this view is generally held, but its extension to the peripheral system is opposed by some on the ground that the axone in peripheral nerves is formed within chains of cells, and is thus strictly speaking not an original product of the neuroblast, though it may be continuous with the axis cylinder process of a neuroblast. This view is discussed under the peripheral nervous system.
Each medullary neuroblast is primarily unipolar and the axone is the original outgrowth. Soon, however, secondary protoplasmic processes arise from the body of the nerve-cell and form the dendrites. These appear first in motor neuroblasts of the ventrolateral portion of the embryonic cord, whose axones enter into the ventral roots of spinal nerves (Fig. 140). The extent and kind of development of these dendritic proFiG. 140. — Transverse section cesses of the nerve-cells varies through the spinal cord of a extraordinarily in different regions; chick on the fourth day of Y\g^. 139, 140, and 141 give an idea
of their rapid development in the motor neuroblasts up to the eighth dav.
The Ganglionic Neuroblasts
The ganglionic neuroblasts are located, as the name implies, in the series of ganglia derived from the neural
It must not be supposed, however, that all of the cells of the ganglia are neuroblasts, for the ganglia contain, in all probability, large numbers of cells of entirely different function. (Sheath-cells, see peripheral nervous system.) It is probable also that the neuroblasts of the spinal ganglia and some cranial ganglia, at least, are of two original kinds, viz., the neuroblasts of the dorsal root and of the sympathetic system. The first kind only is considered here, and they are usually called the ganglionic neuroblasts s.s., because they alone remain in the spinal ganglia. Like the medullary neuroblasts these neuroblasts form outgrowths that become axis cylinder processes; but they differ from the latter in that each ganglionic neuroblast forms two outgrowths, one from each end of the spindle-shaped cells, which are arranged with their long axes parallel to the long axis of the ganglion (Fig. 139). Thus we may distinguish a central process and a peripheral process from each neuroblast (Fig. 139) ; the former corresponds to the axone and the latter to the dendrites of the medullary neuroblast. The central axone enters the dorsal zone of the neural tube, and the peripheral process grows out into the surrounding mesenchyme.
incubation; prepared by the method of Golgi. (After Ramon y CajaL)
C. a., Anterior commissure. D., Dendrite, d. R., Dorsal root. Ep. Z., Ependymal zone. W., White matter (marginal velum). Nbl. 4, Neuroblast of the ventral horn (motor).
Fig. 141. — Transverse section through the spinal cord of a 9-day chick, prepared by the method of Golgi. (After Ramon y Cajal.) Col., Collaterals, d. R., Dorsal root. G., Gray matter. Gn., Ganglion. Nbl. 4, Neuroblast of the ventral horn (motor), v. R., Ventral root. W., White matter.
In the course of the later development the cell-body moves to one side so that the central and peripheral branches appear nearly continuous (Fig. 141). Farther shifting of the cell-body produces the characteristic form of the ganglionic nerve-cell with rounded body provided with stem from which the central and peripheral branches pass off in opposite directions. . The central process enters the marginal velum near its dorsal boundary and there bifurcates, producing two branches, one of which grows towards the head and the other towards the tail in the dorsal column of the white matter. The ascending and descending branches send off lateral branches, collaterals, which pass deeper into the cord, and ramify in the gray matter of the dorsal horn.
Fig. 142. — Six centripetal axones of the dorsal root, rigorously copied from a good preparation prepared according to the method of Golgi. From a longitudinal and tangential section of the dorsal column of the spinal cord of an 8 day chick. (After Ramon y Cajal.)
Col., Collaterals. 1, 2, 3, 4, 5, 6, the axones entering the cord.
Fig. 142 represents six central processes of ganglionic neuroblasts entering the cord and branching as described.
After this preliminary account of the neuroblasts we may take up the development of the spinal cord, brain, and peripheral nervous system.
II. The Development of the Spinal Cord
We have seen that the epithelial cells of the neural tube stretch from the lumen of the central canal to the exterior, and that the nuclei are arranged so as to leave the outer ends free, thus forming the marginal velum.
In the roof and floor the epithelial cells are relatively low, and in the lateral zones much elongated. The epithelial cells are added to at first by transformation of some of the germinal cells; but they do not appear to multiply by division, and as development proceeds they become more and more wideh^ separated, the interstices being filled up by neuroblasts, embryonic glia cells, and fiber tracts. As the wall of the neural tube grows in thickness, the epithelial cells become more and more elongated, seeing that both external and internal connections are retained; and, as the growth takes place mainly external to their nuclear layer, the latter becomes reduced, relative to the entire thickness of the neural tube, to a comparatively narrow zone surrounding the central canal, and is now known as the ependyma (Fig. 143). Cilia develop on the central ends of the ependymal cells in the central canal, and from the outer end of each a branching process extends to the periphery anastomosing with neighboring ependymal processes so as to form a skeleton or framework enclosing the other cellular elements and fibers of the central system.
Beginning with the third day a new layer appears between the nuclei of the epithelial cells and the marginal velum. This layer, known as the mantle layer, is composed of neuroblasts and embryonic glia cells, and represents the gray matter (Figs. 140 and 144). The white matter of the cord is laid down in the marginal velum. The sources of the cells composing the mantle layer may be twofold, viz., from the young epithelial cells or from the germinal cells. According to some authors young epithelial cells may be transformed into either neuroblasts or neuroglia cells. Thus the form of the youngest neuroblasts in Fig. 139 indicates derivation from epithelial cells, but this cannot be regarded as proved. Similarly intermediate stages between epithelial and true glia cells are apparently shown in Fig. 143. However, there can be but little doubt that the principal source of the neuroblasts of the mantle layer is the germinal cells, that migrate outwards between the bodies of the epithelial cells. The germinal cells continue in active division up to at least the eleventh day, and their activity seems sufficient to provide for all the cellular elements of the mantle layer, whereas the epithelial cells apparently do not divide at all. Moreover, mitoses are not infrequent in some cells of the mantle layer itself.
Fig. 143. — Transverse section of the cord of a nine-day chick to show neuroglia and ependymal cells; prepared by the method of Golgi. (After. Ramon y Cajal.)
D., Dorsal. Ep., Ependymal cells. N'gl., Neuroglia cells, v., Ventral.
The form of the gray matter in the cord in successive stages is shown in Figs. 144, 145, and 146, representing sections of the cord at five, eight, and twelve days. It will be seen that the gray matter gains very rapidly in importance between the fifth and the eighth days.
Attention should be directed to a group of neuroblasts situated at the external margin of the white matter just above the ventral roots. This is known as Hoffmann's nucleus; it extends the entire length of the cord (Fig. 146, twelve days).
The white matter of the cord gains in importance at an equal rate (Figs. 144, 145, 146). Its production is due to ascending and descending tracts of fibers derived from medullary and ganglionic neuroblasts. The dorsal and ventral roots of the spinal nerves divide it on each side into three main columns, viz., dorsal situated above the dorsal root, lateral situated between dorsal and ventral roots, and ventral situated below the
Fig. 144. — Transverse section through the cervical swelHng of the spinal cord of a chick, middle of the fifth day. (After V. Kupffer.) bl. v., Blood vessel. C. a., Anterior commissure. C, Central canal, d., Group of axones at the levelof the dorsal root^ Ep., Ependyma. N'bl., Neuroblasts, white matter.
V. Ventral column of ventral roots. The dorsal column begins first as a bundle of fibers at the entrance of the fibers of the dorsal root (Fig. 144). Subsequently, other fibers come in this region and gradually extend towards the dorsal middle line, displacing the ependyma and gray matter (Fig. 145, eight days), but the dorsal columns of the two sides are still separated in the median line by a broad septum of ependymal cells. Later (Fig. 146, twelve da\^s) this septum becomes very narrow, and the accumulation of fibers in the dorsal columns causes the latter to project on each side of the middle line, thus forming an actual fissure between them.
Fig. 145. — Transverse section through the spinal cord, and the eighteenth spinal ganglion of an eight-day chick. Centr., Centrum of vertebra, d. R., Dorsal root. Ep., Ependyma. Gn., Spinal Ganglion. Gn. symp., Sympathetic ganglion. Gr. M., Gray matter, m. N., Motor nucleus. R. com., Ramus communicans. R. d., Ramus dorsalis. R. v., Ramus ventralis. Sp., Spinous process of vertebra, v. R., Ventral root. Wh. M., White matter.
Central Canal and Fissures of the Cord
The central canal passes through a series of changes of form in becoming the practically circular central canal of the fully formed cord. L'p to the sixth day it is elongated dorso-ventrally, usually narrowest in the middle with both dorsal and ventral enlargements. About the seventh day the dorsal portion begins to be ol^hterated by fusion of the ependymal cells, and is thus reduced to an ependymal septum. On the eighth day this process has involved the upper third of the canal; the form of the canal is roughly wedgeshaped, pointed dorsally and broad ventrally (Fig. 145). The continuation of this i^rocess leaves only the ventral division as the permanent canal.
At the extreme hind end of the cord the central canal becomes dilated to form a relatively large pear-shaped chamber with thin undifferentiated walls (Fig. 148); the terminal wall is still fused with the ectoderm at eight days, and the chamber appears to have a maximum size at this time. At eleven days the fusion with the ectoderm still exists, and the cavitv is smaller.
Fig. 146. — Transverse section through the cervical swelling of the spinal cord of a 12-day chick. (After v. Kupffer.)
C, Central canal, d. H., Dorsal horn of the gray matter. Ep., Ej^endyma. N. H., Nucleus of Hoffmann, s. d., Dorsal fissure, s. v., Ventral fissure, v. H., Ventral horn of the gray matter.
The development of the so-called dorsal and ventral fissures is essentially different. The entire ventral longitudinal fissure of the cord owes its origin to growth of the ventral columns of gray and white matter which protrude below the level of the original floor (Figs. 145 and 146), and the latter is thus left between the inner end of the fissure and the central canal. The •dorsal longitudinal fissure on the other hand is for the most part a septum produced by fusion of the walls of the intermediate and dorsal portions of the central canal; there is, however, a true fissure produced by protrusion of the dorsal columns of white matter (Fig. 146). This is, however, of relatively slight extent. The original roof of the canal is therefore found between the dorsal septum and the fissure.
Neuroblasts, Commissures, and Fiber Tracts of the Cord
The medullary neuroblasts may be divided into four groups: (1) The first group, or motor neuroblasts, form the fibers of the ventral roots of the spinal nerves. These are situated originally in the ventro-lateral zone of the gray matter (Figs. 144, 145, 146); they are relatively large and form a profusion of dendrites (Figs. 140, 141). As they increase in number and size they come to form a very important component of the ventral horn of the gray matter and contribute to its protrusion. (2) The second group may be called the commissural neuroblasts. These are situated originally mainly in the lateral and dorsal portions of the mantle layer, but are scattered throughout the gray matter, and their axis cylinders grow ventrally and cross over to the opposite side of the cord through the floor (Figs. 139 and 140), and thus form the anterior or white commissure of the cord. (3) The cells of the fiber tracts are scattered throughout the gray matter, and are characterized by the fact that their axis cylinders enter the white matter of the same side; here they may bifurcate, furnishing both an ascending and a descending branch, or may simply turn in a longitudinal direction. (4) Finally there are found certain neuroblasts with a short axis cylinder, ramifying in the gray matter on the same side of the cord. These are found in the dorsal horn of the gray matter and develop relatively late (about sixteen days, Ramon y Cajal).
III. The Development of the Brain
Unfortunately the later development of the brain of birds has not been fully studied. The following account is therefore fragmentary. It is based mainly on a dissection and sections of the brain of chicks of eight days' incubation.
Fig. 147 is a drawing of a dissection of the brain of an eightday embryo. The left half of the brain has been removed, and the median wall of the right cerebral hemisphere also. The details of the cut surfaces are drawn in from sections. Figs. 148 and 150 show median and lateral sagittal sections of the same stage.
The flexures of the brain at this stage are: (1) the cranial flexure marked by the 'plica encephali ventralis on the ventral surface, (2) the cervical flexure at the junction of myelencephalon and cord, somewhat reduced in this stage, and (3) the pontine flexure, a ventral projection of the floor of the myelencephalon.
Fig. 147. — Dissection of the brain of an 8-day chick. For description see text. The arrows shown in the figure lie near the dorsal and ventral boundaries of the foramen of Monro.
ch. PL, Choroid plexus. Com. ant., Anterior commissure. Com. post., Posterior commissure. C. str.. Corpus striatum. Ep., Epiphysis. H., Hemisphere. Hyp., Hypophysis. L. t., Lamina terminalis. Myeh, Myelencephalon. olf., Olfactory nerve, op. N., Optic chiasma. op. L., Optic lobe. Par., Paraphysis. Paren., Parencephalon. pi. enc. v., Phca encephali ventralis. pont. Fl., Pontine flexure. Rec. op., Recessus opticus. S. Inf., Saccus infundibuli. Tel. med., Telencephalon medium. Th., Thalamus. T. tr., Torus transversus. Tr., Commissura trochlearis.
The lines a-a, b-b, c-c, d-d, e-e, f-f, represent the planes of section A, B, C, D, E, and F of Fig. 151.
The telencephalon is bounded posteriorly, as noted in Chapter VI, by the line drawn from the velum transversum to the recessus opticus. The telencephalon medium has grown but little since the fourth day, but the hemispheres have expanded enormously, particularly anteriorly and dorsally, and their median surfaces are flattened against one another in front of the lamina terminalis, which forms the anterior boundary of the telencephalon medium (Figs. 148, 149). Posteriorly the cerebral hemispheres extend to about the middle of the diencephalon and their lateral faces are rounded. The lateral walls of the hemispheres have become enormously thickened to form the corpora striata (Figs. 147 and 151 A), and the superior and lateral walls have remained relatively thin, forming the mantle of the cerebral hemispheres (pallium). Thus the cavity of the lateral ventricle is greatly narrowed.
Fig. 148. — Median sagittal section of an embryo of eight days.
a. A., Aortic arch. AIL, Allantois. An., Anus. A. o. m., Omphalomesenteric artery. B. F., Bursa Fabricii. b. P., Basilar plate. C. A., Anterior commissure, c. C, Central canal. Ch. op., Optic chiasma. C. p., posterior Commissure. CI., Cloaca. Cr., Crop, d. Ao., Dorsal aorta. D. Hyp., Duct of the hypophysis. Ep., Epiphysis. Fis. Eus., Fissura Eustachii. Hem., Surface of hemisphere barely touched by section. Hyp., Hypophysis. L. t., Lamina terminalis. n. A. 8, neural arch of the eighth vertebra. Nas., Nasal
The dissection (Fig. 147) shows the corpus striatum of the right side forming the lateral wall of the hemisphere, and extending past the aperture (foramen of Monro) between the lateral and third ventricles tow^ards the recessus opticus, where it comes to an end.
The olfactory part of the hemispheres is not well differentiated from the remainder in the chick embryo of eight days. There is, however, a slight constriction on the median and ventral face (Fig. 147) which may be interpreted as the boundary of the olfactory lobe.
The telencephalon medium is crowded in between the hemispheres and the diencephalon; its cavity forms the anterior end of the third ventricle, and communicates anteriorly through two slits, the foramina of Monro, with the lateral ventricles in the hemisphere. In Fig. 147, the upper and lower boundaries of the foramen of Monro, are indicated by the grooves on either side of the posterior end of the corpus striatum. A hair introduced from the third ventricle into the lateral ventricle through the foramen of Monro in the position of the arrow in Fig. 147, can be moved up and down over the whole width of the striatum. The lateral walls of the telencephalon medium are formed by the posterior ends of the corpora striata and are therefore very thick.
cavity. Oes., Oesophagus, p. A., Pulmonary arch, par., Paraphysis. P. C, Pericardial cavity. Rec. op., Recessus opticus. R., Rectum. S. Inf., Saccus infundibuli. T., Tongue. Tel., Med. Telencephalon medium. Tr., Trachea. V. 1, 10, 20, 30, First, tenth, twentieth and thirtieth vertebral centra, r. A.. right auricle. Vel. tr., Velum transversum. V. o. m., Omphalomesenteric vein. V. umb., Umbilical vein.
The lamina terminalis passes obliquely upwards and forwards from the recessus opticus to the region between the foramina of Monro. It is very thin, excepting near its center, where it is thickened to form the torus transversus, containing the anterior commissure. At its dorsal summit it is continuous with the roof of the telencephalon medium, which has formed a pouchlike evagination, the paraphysis. Just behind the paraphysis is the velum transversum, where the roof bends upwards sharply into the roof of the diencephalon. The epithelial wall around the bend is folded to form the choroid plexus of the third ventricle, which is continued forward into the lateral ventricle along the median wall of the hemisphere, ending anteriorly in a free branched tip (Fig. 147, ch. PI.)
Fig. 149. — Median sagittal section of the brain of a chick embryo of 7
days. (After v. Kupffer.)
c., Cerebellum, ca., Anterior commissure, cd., Notochord. ch.. Projection of the optic chiasma. cp., Posterior commissure, e., Epiphysis, e'., Paraphysis. hy., Hypophysis. I., Infundibulum. It., Lamina terminalis. Lop., Optic lobe. M., Mesencephalon. Mt., Metencephalon. opt., Chiasma of the optic nerves, p., Parencephalon. ro., Recessus opticus, s., Saccus infundibuli. se., Synencephalon. tp., Mammillary tubercle, tp., Tuberculum posterius. tr., Torus transversus. Tr., Decussation of the trochlear nerves. Va., Velum medullare anterius. Vi., Ventriculus impar telencephali. vp., Velum medullare posterius.
The principal changes in the telencephalon since the third day comprise: (1) great expansion of the hemispheres and thickening of the ventro-lateral wall to form the corpora striata; (2) origin of the paraphysis which arises as an evagination of the roof just in front of the velum transversum about the middle of the fifth day; (3) formation of the choroid plexus; (4) origin of the anterior commissure within the lamina terminalis; (5) development of the olfactory region. The general morphology of the adult telencephalon is thus well expressed at this time.
The Diencephalon has undergone marked changes since the third day. The roof of the parencephalic division has remained very thin, and has expanded into a large irregular sac (Figs. 147 and 148), situated between the hinder ends of the hemispheres. The attachment of the epiphysis has shifted back to the indentation between parencephalic and synencephalic divisions, and the epiphysis itself has grown out into a long, narrow tube, dilated distally, and provided with numerous hollow buds. In the roof of the synencephalic division the posterior commissure has developed (Fig. 147). In the floor the chiasma has become a thick bundle of fibers, and the infundibulum a deep pocket, from the bottom of which a secondary pocket (saccus infundibuli) is growing out in contact with the posterior face of the hypophysis. Following the posterior wall of the infundibulum in its rise, we come to a slight elevation, the rudiment of the mammillary tubercles; just beyond this is a transverse commissure (the inferior commissure) ; and the diencephalon ends at the tuberculum posterius.
The hypophysis has become metamorphosed into a mass of tubules enclosed within a mesenchymatous sheath; the stalk is continuous with a central tubule representing the original cavity from which the other tubules have branched out (Fig. 148), and it may be followed to the oral epithelium from which the whole structure originally arose. (See note at end of this chapter.)
The lateral walls of the diencephalon have become immensely thickened, both dorsally and ventrally, and a deep fissure (Fig. 147) is found on the inner face at the anterior end, between the dorsal and ventral thickenings. The deepest part of the fissure is a short distance behind the velum transversum; from this a short spur runs forward, a still shorter one ventrally, and the longest arm extends backwards, gradually fading out beyond the middle of the diencephalon. This fissure is not a continuation of the sulcus Monroi, or backward prolongation of the foramen of Monro, but is, on the contrary, entirely independent.
Fig. 150. — Lateral sagittal section of an embryo of 8 days. Right side of the body. All. N., Neck of the allantois. Cbl., cerebellum. Cr., Crop. E. T., Egg
The lateral thickenings of the diencephalon constitute the thalami optici, each of which may be divided into epithalamic, mesothalamic, and hypothalamic subdivisions. In the chick at eight days there is a deep fissure between the epi- and mesothalamic divisions (the thalamic fissure. Fig. 147). The substance of the epithalamus forms the ganglion habenulse. The mesothalamic and hypothalamic divisions are not clearly separated. The transition zone between the diencephalon and mesencephalon is sometimes called the metathalamus.
The mesencephalon has undergone considerable changes since the third day. The dorso-lateral zones have grown greatly in extent, at the same time becoming thicker, and have evaginated in the form of the two large optic lobes. Hence the median portion of the roof is sunk in between the lobes (Fig. 147), and is much thinner than the walls of the lobes. The dorso-lateral zones and roof thus form a very distinct division of the mesencephalon, known as the tectum lohi optici. The ventro-lateral zones and floor have thickened greatly and form the basal division of the mesencephalon. The ventricle of the mesencephalon thus becomes converted into a canal (aqueduct of Sylvius), from which the cavities of the optic lobes open off.
In the metencephalon likewise there is a sharp distinction between the development of the dorso-lateral zones and roof, on the one hand, and the ventro-lateral zones and floor on the other. From the former the cerebellum develops in the form of a thickening overhanging the fourth ventricle. This thickening is relatively inconsiderable in the middle line (cf. Figs. 148 and 150). Thus the future hemispheres of the cerebellum are indicated. The surface is still smooth at the eighth day, but on the tenth and eleventh days folds of the external surface begin to extend into its substance, without, however, invaginating its entire thickness. These are the beginnings of the cerebellar fissures.
tooth. Eust., Eustachian tube. Gn. 1, 13, First and thirteenth spinal granglia. Gon., Gonad. Hem., Hemisphere. Lag., Lagena. Lg., Lung. M., Mantle of Hemisphere. Msn., Mesonephros. Olf. L., Olfactory lobe. Olf. N., Olfactory nerve. P. C, Pericardial cavity. Pz. 5, The fifth post-zygapophysis. R. C. 1, 2, Last two cervical ribs. R. th. 1, 5, First and fifth thoracic ribs. S. pc-per., Septum pericardiaco-peritoneale. S'r., Suprarenal. Symp., Main trunk of the sympathetic. Str., Corpus striatum. V. 1, 10, 20, 30, First, tenth, twentieth and thirtieth vertebral arches. V. C. I., Vena cava inferior. V. L. L., Ventral ligament of the liver.
The floor and ventro-lateral zones of the metencephalon enter into the formation of the pons.
In the roof of the isthmus, or constricted region between cerebellum and mesencephalon, is found a small commissure produced by decussation of the fibers of the trochlearis (Fig. 147).
In the wall of the myelencephalon the neuromeres have disappeared. The thin epithelial roof has become more expanded in the anterior part (Figs. 147 and 148). Floor and sides have become greatly thickened.
The brain commissures existing at eight days are the anterior, posterior, inferior, and trochlearis (Fig. 149). In the next four or five days two more appear, viz., the commissura pallii anterior (Kupffer), corresponding to the corpus callosum of mammalia and the commissura habenularis.
The development of the various nuclei and fiber tracts of the bird's brain is entirely unknown and affords an interesting topic for research.
IV. The Peripheral Nervous System
The peripheral nervous system comprises the nerves which span between peripheral organs and the central nervous system. There are fifty pairs in a chick embryo of eight days, of which twelve innervate the head, and thirty-eight the trunk, distinguished respectively as cranial and spinal nerves. It is convenient for purposes of description to consider cranial and spinal nerves separately, and to take up the spinal nerves first because they are much more uniform in their mode of development than the cranial nerves, and also exhibit a more primitive or typical condition, on the basis of which the development of the cranial nerves must be, in part, at least, explained.
The Spinal Nerves
Each spinal nerve may be divided into a somatic portion related primarily to the somatopleure and axis of the embryo, and a splanchnic portion related primarily to the splanchnopleure and its derivatives. In each of these again a motor and sensory component may be distinguished. Thus each spinal nerve has four components, viz., somatic motor, somatic sensor}^, splanchnic motor, and splanchnic sensory, the two latter constituting the so-called sympathetic nervous system. It is obvious, of course, that the splanchnic components must be missing in the caudal nerves. The somatic and splanchnic components will be considered separately.
Fig. 1.51. — Six transverse sections through the brain of an 8-day chick in the planes represented in Fig. 147. Cbl., Cerebelhim. F. M., Foramen of Monro. Gn. V., Ganghon of the trigeminus. Isth., Isthmus. It. d., Diverticuhmi of the iter. lat. V., Lateral ventricle. Other abbreviations as before (Fig. 147).
Each spinal nerve arises from two roots, dorsal and ventral (Fig. 145). The fibers of the former arise from the bipolar neuroblasts of the spinal ganglia; the fibers of the ventral root, on the other hand, arise from a group of neuroblasts in the ventral portion of the cord. The roots unite in the intervertebral foramen to form the spinal nerve. Typically, each spinal nerve divides almost immediately into three branches, viz., a dorsal branch, a ventral branch, and a splanchnic branch to the sympathetic cord; the last is known as the ramus communicans.
Fig. 145 represents a section passing through the twentieth spinal nerve of an eight-day chick. The dorsal and ventral roots unite just beneath the spinal ganglion; fibers are seen entering the sympathetic ganglion (ramus communicans); the ventral branch passes laterally a short distance where it is cut off; beyond this point it can be traced in other sections in the next posterior intercostal space more than half-way round the body-wall; that is, as far as the myotome has extended in its ventral growth. The dorsal branch arises at the root of the ventral and passes dorsally in contact with the ganglion to branch in the dorsal musculature and epidermis. This nerve may be regarded as typical of the spinal nerves generally.
There are thirty-eight spinal nerves in an embryo of eight days. The first two are represented only by small ventral roots.
The first two spinal ganglia are rudimentary in the embryo and absent in the adult, hence the ganglion illustrated in Fig. 145 is the eighteenth of the functional series (see Fig. 149) ; it lies between the nineteenth and twentieth vertebra?.
The fourteenth, fifteenth, and sixteenth are the principal nerves of the brachial plexus, and have unusually large ganglia. The twenty-third to the twenty-ninth are the nerves of the leg plexus, the thirtieth to the thirty-second innervate the region of the cloaca and the remainder are caudal. The special morphology of the spinal nerves does not belong in this description.
There are one or two vestigial ganglia behind the thirty-eighth nerve, evidently in process of disappearance at eight days.
The early history of the spinal nerves is as follows: The axis cylinder processes of the fibers begin to grow^ out from the neuroblasts about the third day (cf. p. 235). At this time the myotomes are in almost immediate contact with the ganglia; thus the fibers have to cross only a very short space before they enter the myotome. The further growth is associated with the growth and differentiation of the myotome between which and the embryonic nerve there is a very intimate relation of such a sort that the nerve follows the myotome and its derivatives in all changes of position. Thus nerves do not need to grow long distances to establish their connections, but these are formed at a very early period. This accounts for the motor fibers; the way in which the sensory fibers, that arise from the spinal ganglia, reach their termination is not known.
Sheath-cells and Cell-chain Hypothesis. No embryonic nerve consists entirely of axones, but, from the start, each nerve trunk contains numerous nuclei. The latter belong to cells which have been given two radically different interpretations, corresponding to two distinct theories concerning the neuraxone.
(1) The first theory, knowm as the neurone theory, is the one tacitly followed in the preceding description and may be stated as follows: the nerve-cell, dendrites and axone, including the terminal arborization, constitute a single cellular individual or unit, differentiated from the neuroblast alone. The nuclei in the embryonic nerves therefore belong to cells that are foreign to the primary nerve. Their function is to form the various sheaths of the nerves, viz., the sheaths of the individual axones and the endo-, peri-, and epineurium. The sheath of Schwann arises from such cells that envelop the individual fibers at suitable distances and spread longitudinally until neighboring sheath cells meet; each such place of meeting constitutes a node of Ranvier. Until recently it has been universally believed that the sheath cells arose from the mesenchvme; but recent observations on Amphibia and Selachia have shown that they arise from the ganglia in these forms; their original source is therefore the ectoderm. It is probable that they have the same origin in the chick, though this has not been demonstrated by direct observation or experiment.
(2) The second theory is known as the cell-chain hypothesis.
According to this the axones of peripheral nerves arise as differentiations of the sheath-cells in situ; continuity of the axone is established by arrangement of these cells in rows, and union with the neuroblast is essentially secondary. The entire axone is thus by no means an outgrowth of the neuroblast; at most its proximal portion is.
Bethe (1903) expresses the idea thus: "Between the cord of the embryo and the part to be innervated there is formed primarily a chain of nuclei around which the protoplasm is condensed. This is fundamentally an extended syncytium in which the nuclei of the neuroblasts and of the nerve-primordium lie. Within the denser protoplasm which appears as the body of the nervecells, axones differentiate by condensation, and these extend from one cell to the next, and so on to the condensations which are called neuroblasts. The differentiated axones tend more and more to occupy the center of the embryonic nerve, where they appear to lie free, though as a matter of fact they are still embedded in the general plasma which is no longer distinctly visible on account of its lesser density. Since the axones remain in firm connection with the neuroblasts, it appears in later stages as if they were processes of these and had nothing to do with their original formative cells."
This view is essentially that of Balfour, Beard, and Dohrn; the neurone hypothesis was first clearly formulated in embryological terms by His, and has been supported by the investigations of a considerable number of observers, notably Ramon y Cajal, Lenhossek and Harrison.
The neurone hypothesis has far stronger embryological support than the cell-chain hypothesis; moreover, it is the only possible hypothesis of the development of nerve tracts in the central system, because cell-chains are entirely lacking here during the formation of these tracts. In recent years it has been demonstrated that isolated neuroblasts in culture media produce complete axones, sheath cells being entirely absent. Thus the cell-chain hypothesis has received its final quietus, and is now of historical interest only. (Burrows 1911, Lewis and Lewis 1911.) Splanchnic Components (Sympathetic Nervous System). Two views have been held concerning the origin of the sympathetic nervous system: (a) that it is of mesenchymal origin, its elements arising in situ; (b) that it is of ectodermal origin, its elements migrating from the cerebro-spinal ganglia to their definitive positions. The first view was held by the earlier investigators and was originally associated with the extinct idea that the spinal ganglia were mesenchymal in origin; the view has been entirely abandoned. The second view was partly established with the discovery that the spinal ganglia are of ectodermal origin, and that the ganglia of the main sympathetic trunk arise from the spinal ganglia; but there is some difference of opinion yet in regard to the peripheral ganglia of the symphathetic system, and especially the plexuses of Meissner and Auerbach in the walls of the intestine. However, the preponderance of evidence and logic favors the view of the ectodermal origin of the entire sympathetic nervous system.
The first clear evidences of the sympathetic nervous system of the chick are found at about the end of the third or the beginning of the fourth day; at each side of the dorsal surface of the aorta there is found in cross-section a small group of cells massed more densely than the mesenchyme and staining more deeply. Study of a series of sections shows these to be a pair of longitudinal cords of cells beginning in the region of the vagus, where they lie above the carotids, and extending back to the beginning of the tail; the cords are strongest in the region of the thorax, and slightly larger opposite each spinal ganglion. Cells similar to those composing the cords are found along the course of the nerves up to the spinal ganglia, and careful study of earlier stages indicates that the cells composing the cords have migrated from the spinal ganglia. The two cords constitute the primary sympathetic trunks.
Fig. 152 is a reconstruction of the anterior spinal and sympathetic ganglia of a chick embryo of four days. The primary sympathetic trunk is represented by a cord of cells enlarged opposite each ganglion and united to the spinal nerve by a cellular process, the primordium of the ramus communicans. In the region of the head the segmental enlargements are lacking.
No other part of the sympathetic nervous system is formed at this time with the exception of a group of cells situated in the dorsal mesentery above the yolk-stalk; these are destined to form the ganglion and intestinal nerves of Remak. They have not been traced back to the spinal ganglia, but it is probable that such is their origin.
In the course of the fourth and fifth days aggregations of sympathetic gangUon cells begin to appear ventral to the aorta, and in the mesentery near the intestine. The connection of these with the primary cord is usually rendered evident by agreement in structure, and by the presence of intervening strands of cells; moreover, in point of time they always succeed the primary cord, so that their origin from it can hardly be doubted.
About the sixth day the secondary or permanent sympathetic trunk begins to appear as a series of groups of neuroblasts situated just median to the ventral roots of the spinal nerves. They are thus separated from the spinal ganglia only by the fibers of the ventral roots between which neuroblasts may be found, caught apparently in migration from the spinal to the sympathetic ganglion. The number of these secondary sympathetic ganglia is originally 30, one opposite the main vagus ganglion, and each spinal ganglion to the twenty-ninth (Fig. 150). Soon after their origin they acquire three connections by means of axones, viz., (a) central, with the corresponding spinal nerve root by means of strong bundles of fibers; (b) peripheral, with certain parts of the original primary sympathetic cord; (c) longitudinal, the entire series being joined together by two longitudinal bundles of fibers uniting them in a chain. The central connections constitute the rami communicantes , and are as numerous as the sympathetic ganglia themselves; but so close is the approximation of the sympathetic ganglion to the roots of the spinal nerves that they are not visible externally, the ganglion appearing to be sessile on the root (Fig. 145); sections, however, show the fibers. The peripheral connections constitute the various nerves of the abdominal viscera; these are not metameric; their number and arrangement is shown in Figure 153.
Fig. 152. — Reconstruction in the sagittal plane of the anterior spinal and sympathetic ganglia of a chick embryo of 4 days. (After Neumayer.) Ceph. S., Cephalic continuation of the sympathetic trunk. S. C, Sympathetic cord. Sg., Sympathetic ganghon. sp., Spinal nerve, spg., Spinal ganglion. R. C, Ramus communicans.
In the period between the fourth and the eighth da}^ the primary sympathetic cord becomes resolved into the various ganglia and nerves constituting the aortic plexus, the splanchnic plexus, and the various ganglia and nerves of the wall of the intestine. Remak's ganglion has grown and formed connections with the splanchnic plexus, and other parts of the primary sympathetic cord. The details of these various processes are too complex for full description; they are included in part in Figs. 153 and 154.
Ganglia and Nerves of the Heart. The development of the cardiac nerves is of special interest on account of its bearing on the physiological problem of the origin of the heart-beat. The heart of the chick begins to beat long before any nervous connections with the central system can have been established; indeed, the rhythmical pulsation begins at about the stage of 10 somites when the neural crest is yet undifferentiated, and no neuroblasts are to be found anywhere. Either, then, the heart-beat is of muscular origin (myogenic), or, if of nervous origin, the nerve-cells concerned must exist in the wall of the cardiac tube ah initio.
The first trace of nerve-cells is found in the heart of the chick about the sixth day. These cells are at the distal ends of branches of the vagus, with which they have grown into the heart. Previous to this time these neuroblasts are found nearer to the vagus along the course of the arteries. There can be but little doubt that they have arisen from the vagus ganglion and that they reach the heart by migration. Such an origin has been demonstrated with great probability for all the known nervous elements of the heart of the chick. (See Wilhelm His, Jr., Die Entwickelung des Herznervensystems bei Wirbelthieren.)
If any cardiac nervous elements arise in situ, they certainly remain undifferentiated until those that have a ganglionic origin have already entered the heart.
The Cranial Nerves. Tlie nerves of the head exhibit a much greater degree of heteronomy than the spinal nerves, and, in spite of much study, knowledge of their embryonic development is still in a very unsatisfactory condition. The same principles, however, apply to the development of both cranial and spinal nerves; the axones of the former like those of the latter arise either from medullary or ganglionic neuroblasts which are respectively unipolar and bipolar; but the cranial ganglionic and
Fig. 154. — Diagram of the relations of the parts of the sympathetic nervous system as seen in the cross-section. (After His, Jr.)
M., mesentery. Msn., Mesonephros. Other abbreviations same as Fig. 153.
medullary nerve-nuclei are not similarly segmented, as in the case of the spinal nerves, and hence the axones are not related as dorsal and ventral roots of single nerve trunks; nor has the attempt to interpret the cranial nerves as homologues of dorsal and ventral roots respectively been successful in the case of the most important nerves. Moreover, the olfactory and optic nerves differ from the spinal type even more fundamentally. The olfactory is a sensory nerve that arises apparently from the olfactory epithelium, and the optic is really comparable to an intramedullary nerve tract, seeing that its termination lies in a part of the original wall of the neural tube, viz., the retina.
Groups of medullary neuroblasts giving rise to axones of motor cranial nerves are located in the brain as follows, according to His:
Oculo-motor nucleus in the mid-brain.
Trochlearis nucleus in the isthmus.
Motor trigeminus nucleus in the zone of the cerebellum, including the descending root. Abducens and facialis nuclei, beyond zone of greatest width of the fourth ventricle (auditory sac zone). Glossopharyngeus, vagus, in the region of the calamus scrip torius. Accessorius and hypoglossus, in the region extending to the cervical flexure.
These constitute the cranial motor nerve nuclei, and are more or less discontinuous.
The ganglionic nerves or nerve-components of the head arise from the following primitive embryonic ganglion-complexes:
1. Complex of the trigeminus ganglia.
2. Complex of the acustico-facialis ganglia.
3. Complex of the glossopharyngeus ganglia.
4. Complex of the vagus ganglia.
The early history of these ganglion-complexes has already been considered; they are called complexes because each forms more than one definitive ganglion. It is probable also that each contains sympathetic neuroblasts, which may separate out later as distinct ganglia, thus reseml)ling the spinal sympathetic neuroblasts.
There is no close agreement in the segmentation of the motor neuroblasts within the brain and that of the ganglion complexes. For instance, in the region of the trigeminal ganglionic complex, the motor nuclei of the oculo-motor, trochlearis, and trigeminus are found, and in the region of the vagus ganglionic complex, the motor nuclei of vagus, accessorius, and hypoglossus. Thus the medullary and ganglionic nerves of the head are primitively separate by virtue of their separate origins. They may remain entirelv so, as in the case of the olfactory, trochlearis, and abducens, or they may unite in the most varied manners to form mixed nerves.
The motor nuclei of the oculo-motor, trochlearis, abducens, and hypoglossus nerves He in the same plane as the motor nuclei of the spinal nerves, i.e., in the line of prolongation of the ventral horns of the gray matter. The motor nuclei of the trigeminus, facialis, glossopharyngeus, vagus, and spinal accessory on the other hand lie at a more dorsal level, and the roots emerge therefore above the level of origin of the others. It will be noted that these are the nerves of the visceral arches, whereas those cranial nerves that continue the series of spinal ventral roots innervate myotomic muscles, like the latter. Similarly the ganglia of the pharyngeal nerves (V, VII, IX, and X) differ from spinal ganglia in certain important respects: the latter are derived entirely from the neural crest, whereas a certain portion of each of the primary cranial ganglia is derived from the lateral ectoderm of the head, as noted in the preceding chapter. Thus the pharyngeal nerves form embryologically a class by themselves, both as regards the medullary and also the ganglionic components.
1. The Olfactory Nerve. The embr3'onic origin of the olfactory nerve has been a subject of much difference of opinion: thus it has been maintained by a considerable number of w^orkers that it arises from a group of cells on each side situated between the fore-brain and olfactory pits; some of these maintained that these cells arose as an outgrowth from the fore-brain, others that they came from the epithelium of the olfactory pit, and yet others that this group of cells, or olfactory ganglion, was derived from both sources. This group of cells was supposed by some to include a large number of bipolar neuroblasts, one process of which grew towards the olfactory epithelium and the other towards the fore-brain, entering the olfactory lobe and ending there in terminal arborization. This view is, however, in conflict with the ascertained fact that the fibers of the fully formed olfactory nerve are centripetal processes of olfactory sensory cells situated in the olfactory epithelium.
The most satisfactory account of the origin of the olfactory nerve in the chick is that of Disse. This author finds two kinds of cells in the olfactory epithelium of a three-day chick, viz., epithelial cells, and germinal cells which become embryonic nerve-cells or neuroblasts. At this time the olfactory epithelium is separated from the w^all of the fore-brain by only a very thin layer of mesenchyme. Early on the fourth day axones arise from the central ends of the neuroblasts and grow into the mesenchyme towards the fore-brain. At the same time groups of epithelial cells free themselves from the inner face of the olfactory epithelium, and come to lie between this and the forebrain. The axones of the neuroblasts grow between these cells until they reach the base of the fore-brain over which they spread out, entering the olfactory lobe about the sixth day (Figs. 155 and 156). In the meantime the peripheral ends of the olfactory neuroblasts have extended out as broad protoplasmic processes to the surface of the olfactory epithelium, and thus form the percipient part of the olfactory sense-cells.
Fig. 155. — Olfactory epithelium of a chick embryo of 5 days, prepared by the method of Golgi. (After Disse.) a, b, and c indicate different forms of neuroblasts in the olfactory epithelium.
The epithelial cells between fore-brain and olfactory pit, through which the axones of the olfactory neuroblasts grow, are for the most part supporting and sheath-cells of the nerve, but they include a few bipolar neuroblasts (Fig. 156). The latter are to be considered as olfactory neuroblasts with elongated protoplasmic processes.
Rubaschkin finds a ganglion, which he calls ganglion olfactorium nervi trigemini, situated beneath the olfactory epithelium in a nineday chick. The bipolar cells send out processes peripherally which end in fine branches between the cells of the olfactory mucous membrane, and centrally, which go by way of the ramus olfactorius nervi trigemini towards the Gasserian ganglion.
2. The Second Cranial or Optic Nerve. The course of this nerve is entirely intramedullary, the retina being part of the wall of the embryonic brain; its development will therefore be considered in connection with the development of the eye.
Fig. 156. — Sagittal section through the head of a chick embryo of 5 days, showing the floor of fore-brain, olfactory pit, and developing olfactory nerve between. (After Disse.)
a., Unipolar neuroblasts near the olfactory epithelium, b., Bipolar cell in the olfactory nerve, c, Unipolar cell near the brain. F. B., Floor of fore-brain. N'bl., Neuroblast in the olfactory epithelium, olf. Ep., Olfactory epithelium, olf. N., Olfactory nerve, olf. P., Cavity of olfactory pit.
3. The third cranial or oculo-motor nerve arises from a group of neuroblasts in the ventral zone of the mid-brain near the median line, and appears external to the wall of the brain at about sixty hours (about 28-30 somites). At this time it appears as a small group of axones emerging from the region of the plica encephali ventralis, and ending in the mesenchyme a short distance from its point of origin. At seventy-two hours the root is much stronger, interpenetrated with mesenchyme and ends between the optic cup and floor of the brain behind the optic stalk (cf. Fig. 101). At ninety-six hours the root is broad and fan-shaped, the nerve itself is comparatively slender, and passes downwards and backwards behind the optic-stalk where it enters a welldefined ganglion situated just median to the ophthalmic branch of the trigeminus; this is the ciliary ganglion; beyond it the fibers of the oculo-motor turn forward again to enter the region of the future orbit.
According to Carpenter (1906) the ciliary ganglion arises from two sources: (a) migrant medullary neuroblasts that pass out into the root of the oculo-motor, and follow its course to the definitive situation of the ciliary ganglion, and (b) a much smaller group of neuroblasts that migrate from the ganglion of the trigeminus along the ophthalmic branch, and by way of a ramus communicans to the ciliary ganglion. The adult ciliary ganglion show\s correspondingly two component parts: (a) a larger ventral region composed of large bipolar ganglion cells, and (6) a smaller dorsal region containing small ganglion cells with many sympathetic characters. It is probable that the medullary fibers of the oculo-motor nerve are distributed entirely to the muscles innervated by it, viz., the superior, inferior, and internal rectus and inferior oblique muscles of the eye. The fibers arising from the neuroblasts of the ciliary ganglion terminate peripherally in the intrinsic muscles of the eye-ball, and centrally (in the case of the bipolar cells) in the brain, which they reach by way of the medullary nerve. The motor branches leave the trunk of the nerve a short distance centrally to the ciliary ganglion.
4. The trochlearis or fourth cranial nerve is peculiar inasmuch as it arises from the dorsal surface of the brain in the region of the isthmus. It arises entirely from medullary neuroblasts and innervates the superior oblique muscle of the eye. Marshall states that it may be readily seen in a five-day embryo; in an embryo of eight davs it is a slender nerve arising from the dorsal surface of the isthmus immediately in front of the cerebellum ; the fibers of the two sides form a commissure in the roof of the isthmus (Fig. 148).
5. The trigeminus or fifth cranial nerve consists of motor and sensory portions. The latter arises from the trigeminal ganglion, the origin of which has already been described. The ganglionic rudiment appears roughly Y-shaped even at an early stage (cf. Figs. 105 and 117), the short stem lying against the wall of the brain and the two branches diverging one in the direction of the upper surface of the optic cup (ophthalmic branch) and the other towards the mandibular arch. The original connection of the ganglion with the roof of the neural tube is lost during the second day and permanent connection is established during the third day, presumably by growth of axones into the wall of the brain. The new connection or sensory root of the trigeminus is attached to the myelencephalon in the region of greatest width of the fourth ventricle near the ventral portion of the lateral zone.
During the fourth day the peripheral axones follow the direction of the ophthalmic and mandibular branches of the ganglion and grow out farther as the ophthalmic and mandibular nerves; the former passes forward between the optic vesicle and the wall of the brain; the latter runs ventrally towards the angle of the mouth, over which it divides, a smaller maxillary branch entering the maxillary process of the mandibular arch, and a larger one, the mandibular nerve, runs into the mandibular arch. (For an account of the branchial sense organ of the trigeminus, see Chap. VI.)
A medullary component of the trigeminal nerve arises from the wall of the brain just median to the ganglionic root during the fourth day; it runs forward parallel to the ganglionic ophthalmic branch, and sends a twig to the ciliary ganglion. Beyond this point it unites with the ganglionic branch.
A connection of the trigeminus with the olfactory sensory epithelium is described under the olfactory nerve.
6. The sixth cranial or abducens nerve is stated to arise about the end of the fourth day. It is a purely motor nerve, and has no ganglion connected with it; it innervates the external rectus muscle of the eye. At 122 hours it arises by a number of slender roots attached to the myelencephalon near the mid- ventral line, beneath the seventh nerve. Its roots unite into a slender trunk that runs directly forward beneath the base of the brain to the region of the orbit. The sixth nerve thus corresponds more nearly than any other cranial nerve to a ventral spinal nerveroot.
7 and 8. The Facial and Auditory Nerves. The ganglia of these nerves at first form a common mass, the acustico-facialis. But during the course of the fourth day the anterior and ventral portion becomes distinctly separated from the remainder and forms the geniculate ganglion; the remainder then forming the auditory ganglia (cf. Fig. 102). The acustico-facialis ganglion complex moves from its original attachment to the dorsal surface of the brain and acquires a permanent root during the third day, attached ventrally just in front of the auditory sac.
(a) The seventh cranial or facialis nerve arises during the fourth day from the geniculate ganglion which is situated just above the second or hyomandibular branchial cleft. It grows first into the hyoid arch (posttrematic branch), but towards the end of the fourth day a small branch arises just above the cleft and arches over in front of it and runs down the posterior face of the mandibular arch (pretrematic branch). The origin of the motor components is not known.
(h) The further history of the auditory nerve is considered with the development of the ear.
9. The ganglion of the ninth cranial or glossopharyngeal nerve (ganglion petrosum cf. Fig. 102) arises from the anterior part of the postotic cranial neural crest as already described. Early on the fourth day the ganglionic axones enter the base of the brain just behind the auditory sac and establish the root, which consists of four or five parts on each side. From the ganglion which is situated at the summit of the third visceral arch a strong peripheral branch develops on the fourth day, and extends into the same arch; a smaller anterior branch develops a little later which passes over the second visceral pouch and enters the second visceral arch. About the same time an anastomosis is formed with the ganglion of the vagus.
10. The tenth cranial or vagus (pneumogastric) nerve is very large and complex. Its ganglion very early shows two divisions, one near the roots (ganglion jugulare) and the other above the fourth and fifth visceral arches (ganglion nodosum cf. Fig. 102). It arises by a large number ot fine rootlets on each side of the hind-brain behind the glossopharyngeus, and the roots converge in a fan-like manner into the proximal ganglion; from here a stout nerve passes ventrally and enters the ganglion nodosum situated above the fourth and fifth visceral arches. Branches pass from here into the fourth and fifth arches, and the main stem is continued backward as the pneumogastric nerve s.s. From the hinder portion of the spreading roots a strong commissure is continued backward parallel to and near the base of the neural tube as far as the fifth somite; this is provided with three small ganglion-like swellings. This condition is found about the end of the fourth day. Later this commissure unites with the main sympathetic trunk, and part of the vagus ganglion separates from the remainder as the ganglion cervicale primum of the sympathetic trunk.
During the fifth and sixth days the main stem of the vagus grows farther back and innervates the heart, lungs, and stomach. Neuroblasts of the sympathetic system accompany the vagus in its growth, and form the various ganglion cells of the heart, and other organs innervated by the vagus.
During the fifth and sixth days the ganglion nodosum, which originally lay at the hind end of the pharynx, is carried down with the retreat of the heart into the thorax, and on the eighth day it is situated at the base of the neck in close contact with the thymus gland.
11. The Eleventh Cranial or Spinal Accessory Nerve. No observations on the development of this nerve in the chick are known to me.
12. The twelfth cranial or hypoglossus nerve appears on the fourth day as two pairs of ventral roots opposite the third and fourth mesoblastic somites; each root is formed, like the ventral roots of the spinal nerves, of several bundles that unite in a common slender trunk; ganglia are lacking, as in the first and second cervical nerves. The roots of the hypoglossus are a direct continuation of the series of ventral spinal roots, and as they are related to somitic muscle plates in the same way as the latter, there can be no doubt of their serial homology with ventral roots of spinal nerves. The first four mesoblastic somites are subsequently incorporated in the occipital region of the skull, and thus the hypoglossus nerve becomes a cranial nerve. No nerves are formed in connection with the first and second mesoblastic somites. As the occipital region of the skull forms in the region of the occipital somites, two foramina are left on each side for exit of the roots of the hypoglossus (Figs. 150 and 244).
During the fourth and fifth days the nerve grows back above the roof of the pharynx, then turns ventrally behind the last visceral pouch and forward in the floor of the pharynx.
According to Chiarugi minute ganglia are formed in the second, third, and fourth somites: but they soon degenerate (fourth day) without forming nerves.
Note: The structure called "hypophysis" on p. 249, sometimes called Rathke's pouch, forms only the anterior lobe or glandular part of the hypophysis of adult anatomy; the posterior lobe of the hypophysis is derived from the structure called " saccus infundibuli " in this chapter, which is a derivation of the floor of the brain.
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