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Streeter GL. The Development of the Nervous System. (1912) chapter 14, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.

XIV. Development of the Nervous System: Histogenesis of Nervous Tissue | Central Nervous System | Peripheral Nervous System | Sympathetic Nervous System | Manual of Human Embryology II
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III. Peripheral Nervous System

Embryology History George Streeter
George Linius Streeter (1873-1948)

By George L Streeter, Ann Arbor, Mich.

Spinal Nerves

The ventral roots of the spinal nerves are derived from the mantle layer of the neural tube, as has been previously referred to. Processes from the neuroblasts situated in the mantle layer are assembled into rootlets which emerge in a continuous longitudinal series along the ventrolateral border of the tube. Outside of the tube these rootlets are grouped into segmental bundles, and after being joined by the fibres of the dorsal roots they constitute complete segmental nerves. The direction taken by the ventral root fibres on emerging from the tube varies according to the size and position of the ganglion crest. In the human embryo it is almost directly lateral. In some sections of the 5.5 mm. pig in the author's possession the ventral roots extend dorsalward through the mesenchyma at an angle of 30° to reach the ventral border of the ganglion mass.

Keibel Mall 2 082.jpg

Fig. 82. — Reconstruction of a portion of the peripheral nerves of a human embryo 4 mm. long. (Hertwig collection, No. 137). Enlarged 22 : 1. Ot. v., ear vesicle.

The dorsal roots are derived entirely from the ganglion crest. This structure can be seen in the 4 mm. embryo, Fig. 82, as a flattened cellular band which extends caudalward from the auditory vesicle along the lateral border of the neural tube to its extreme tip. As will be referred to later, the ganglion crest of the hindbrain is similar to and by some described as continuous with that of the spinal cord (p. 139). That part of the crest which corresponds to the spinal cord is characterized at this time by segmental incisures along its ventral border. The dorsal border of the crest remains intact until the appearance of the dorsal rootlets, in the meantime constituting a cellular bridge connecting the more ventral ganglionic clumps. In embryos at the end of the fourth week _ B fibrous processes can be seen cropping out from the dorsal border of the crest and attaching themselves to the spinal cord. They appear first in the cervical region and somewhat later can be seen in the more caudal part of the crest. These are the primitive dorsal rootlets. They enter the marginal zone of the tnbe and eventually form a longitudinal bundle corresponding to the dorsal funiculus of the adult cord. Peripherally they can be traced back to cell clusters in the crest, the processes from ^ral cells uniting in a common fibrous strand. With the formation of these rootlets there is a gradual disappearance of the dorsal bridge, and there is thereby produced a complete segmentation of the ganglion crest. The ventral end of the ganglia extend forward and end diffusely among the fibres of the ventral roots. The cells are in a state of active differentiation, and the developing fibrous pro' can be seen joining the more precocious ventral root

Keibel Mall 2 083.jpg

Fig. 83. a of the peripheral nerves of a human embryo 6.9 mm long (His collection. Enlarged 16.7 : 1.

In embryos 9 to 10 mm. long (Fig. 84) the differentiation of the ganglion-cells and their fibrous processes has advanced to a point where the chief parts of a typical spinal nerve may be recognized. There is the dorsal root and its sharply outlined ganglion and a well-defined ventral root joins it, the two together forming the nerve-trunk. At the same time that the dorsal and ventral roots unite to form the main trunk, they both give off lateral fibres which form the dorsal branch, the so-called posterior primary division, which breaks up among the cells that are to form the long muscles of the back, supplying these and extending through to reach the integument. The remainder of the nervetrunk is continued forward as the ventral branch, or anterior primary division. From its median side there is given off the ramus communicans, which extends toward the aorta and ends in the sympathetic ganglion cord. The main trunk terminates in two branches, the anterior and lateral terminal branches, from which arise the anterior and lateral cutaneous branches of the adult, and which in the thoracic and abdominal regions give off branches to the musculature of the front and lateral body wall. The relation of these branches to the individual muscles is shown at a later stage in Fig. 85. In the same figure can be seen the loop-formation in the intercostal space that occurs before the bifurcation of the trunk into the lateral and anterior terminal branches is completed.

Keibel Mall 2 084.jpg

Fig. 84. Diagrammatic transverse section through a thoracic segment of a 9 mm human embryo. Enlarged 25 : 1. (After Bardeen and Lewis, 1901.)

Throughout the spinal region there is a tendency for the ad cent nerve-trunks to unite at the place where the lateral term; branches arise, and there is formed thereby a ieri inters mental loops. This loop- or plexus-formation may involve either the lateral or the anterior terminal branches, or both. Its degree of development depends upon the complexity of the parts supplied.

Keibel Mall 2 085.jpg

Fig. 85. — Diagrammatic transverse section through a thoracic segment of a 17 mm human embryo (Huber collection, No. 14), showing a typical thoracic nerve. Enlarged 15 : 1.

In three regions it is particularly marked, and there are thus produced the cervical, brachial, and lumbosacral plexuses.

Cervical Plexus

In the cervical region the anterior and lateral terminal branches form two separate plexuses; the former produces the deep cervical plexus, and the latter the superficial cervical plexus. The superficial cervical plexus consists of the union of the lateral terminal branches into loops, from which are given off the cutaneous branches to the auricular, cervical, and occipital regions. The deep plexus results in the formation of the ansa hypoglossi and the phrenic nerve. The former is produced by the fusion of the second and third cervical nerves into the descendens cervicis, which unites in a loop with the hypoglossal, together with which the first cervical has been incorporated above. From this loop are given off the short branches which end among the cells that are to form the hyoid musculature.

The phrenic nerve is formed by anterior terminal branches principally from the fourth and fifth cervical nerves. A contribution on the part of the sixth and third nerves may occur. This can be seen through the transparent arm in Fig. 8G. Owin to the position of the diaphragm at this time the course of the nerve is almost directly ventral. Later, as pointed out by His and Mall (Mall, 1901), the points of origin and insertion of the nerve draw gradually apart, due on the one hand to the descent of the diaphragm and the lengthening of the thoracic cavity, and on the other hand to the subsequent elevation of the cervical nerves which accompanies the development of the structures of the neck. It is thus that there results the long caudal course of this nerve that is characteristic of the adult.

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Fig. 86. Reconstruction of the peripheral nervous system of a human embryo 10 mm long (Huber collection, No. 3). The arm and leg are represented as transparent masses, into the substance of which the branches of the brachial and lumbosacral plexuses may be followed. Enlarged 12 : 1.

Nerves of the Arm and Leg

In the arm and leg we meet with special conditions which will be better understood if we first refer to two essential factors that were established by the recent experimental work on amphibian larvae by Harrison (1907). In the first place, he has shown that the nerves which take part in the innervation of a limb are determined by the position and width of the limb bud. A limb bud transplanted to some other part of the body acquires a complete system of nerves, supplied by the region in which the limb is implanted. In the second place, the distribution of the nerves within a limb is determined by its own component structures. The segregation of the developing structures within the limb has a directive action upon the growing nerve-fibres, and determines their grouping into definite characteristic bundles. Even foreign nerves entering a transplanted limb bud are likewise controlled so that they form intrinsic nerves to the limb and assume normal terminal ramifications. These two factors are to be kept in mind in interpreting the normal embryology of these nerves. Our knowledge concerning the details in the development of the nerves of the arm in the human embryo is based principally upon the work of Lewis (W. H., 1902) and of the leg on the work of Bardeen (1907). Large use has been made of their papers in the following description.

When the limb buds first form they consist, to all appearances, of homogeneous mesenchyma and contain no nerves. Very soon, however, opposite the base of each limb bud, presumably stimulated by its presence, the anterior primary divisions of the spinal nerves undergo an exuberant growth and form a solid sheet or plexus of fibres extending toward the base of the limb. In embryos a few days older, coincident with the condensation of the skeletal core, branches from this nerve-plexus can be seen advancing into the limb bud and entering the areas of premuscle tissue that in the mean time have formed a sheath around the skeletal core. The premuscle sheath is not evenly distributed, but from the very start is arranged in the form of muscle groups, and it is between these groups through the intermuscular spaces that the nerves make their way. As the differentiation of the limb continues the nerve trunks extend distalward in the limb and give off muscle branches which enter the muscle groups and supply the individual muscle anlages. The site of nerve entry into a muscle is constant. It is situated near the centre of the anlage on the side toward the main trunk. This point is the seat of the earliest differentiation of the muscle, and according to the Nussbaum law muscle growth extends from here in the direction of the intramuscular nerve branching. Though nerve-fibres and muscle groups seem to make their appearance simultaneously, and even at times the nerves seem to precede the muscles, yet it must be remembered that the experimental evidence clearly shows that the situation and branching of the nerves are entirely dependent on the structural segregation of the developing muscles and skeleton, and this is why the main nerve-trunks are developed in paths situated in the intermuscular areas, and likewise why the larger nerve branches are in the intramuscular septa of individual muscles. As we do not have a metameric distribution of the muscles of a limb, we consequently do not have a true metameric distribution of the nerves.

Keibel Mall 2 087.jpg

Fig. 87. Diagram illustrating the influence of the direction of the fibre bundles in different muscle types upon the character of their nerve supply. (After Bardeen, 1907.)

Within the muscle the course of the chief branches is determined by the direction of the fibre bundles, whether they run parallel or are transverse to the main trunk from which the nerve arises. When the fibre bundles of a muscle are transverse to the main nerve-trunk, its nerve and chief branches pass across the fibre bundles midway between the points of attachment, giving off branches on each side which go to form the intramuscular nerve-plexus. When the fibre bundles of a muscle run parallel with the main nerve-trunk, the branches to this muscle, as a rule, enter the proximal third of the muscle belly and extend distally parallel with the muscular fibres, giving off at the same time the branches to the intramuscular plexus. This relation between the direction of the muscle-fibres and the course of the supplying nerve is shown for different muscle types in Fig. 87. A further influence on the course of nerves is exerted by the migration of the muscles which they supply. The muscle masses, having received their nerves at an early stage, may by subsequent migration draw the nerves a long way out of their original course. This is illustrated by the latissimus dorsi, the trapezius, the diaphragm, and the muscles of the tongue.

The arm bud develops somewhat in advance of the leg bud. In the 4.5 mm. embryo there is a well-defined arm bud, consisting of an apparently homogeneous mesenchyma and having as yet no nerves. A leg bud in a similar stage of development is not met with until we come to embryos about 7 mm. long. The base of the arm bud is usually situated opposite the lower four cervical and first thoracic vertebrae, and the leg bud opposite the five lumbar and first sacral. There is some variation in the position of the limb buds relative to the spinal axis at the time of entrance of the spinal nerves. Consequently there may be a variation of as much as three segments in the origin of the spinal nerves which eventually enter a particular limb, the more cephalic nerves supplying the limb when the limb bud has a more cephalic position and vice versa.

In the 9 mm embryo the central mesenchyma of the leg bud is condensed into sclerogenous tissue corresponding to the hip-bone and proximal part of the femur. This sclerogenous tissue divides the bundles of nerve-fibres streaming into the leg into the main nerve-trunks. The lumbosacral plexus in an embryo of about this age is shown in Fig. 86. The nerves forming it unite into a flattened mass or sheet of fibres which enters into the base of the leg bud, the division into anterior and lateral terminal branches being lost in the formation of the plexus. The further course of the fibres is determined by the framework of the leg. Owing to the cell masses of the bony pelvis and the femur the fibres become grouped into four bundles arranged in two pairs, each consisting of a median and lateral trunk. Of the upper pair the median trunk corresponds to the n. obturator, and the lateral to the n. femoralis. The lower pair represent the n. sciaticus, the median bundle constituting the future n. tibialis, and the lateral the n. peronseus communis. In the 11 mm. embryo, as shown in Fig. 88, the leg is differentiated externally into foot-plate, cms, and thigh, and internally, surrounding the skeletal core, a distinct myogenous zone can be recognized, consisting of muscle groups with intervening intermuscular spaces. The main nerve-trunks have grown well down into the limb between the muscle groups, and the chief muscular and cutaneous branches can be seen. In Fig. 89 is represented a median view of the leg and the adjoining part of the trunk C)f an embryo 20 mm. long, showing the relations of the thoracic, lumbar, and sacral nerves to the abdominal musculature and the skeleton of the leg. Both muscular and sensory branches are shown, and it will be seen that the nerve supply of the leg at this time must be considered as essentially complete.

The nerves of the arm in a 9 mm embryo are shown in Fig. 90. The brachial plexus consists of a continuous sheet of fibres which on reaching the developing humerus is split into a dorsal and ventral division, the former corresponding to the posterior cord and the latter to the outer and inner cords. These cords are immediately broken up into the large branches which pass down in the intermuscular spaces, where the fibres abruptly fray out to enter the premuscle masses. The formation and branches of the brachial plexus, as seen in the 10 mm. embryo, are shown in Fig. 86. The brachial plexus is split by the skeletal anlage into two laminae from which the various nerves arise. From the anterior or ventral lamina arise the n. musculocutaneus, n. medianus, and n. ulnaris, and from the posterior or dorsal lamina the n. axillaris and n. radialis. As compared with the lumbosacral plexus in the same embryo it is considerably in advance. It is not until later (20 mm), secondary to the caudal migration of the arm, that we meet with a decided posterior inclination of the brachial plexus ; but, aside from this and aside from the proportionate large size of the nerves as compared with other structures, there is little in their gross morphology to distinguish the nerves of the arm at the end of the first fetal month from those of the adult. In the following table is given the origin of the fibres of the larger nerves of the arm as traced by Lewis (1902) in an embryo of this age:

Keibel Mall 2 088.jpg

Fig. 88. Nerves of the leg in an embryo 11 mm long, age about five weeks. Enlarged 17 : 1. The principal muscle anlages are shown in lighter color. Isch., os ischii; Mm. add., musculi adductores; Mm. cr. post, sup., musculi cruris post, superficiales; Mm. cr. post, prof ., musculi cruris post, profundi; Mm. fern, post., musculi femorales posteriores; M. obt. int., muse, obturator int.; M. quadr. fern., muse, quadratus femoris; M . rect. abd., muse, rectus abdominis; N. bi. prox. port, et semit., n. proximalis portionis muse, bicipitis et muse, semitendinosi; N. cocc., n. coccygeus; N. cut. ant., n. cutaneus femoris anterior; N . cut. fern, post., n. cutaneus femoris posterior; N. cut. lot., n. cutaneus femoris lateralis;, n. iliohypogastricus; N. il. ing., n. ilio-inguinalis; iV. I. ing., n. lumbo-inguinalis; N. pud., n. pudendus; N. saph., n. saphenus; N. sart., n. muse, sartorii; N. sp. ext., n. spermaticus ext.; N. thor. 11, n. thoracalis 11; Pu„ os pubis. (After Bardeen, 1907.)

Keibel Mall 2 089.jpg

Fig. 89. Nerves of the leg and adjacent abdominal wall in an embryo 20 mm. long.'age about' seven weeks. Enlarged 10 : 1. (After Bardeen, 1907.) For the abbreviations see Fig. 88.

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Fig. 90. Reconstruction of the nerves and skeleton of the arm in a human embryo 9 mm long. (After W. H. Lewis, 1902.) Cervical.


N. suprascapularis IV, (VI) N. subscapularis V, VI, VII N. thoracalis longus VII, VLTl Nn. thoracales anteriores V, VI, VII, VIII N. musculocutaneus V, VI, (VII) N. medianus V, VI, VII, VIII N. axillaris V, VI, VI I N. radialis V, VI, VII, VIII N. ulnaris (VI), VII, VIII

According to Bardeen (1907) the cutaneous nerves first approach the superficial fascia along the line corresponding to the primary margin of the limb bud, and from these areas send branches of distribution over the dorsal and ventral surfaces of the developing limb, as shown in Fig. 91. There exi-N a considerable variation in the extent of distribution of these branches to the skin. The extensive development of one nerve tends to retard the growth of the neighboring nerves, and diminished development stimulates them to more active growth. A further source of variation, which is equally true of motor nerves, is that any two nerves that arise in succession — such as the twelfth thoracic and hypogastric, the hypogastric and inguinal, or the lateral cutaneous and femoral— may be combined into a single trunk for a greater or lesser part of their course. On the other hand, two or more nerve-trunks may carry fibres ordinarily assembled in a single trunk, such as extra iliac and genital branches or an accessory obturator.

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Fig. 91. Diagram of the cutaneous nerves of the lumbosacral plexus, showing the superficial areas along the margin of the limb bud which are first reached by the tips of the growing nerves. Extending from these foci branches spread eventually over the dorsal and ventral surfaces of the limb. A, ventral group; B, dorsal group. (After Bardeen, 1907.)

Cerebral Nerves

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Modern Notes: cranial nerve

Cranial Nerve Links: Neural | Neural Crest | CN I | CN II | CN III| CN IV | CN V | CN VI | CN VII | CN VIII | CN IX | CN X | CN XI | CN XII | placodes | Category:Cranial Nerve
Historic Embryology Cranial Nerves 
1908 10 mm Human Embryo | 1912 Cerebral Nerves | 1925 Rat Ganglia | 1927 Oculomotor | 1938 Spinal Accessory | 1938 Hypoglossal | 1941 Olfactory | 1943 Oculomotor | 1947 Facial

For purposes of description the nerves of the head will be grouped according to their function, following as far as possible the functional systems of Gaskell ; i.e., the activities of the organism are separated into somatic and visceral, in each of which there is the double activity on the part of the nervous system, motor and sensory, making in all four primary functional divisions. Some of the cranial nerves consist of elements belonging exclusively to one functional division, — for example, the n. abducens, which consists entirely of somatic motor fibres, — while others are complex nerves containing elements of more than one system, such as the n. vagus, which contains elements from three functional divisions, somatic sensory, visceral sensory, and visceral motor. The nerves will therefore be grouped according to the predominance of their functional elements as follows: Somatic sensory. Somatic motor. Visceral (motor and sensory).

Olfactory Oculomotor Trigeminal Optic Trochlear Facial Acoustic Abducens Glossopharyngeal Hypoglossal Vagus and accessory In general the basal plate of the neural tube is motor and the lateral or alar plate is sensory; thus, the somatic motor group arises entirely from the basal plate, while the visceral group is connected in larger part (sensory) with the alar plate and in lesser part (motor) with the basal plate. The nerves included under the somatic sensory group are all specialized nerves and have individual processes or lobes of the nervous system devoted to them, — i.e., the olfactory bulb, the eye bulb and stalk, and the tuberculum acusticum.

Nerves of Special Sense Organs. (Special Somatic Sensory. ) These nerves belong to the group of afferent nerves which connect the integument with the central nervous system, and the union of nerve and integument has resulted in the formation of special sense organs, — that is, the olfactory organ, the eye. the ear, and the lateral line system, composed partly of nerve elements and partly of integument. This nerve group is shown in Fig. 92, which represents schematically a typical vertebrate &ead in which the mtegumental part of the special sense organs is shown in red. The nose, eye, and ear are the same as seen in man. The lateral line system, however, is absent in man except for a temporary trace which may be seen in 7 mm. embryos in the form of areas of thickened epidermis situated over the second, third, and fourth branchial arches, probably representing the lateral line ganglia incorporated with the seventh, ninth, and tenth cranial nerves of lower- vertebrates. By comparing the nerve portions of these organs in Fig. 92 it can be seen that the olfactory nerves, the retinal ganglion-cells, and the acoustic nerve, though differing so widely in their adult morphology, must all be considered as analogous structures.

The nn. olfactorii and the n. opticus will be described with the special sense organs to which they belong.

The n. acusticus develops from a small group of ganglion-cells which can be recognized by the end of the third week, lying closely against the cephalic border of the ear vesicle. Nerve-fibres arise from the proximal pole of the mass connecting it with that part of the neural tube which is to form the tuberculum acusticum, and from the distal pole connecting it with the auditory vesicle. Concerning the origin of these ganglion-cells there still remains some uncertainty. They are evidently not derived from the neural crest; but whether they migrate out from the brain wall or the walls of the developing ear vesicle, or are derived from the ectoderm immediately adjacent to the auditory pit, remains to be determined.

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Fig. 92. — Diagrammatic vertebrate head showing the special sense organs that are formed by the union of nerve and integument, the in tegumental portion being shown in red.

The successive* stages in the development of the acoustic ganglion mass and its branches are represented in Fig. 93, which shows its appearance in embryos 4, 7, 9, 20, and 30 mm. long. This should be compared also with Fig. 86. The first step consists in the elongation of the ganglion and partial subdivision into a pars superior and pars inferior, each of which develops its own separate group of peripheral nerve branches. The pars superior forms the ganglion of the nerves to the utricle and the ampullar of the superior and lateral semicircular canals. The pars inferior forms the ganglion of the nerves to the saccule and the ampulla of the posterior canal. In addition there is derived from the pars inferior a cell mass that becomes differentiated into the ganglion spirale. This makes its appearance in embryos about 9 mm. long, where it can be seen that some of the ganglion-cells on the ventral border of the pars inferior have become massed together to form the anlage of this ganglion; in other words, the ganglion acusticum at this stage consists of an upper division entirely vestibular and a lower division partly vestibular and partly cochlear. The vestibular part of the pars inferior constitutes the so-called Zwi sell en ganglion of His, jun.

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Fig. 93. — Development of the left ganglion acusticum and its nerve branches. The vestibular portion is shown by fine dots and the cochlear (ganglion spirale) by large dots. In the 9, 20 and 30 mm. stages a median view is shown on the left and a lateral view on the right.

As the differentiation proceeds and the fibres elongate, the group of cells forming the ganglion spirale becomes separated from the parent ganglion mass, and eventually assumes the spiral form of the adult. The pars superior and pars inferior usually become completely separated, accompanying the subdivision of the vestibular nerve. The embryonic connection between the two may, however, persist. Likewise the path of separation between the pars inferior and the ganglion spirale may be more or less completely bridged over in the adult by a persistent connecting chain of ganglion-cells.

It will be seen that the n. cochlearis is made up entirely of fibres derived from the ganglion spirale, while the n. vestibularis consists of two portions derived respectively from the pars superior and pars inferior of the ganglion vestibulare. Owing to the contiguity of the pars inferior and the n. cochlearis, they become closely united by the developing mesenchymal elements, and this gives rise to the misleading appearance in the adult of the saccule and posterior ampulla being supplied by the cochlear nerve. The vestibular and cochlear divisions of the acoustic complex present the following contrasting characteristics : The ganglia belonging to the vestibular division develop midway along the trunk of the nerve; the ganglion of the cochlear division is situated at the extreme distal end of the nerve and lies closely against the cochlear duct, being later incorporated with it in the cartilaginous capsule, and hence the cochlear terminal branches are short and form a continuous fringe of fibres, while the vestibular terminal branches form discrete nerves of some length; the main trunk of the cochlear division is characterized by the compactness of its fibres and their spiral arrangement, which is already apparent in the 30 mm. embryo, while the fibres of the vestibular division are less compactly bundled and do not have a spiral character. In these particulars the vestibular nerve is the more primitive of the two, and the cochlear nerve must be considered as a portion of the former that has undergone special development.

Somatic Motor Group

This group consists of the hypoglossal and the three nerves to the extrinsic eye muscles (nn. oculomotorius, trochlearis, and abducens), and in common they all arise from the basal plate and maintain their position near the median line directly beneath the floor of the ventricle. Their nuclei of origin are considered as a cephalic continuation of the ventral motor column of the spinal cord. The series is shown in red in Fig. 100.

The n. liypoglossus, as it appears in the 4 mm. embryo, is shown in Fig. 82. Its rootlets arise from the basal plate of the neural tube in three or four segmental groups in a longitudinal series directly continuous with the ventral roots of the cervical nerves. During the fourth week they grow forward and fuse in a common trunk, which by the end of the first month has made its way around the lateral border of the ganglion nodosum, and there breaks up in its terminal branches in the anlage of the tongue. It is now generally considered that the hypoglossal is a composite nerve made up of the ventral roots of three or four segmental spinal nerves which in the course of phylogenesis have become inclosed by the bony cranium (occipitospinal nerves). This view is supported by the identity in appearance that exists in the earliest stages (Fig. 82) between the hypoglossal rootlets and the ventral roots of the spinal nerves. Furthermore the nucleus of origin of the hypoglossal forms a continuous column with the ventral horn of the spinal cord, as seen in Fig. 100. The fact that there are no dorsal roots and ganglia as in the other spinal nerves is explained on the ground of retrogression of the sensory part of these nerves, involving especially their more cephalic rootlets. Occasionally in the embryo a ganglion, and at times also a dorsal root, is found in connection with the more caudal rootlets of the nerve (Froriep's ganglion), as in Fig. 94. In other cases, where the sensory retrogression is more extreme, there not only is no ganglion of the hypoglossal, but the ganglion of the first cervical is also partially or wholly absent.

In the course of its development the hypoglossal unites with the cervical nerves in the formation of the ansa hypoglossi. The steps in the formation of this plexus may be seen in Figs. 82, 83, and 94. The fibres of the hypoglossal and the upper three cervical nerves start out perpendicularly from the neural tube, and due to the curve of the latter they are brought together at a common focus, like spokes in a wheel, and enter together- the muscle mi Froriep's Schulterzungenstrang, that is to form the tongue and hyoid muscles. With the formation of nerve sheathe adjoining fibres become bound together, so that, as the individual muscles take form and draw apart, the nerves are separated out in the plexus that is characteristic of the nerves supplying these mus< in the adult. As seen in Fig. 86, the essential features of ansa hypoglossi are completed in the 10 mm. embryo. The f cervical unites with the trunk of the hypoglossal, and lower down leaves it as the descendens hypoglossi, carrying along a var] number of hypoglossal fibres. The descendens hypoglossi on with the descendens cervicis, a fused branch from the second and third cervical nerves, and thereby forms a loop, the ansa hypoglossi, from which are .given off branches to the hyoid musculature. The n. oculomotorius arises from a group of neuroblasts situated in the ventral part of the mantle layer of the mesencephalon. These neuroblasts converge to form small rootlets, which emerge on the ventral surface of the neural tube in the concavity of the cephalic flexure. Here they unite, as shown in Figs. 10, 83, and 86, in a common trunk, which passes ventralward to the region median to the first and second divisions of the trigeminal nerve, where it breaks up in the cellular mass that is to form the eye muscles. It eventually supplies a root to the ciliary ganglion. There is no sensory ganglion in connection with the oculomotor in the human embryo. In the chick and torpedo, however, large bipolar cells have been described as migrating along its trunk to take part in the formation of the ciliary ganglion (Froriep, 1902, Carpenter, 1906).

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Fig. 94. — Reconstruction of the peripheral nerves on the left side of a human embryo 14 mm. long (Mall collection, No. 144). Enlarged 16.7 : 1.

The ii. trochlearis arises from a cluster of neuroblasts similar and lying just caudal to those of the oculomotor. The rootlets derived from them, instead of emerging directly ventralward, curve dorsalward over the roof of the aqueduct, where they decussate and emerge as a slender trunk which passes ventralward to reach the anlage of the superior oblique muscle. No satisfactory explanation has ever been given for the peculiar dorsal decussation of this nerve.

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Fig. 95. — Composite sagittal section through a human embryo 10 mm. long, the same shown in Fig. 86, and representing the rhombic grooves and their nerve connection with the branchial arches. The nerve arising from groove d is the n. abducens, which passes median to the ganglion semilunare.

The n. abducens arises from a group of neuroblasts in the median part of the mantle zone directly beneath the fourth rhombic groove or neuromere (Figs. 95 and 100). These neuroblasts converge and form rootlets which pass through the marginal zone and emerge on the ventral surface of the neural tube. On emerging, as can be seen in the 10 mm. embryo, they are gathered together in a single trunk, which immediately bends forward at an angle of 90°, and passes forward mesial to the semilunar ganglion to reach the anlage of the external rectus muscle. It has been shown by Bremer (1908) and Elze (1907) that it is not uncommon for this nerve to have a series of multiple rootlets, with a corresponding caudal prolongation of its nucleus of origin backward into the region of the fifth rhombic groove. On the other hand, it has been further shown by Bremer (1908) that the nucleus of origin of the n. hypoglossus may extend forward and more or less completely bridge in the gap existing between it and the n. abducens. The root fibres arising in such cases from the extreme cephalic end of the hypoglossal nucleus and from the extreme caudal end of the abducens nucleus, instead of joining with their respective nervetrunks, show a tendency to form "aberrant roots," which pass out in various directions and are lost in the loose mesenchyme. These aberrant roots eventually (during the second month) entirely disappear.

Visceral Group

The facial, glossopharyngeal, and vagus form a series of similar nerves which consist almost wholly of visceral fibres. Their motor visceral fibres arise from a column of neuroblasts (nucleus ambiguus and nucleus facialis) continuous with the lateral horn cells of the cord. Their sensory visceral fibres arise from the peripheral ganglia and enter the alar plate of the neural tube and form a longitudinal strand which in the adult we know as the tractus solitarius. In addition to these visceral fibres there are a few somatic sensory fibres for the supply of the integument of the adjoining region, which arise and have a course similar to the visceral sensory fibres. In aquatic vertebrates there are also the special somatic sensory fibres of the lateral line system, whose fibres join the roots of the facial, glossopharyngeal, and vagus to reach the brain, and the ganglia from which these fibres are derived become incorporated in the geniculate, petrosal, and nodosal ganglia. A trace of these organs is seen in the human embryo in the form of a temporary thickening of the ectoderm directly over the ganglia of these three nerves. The fourth member of this group, the trigeminal nerve, is distinguished by a larger admixture of somatic sensory fibres. The ganglia of all four nerves are derived from the neural crest.

The n. facialis is characterized by a large predominance of visceral motor fibres which make up the large motor root of the adult. These motor fibres can be seen in the 10 mm. embryo (Figs. 96 and 100) arising from a group of neuroblasts situated beneath the third rhombic groove or neuromere. The fibre bundles are assembled and pass directly lateral under the floor of this groove and gradually converge to form a solid trunk which emerges from the neural tube just median to the acoustic ganglion. On emerging the motor trunk curves caudalward (Fig. 100) and terminates among the cells of the hyoid arch which are destined to form the muscles of expression.

The sensory fibres of this nerve spring from the geniculate ganglion, which is apparently a derivative of the neural crest. It can be clearly made out toward the end of the third week, lying in front of and separate from the ganglion acusticum. Shortly after a path of loosely grouped fibres can be seen extending from it to the neural tube constituting its proximal root, the n. intermedins. On entering the alar plate it forms, as can be seen in the 10 mm. embryo (Figs. 96 and 100), a fibre path which bends caudalward to join the tractus solitarius. From the peripheral end of the ganglion fibres pass down as the chorda tympani, which finally leaves the main trunk of the nerve to enter the mandibular arch, eventually joining the third division of the trigeminal nerve. The great superficial petrosal nerve is another peripheral derivative of this ganglion, which makes its appearance shortly after the chorda tympani and extends forward to reach the anlage of the sphenopalatine ganglion. Most authors in comparing the seventh nerve with a typical branchial nerve describe the great superficial petrosal as representing the prsetrematic branch and the chorda tympani as the posttrematic branch. In the early embryo the sensory root or pars intermedia, as compared with the motor root, is larger than in the adult. The subsequent marked growth of the motor root and the relative, standstill of the sensory root result in the former becoming the main trunk of the nerve.

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Fig. 96. — Reconstruction of the left facial, glossopharyngeal, and vagus nerves of the same embryo shown in Fig. 86. A transverse section of the neural tube is included in the reconstruction to show it* relation to the different nerve-roots. This should be compared with Fig. 100.

Owing to the close relation existing between the facial and acoustic nerves the two are frequently classed as the facial-acoustic complex. However, aside from the fact that the acoustic nerve, in its development in the higher vertebrates, crowds in against the facial nerve and becomes more or less fastened together with it by mesodermal elements, it has no other thing in common, the two being nerves of entirely different embryological and functional significance. The relation between the facial and abducens becomes reversed in the adult from that of the early embryo. The facial at first lies directly under the third rhombic groove, while the abducens is more caudal and is under the fourth rhombic groove. As shown in Fig. 97, the two nerves gradually shift their relative positions, the abducens moving forward. This migration results in the bending of the motor root of the facial out of its original course and produces the genu facialis.

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Fig. 97. — Diagram illustrating three stages in the development of the genu facialis, the youngest, A, corresponding to the 10 mm. embryo, and the oldest, C, the new-born child. The relative position of the nucleus of the n. abducens is represented in outline. Sulcus, sulcus medianus fossa? rhomboidese.

The n. gloss opharyng eus possesses a ganglion of the root and ganglion of the trunk, the latter being temporarily connected with the placode over the third arch. As can be seen from the relative size of the ganglia in Fig. 86, the nerve consists almost wholly of sensory fibres, connected peripherally with the structures developing from the second (r. tympanicus) and third (r. lingualis) arches. The tympanic branch is not well defined until we come to embryos between 12 and 14 mm. long. Centrally the rootlets enter the brain wall and, joining with the fibres of the facial, extend caudally (Fig. 96) as the tractus solitarius. The motor rootlets of this nerve arise from a group of neuroblasts in the nucleus ambiguus series, situated beneath the floor of the fifth rhombic groove. The motor bundles extend directly lateral beneath this groove and pass under the spinal tract of the trigeminal and then emerge from the brain wall and join the main trunk of the nerve. This nerve forms a more typical branchial nerve than either the facial or vagus. Its tympanic branch is regarded as the prsetrematic branch and the lingual as the posttrematic branch.

The vagus complex (n. vagus et n. accessorius) represents several branchial nerves, the motor fibres of which in man have undergone special development for the purpose of supplying the group of muscles derived from its branchial arches. The facial nerve is also a similar nerve; its large motor trunk, as we have seen, is distributed to the muscle-cells of the hyoid arch, and, as these cells group themselves into the muscles of expression and spread forward over the face, the facial branches are drawn along with them. In a similar way the more caudal rootlets of the vagus become predominantly motor, and form a distinct bundle which we know as the spinal accessory nerve, and this bundle is distributed to a group of muscle-cells originally belonging to the more caudal branchial arches, and in man is destined to form muscles for the arm girdle, the mm. sternocleidomastoideus and trapezius. As these muscles spread out into their eventual position the nerve is drawn down across the neck with them. Coincident with the increased importance of this musculature as we ascend the vertebrate scale we meet with increased development of the accessory nerve, and it obtains additional rootlets of origin by spreading down into the region of the spinal cord. As can be seen in Fig. 86, it may reach as far down as the fourth cervical segment. The nucleus of origin of the spinal accessory and other motor rootlets of the vagus constitutes the nucleus ambiguus of the medulla oblongata and a portion of the lateral horn of the spinal cord, the two being continuous (see Fig. 100).

The early stages in the growth of the glossopharyngeal and vagus nerves are shown in Figs. 82, 83, 86, and 94-. Their sensory elements are derived from the ganglion crest of the hind-brain in the same manner that spinal ganglia are derived from the ganglion crest of the cord. The crest of the hind-brain and that of the cord are described by some as continuous (Dohrn, 1901), but Froriep (1901) distinguishes between a ganglion crest of the head and one of the trunk, the two overlapping in the occipito spinal region. The cranial ganglion crest migrates ventrally down the side of the neural tube and is soon joined by visceral motor fibres that emerge from the lateral border of the neural tube. In the 4 mm. embryo, Fig. 82, a bundle of such fibres can be seen running along the ventral border of the crest and constituting the primitive n. accessorius. Soon after this the cells of the crest show signs of differentiation and are gradually converted into sensor} glion-cells with fibrous processes, which attach themselves to the neural tube on the one hand and extend peripherally on the other, — i.e., dorsal rootlets. This fibre development results in the break up of the ganglion crest into cell masses, which is not a metameric segmentation such as appears in the spinal region. These masses constitute the ganglia of the roots. The ninth nerve has one. the ganglion of Ehrenritter ; while the tenth, being a composite nerve, has a series of them, the most oral one being the largest. The ganglion of the trunk (ganglion nodosum), as is also true of the ganglion petrosum of the glossopharyngeus, when first identified does not seem to be definitely connected with the root ganglia. Furthermore it differs from the root ganglia in being connected with a rudimentary sense organ (epibranchial placode), as shown in Fig. 98. It, however, is generally considered as a derivative of the ganglion crest.

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Fig. 98. — Reconstruction of the nerves in the occipital region of a 7 mm. human embryo (Mall collection, No. 2), showing sensory placodes in connection with the petrosal and nodosal ganglia. Enlarged 16.7 : 1. Ot. v., ear vesicle; gang, crest., ganglionic crest.

The formation of the vagus rootlets at a somewhat later stage is shown in Figs. 96 and 100. The motor neuroblasts point dorsolateralward and assemble in a series of rootlets which emerge just ventral to the entrance of the sensory roots. After emergence they turn forward and form a common trunk, which in the spinal region lies between the dorsal roots and the side of the spinal cord. Connected with the motor roots are the sensory roots and ganglia. The development of the ganglia is more pronounced in the cephalic end of the nerve. The more caudal ganglia usually disappear in the adult, except for traces of scattered ganglion-cells found occasionally on the rootlets of the accessory division. Owing to the tendency to regression on the part of the more caudal of the vagus root ganglia, the vagus complex becomes differentiated into a fore part or vagus division which is predominantly sensory, and a hind part or accessory division which is almost wholly motor. In other words, in the course of phylogenesis as the vagus invades the spinal region it is the motor elements that play the prominent part. The more caudal vagus ganglia are not to be mistaken for the Froriep ganglion, which represents a persistent precervical ganglion. In the one case we have a series diminishing from I head toward the tail, and in the other it is in the opposite direction. The sensory fibres derived from these ganglia enter the wall of the neural tube and immediately nnite in a longitudinal tract continuous with similar fibres from the facial and glossopharyngeal, thus completing the formation of the tractus solitarius, whose form and relation to a section through the oblongata region are shown in Fig. 96.

The n. trigeminus possesses on its sensory root the largest ganglion of the whole embryo, the ganglion semilunare. Processes which grow out from its constituent cells connect it with the brain on the one hand, and extend as three large trunks on the other into the ophthalmic, maxillary, and mandibular region-. 'This ganglion has generally been considered as a single undivided mass of cells derived entirely from the ganglion crest (Dixon, 1896). A human embryo of 15 segments has, however, been described by Giglio-Tos (1902), in which the anlage of the semilunar ganglion consists of three separate proganglia, " proganglia neurales, ,; which are connected by a cellular lamina with three other distal or epibranchial proganglia, the whole group being eventually fused into a single ganglion mass. If this is the case, it is probable that the semilunar ganglion arises in part from the ganglion crest and in part from the epibranchial ectoderm. Such a composite character would correspond in some degree with the condition found in lower vertebrates. It has been suggested by Johnston (1908) and others that perhaps the sensory ganglion-cells belonging to the midbrain and the most oral part of the hind-brain become included in the wall of the neural tube, instead of becoming detached with the neural crest. Such cells then never become incorporated in the semilunar ganglion. It is these cells that are supposed to form the mesencephalic root of the trigeminal nerve, their processes extending caudalward beneath the central gray substance to join the main sensory trunk of the nerve at its point of entrance into the wall of the tube.

Processes grow out from the constituent cells of the ganglion and extend peripherally as three large trunks or divisions (see Figs. 83, 99, and 86). The ophthalmic division passes forward and soon subdivides into the frontal and nasociliary nerves ; the latter in the 10 mm. embryo is a well-defined branch just dorsal to the eye stalk. The maxillary and mandibular divisions extend downward and break up in their terminal branches among the cells of the maxillary process and mandibular arch respectively. By the beginning of the sixth week the chief branches of these divisions can be recognized. Centrally the ganglion becomes connected with the brain by a large single root, consisting of both somatic and visceral sensorv fibres. This enters the wall at the pontine flexure opposite the first and second rhombic grooves. Wiithin the wall in the marginal zone the fibres form a flattened longitudinal tract, part of which extends caudally as the spinal tract, and part extends forward and upward to enter the cerebellar ridge as shown in Fig. 100.

In its motor elements the trigeminal nerve departs somewhat from the type represented in the other three nerves of this visceral group. In the others the nucleus of origin is in the basal plate, and the nerve rootlets exhibit a characteristic curved course to reach the point of emergence ; while in the trigeminal the nucleus is more lateral and lies directly against the entering sensory fibres, so that the fibres of the motor root pass directly ventralward to fuse with the mandibular division. Its nucleus corresponds to the dorsal motor nuclei found in the adult ninth and tenth nerves, though it is much larger. On analysis it is found that a typical visceral cranial nerve has three central terminations. — sensory root (tractus solitarius), ventral motor root (nucleus ambiguus), dorsal motor root (nucleus nervi dorsalis). The ventral motor root always has a characteristic curved course between its nucleus and point of emergence, while the nucleus of the dorsal motor root is clustered near the sensory division of the nerve, and from it the root extends to the surface of the brain in a straight line. These three elements are represented in the different nerves in different proportions. The ninth nerve approaches the mean and all elements are fairly represented. In the vagus the curved ventral motor roots are increased in the caudal portions and form the spinal accessory. In the facial the sensory root (n. intermedius) is diminutive, while the curved ventral root becomes the main trunk of the nerve. In the trigeminal it is the straight dorsal motor root that forms the principal motor supply, while the curved ventral motor root must be considered as absent. It should be mentioned, that though the motor root eventually fuses with the mandibular division, in the embryo the motor fibres corresponding to the n. masticatorius have been observed passing on the median side of the ganglion and extending from its distal border, not with the mandibular division, but as a separate trunk (Streeter, 1904). The cranial sympathetic ganglia derived from the trigeminal nerve will be described under the sympathetic system.

Keibel Mall 2 099.jpg

Fig. 99. — Reconstruction of the facial and trigeminal nerves of a human embryo 14 mm. long (Mall collection. No. 144), showing motor and division of the facial sensory. Enlarged 8 : 1.

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Streeter GL. The Development of the Nervous System. (1912) chapter 14, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.

XIV. Development of the Nervous System: Histogenesis of Nervous Tissue | Central Nervous System | Peripheral Nervous System | Sympathetic Nervous System | Manual of Human Embryology II
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