Human Embryology and Morphology 8

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures


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Chapter VIII. The Mid- and Hind-Brains

When the neural tube is traced forwards into the head region, it is seen to undergo a marked change in form — a transformation due to a change in function. In the spinal cord the nerves arose in two rows — a dorsal sensory and a ventral motor ; here the dorsal and ventral series are still represented, but a third or intermediate series has been added. This series is represented by the spinal accessory (XI), vagus (X) and glossopharangeal (IX), facial (VII) and fifth (V) pairs of nerves. They arise from an intermediate column of cells representing in an exaggerated degree the splanchnic or visceral nerve columns of the spinal cord. Further, the central canal becomes enlarged to form the 4th ventricle. Part of the roof of the neural tube becomes reduced to a membranous lamina, forming the medullary velum and choroid plexus — a secretory mechanism. Part of the roof is specialized to form a complex mechanism (the cerebellum) for the coordination of the impulses dispatched to the motor cells of the spinal cord. This high degree of specialization almost obliterates the original simple nature of that part of the neural tube which forms the mid- and hindbrain. In the human embryo at the beginning of the 4th week it is seen that this part of the central nervous system retains its tubular character, while that part which is to form the hind-brain, even at this early stage, shows an imperfect segmentation into nine neuromeres. Further, the neural tube in the regions of the mid- and hind-brain, as in the spinal cord, lies over the notochord (Fig. 80). The notochord ceases at the junction of the mid- and fore-brain. The developing walls of the mid- and hindbrain show the same three zones as were seen in the spinal cord — inner or ependymal, middle or mantle and outer or marginal. We shall find, too, the same division of each lateral neural plate into basal and alar laminae.


A reference to the relationships of the hind-brain, during the 4th week of develo]Dment (Fig. 80), serves to explain why the vital centres of the body — those which are concerned in the regulation of respiration, circulation, deglutition and digestion, come to be placed in its walls. At this time the hind-brain lies over the pharynx, with its aortic arches, and its gill-pockets — representing the breathing mechanism of fishes. When lungs arise the control of respiration still lies in the original respiratory centres of the hind-brain. The heart, too, lies directly under, or ventral to, the hind-brain (Fig. 80) ; hence the centres for circulation are placed there.


The fore-gut, from which the mouth, pharynx, oesophagus, trachea, lungs, stomach and liver are to arise is also placed in the territory of the hindbrain. Its relationship to the otocyst, however, is to prove the most important. From that structure is to arise a vestibular or balancing mechanism designed to supply information concerning the position and movements of the head. The cerebellum and pons which so transform the simple tubular hind-brain, arise in connection with the vestibular nuclei. One other point may be noted before proceeding to follow the transformation of the hind-brain into medulla oblongata, cerebellum and pons. The mid-brain is interpolated between the spinal cord on the one side and the mid- and fore-brain on the other ; hence it becomes the great highway for the nerve tracts which are developed to link brain and spinal cord into a functional whole. Throughout the greater part of the second month the hind-brain forms a little less than half of the total neural tube.

Fig. 80 Showing the tubular form, the neuromeres and relations of the Mid- and Hind-Brain in a Human Embryo in which there were 18 body somites

Fig. 80. Showing the tubular form, the neuromeres and relations of the Mid- and Hind-Brain in a Human Embryo in which there were 18 body somites — in the 4th week of development. (Crawford Watt.)

The Fourth Ventricle

The cavity or neural canal of the hind-brain becomes the fourth ventricle. In its floor are developed, out of the basal or ventral and alar or dorsal laminae (Fig. 81) of the neural plates, the pons and medulla. In its roof are developed the cerebellum, the superior and inferior medullary vela.


Basal and Alar Laminae of the Medulla

The basal and alar laminae of the neural tube become flattened out to form the floor of the hind-brain. At the end of the 4th week each medullary plate shows three zones : an inner or ependymal where new cells are being produced ; a middle or mantle zone in which neuroblasts, neuroglial fibres and young nerve fibres are being differentiated and an outer or marginal zone. By the 6th week the disposition of the nuclei connected with the cranial nerves in the mantle zone can be made out. The grouping of the nuclei as seen in a diagrammatic section across the hind-brain is shown in Fig. 82. In the mantle zone of the basal lamina are three columns of motor cells — the columns being much interrupted as they are traced from the lower to the upper end of the hind-brain. These are : (1) the somatic motor, continuing upwards the somatic cells of the anterior horn and supplying muscles derived from the body somites ; from this column arise the XII and VI nerves. (2) The lateral somatic motor, supplying striped muscle which was first evolved for the movement of gill-arches ; from this column arise motor fibres of XI, X, IX, VII and V. The nucleus ambiguus forms part of the column. (3) The splanchnic motor nuclei, giving origin to fibres distributed to the musculature of the heart, lungs and alimentary canal — represented by the dorsal nuclei of IX and X. In the alar lamina are differentiated two main groups or columns of sensory or reception nuclei : (1) splanchnic, which receive the ingrowing fibres of the IX-X nerves — and therefore are in connection with the pharynx, heart, lungs and alimentary canal, receiving afferent impulses from all including those of taste ; (2) somatic, corresponding to the posterior horn cells of the spinal cord and receiving fibres in series with the posterior roots of spinal nerves. The posterior root fibres in the cranial series are represented by the sensory root of the Vth and by tbe Vlllth nerve — botb vestibular and cocblear divisions. The Vlllth nerve and its ganglia were probably derived from the same system as gave rise to the complex sensory organs of the lateral line of fishes, and should be distinguished from the ordinary somatic group. We have seen how the posterior funiculi are formed in the marginal zone of the spinal cord by fibres of the posterior roots. The sensory fibres of the cranial nerves also form tracts in the marginal zone ; the solitary tract is formed by fibres of the IXth and also of the Xth and Vllth. The vestibular and fifth nerves also form tracts • — the latter being particularly extensive. At first these tracts lie near the surface of the hind-brain, but in the sixth week they become overwhelmed and buried by vast migrations of neuroblasts.

Fig. 81 Section across the Hind-Brain of a Human Embryo in the 6th week.

Fig. 81. Section across the Hind-Brain of a Human Embryo in the 6th week.

Fig. 82 Diagrammatic section across the Hind-Brain to show the grouping of cranial nerves and their nuclei.

Fig. 82. Diagrammatic section across the Hind-Brain to show the grouping of cranial nerves and their nuclei. (After Elliot Smith.)

Neurobiotaxis

In Fig. 83, B is given a diagrammatic section across the right half of the neural plate of the hind-brain at the sixth week of development, showing the solitary tract in the marginal zone of the ventral surface ; in Fig. 83, A is given the condition in the 8th week, showing the fasciculus solitarius buried deeply — much nearer the dorsal than the ventral aspect of the medulla. What has happened is this : swarms of neuroblasts have been produced in the ependymal zone near the dorsal margin of the alar lamina — at the rhombic lip marked by an asterisk in Fig. 83, B. In the spinal cord the posterior horn is the latest site of neuroblastic production ; in the hind-brain this tendency to new production of neuroblasts in the dorsal margin of the alar lamina has become enormously heightened. The arrow in Fig. 83, A shows the direction of the swarm ; they invade the marginal zone, burying the solitary tract, and group themselves as they approach the floor plate in the middle line, to form the inferior olivary body ; the superior olivary body and the great terminal or receptive nuclei — the gracile and cuueate nuclei — are formed in the same way.


Fig. 83 Foetal Medulla and Alar and Basal Laminae of the Hind-Brain

Fig. 83, A. Diagrammatic Section of a Foetal Medulla to show the relative positions of the Nuclei connected with the Somatic and Splanchnic Nerves, and the Origin of the Olivary Body. The Motor Nerves, both Somatic and Splanchnic, are represented black. The arrow indicates the route of migration of the Cells of the Olivary Body.


Fig. 83, B. The Alar and Basal Laminae of the Hind-Brain at the beginning of the 6th week to show the superficial position of the sensory root of the Vagus. (Compare with A.) The rhombic lip and the point at which the root of the 5th Nerve and Restiform Body will be formed is indicated by an asterisk. (After His.)


Dr. Aliens Kappers[1] in his studies on the medulla in 1907 was struck by the apparent evolutionary and developmental movement of the nuclei of motor nerves ; they were drawn towards the terminal nuclei from which they received their chief incoming stimuli or messages.


For example he noted a forward movement of the motor nucleus of the Xllth towards the receptive nuclei of the IXth and Xth ; of the Vllth towards the descending root of the Vth, while the nuclei of the Xlth for the sternomastoid and trapezius tend to spread backwards in the spinal cord towards the receptive nuclei of the neck and shoulder. To the law or force which regulates the mass-movement or migration of neuroblasts Kappers gave the name — Neurobiotaxis. We have just mentioned the migrations which give rise to the olivary nuclei of the medulla, but we shall find, as we ascend the brain stem — to cerebellum and pons, to mid-brain and basal ganglia and particularly to the cerebrum itself — that neuroblastic migration is the basal principle of development and transforms the simple embryonic neural tube into the complexities of the adult brain. In the spinal cord neuroblasts are confined to the mantle zone, but in the hind-, mid- and fore-brains they invade the marginal zone and there establish their chief centres. The cortex of the cerebellum and cerebrum are produced by a neuroblastic invasion of the marginal zone. Nor are the massmigration of nerve-cells really different from other manifestations of living cells. Outgrowing processes from the neuroblasts of the spinal ganglia and spinal cord spread into the limb buds and reach their destinations unerringly — drawn and regulated by some obscure force ; Dr. Eoss Harrison found that if a limb-bud was transplanted, the strange nerve fibres which entered it were attracted and moulded to a normal supply by some influence in the tissues of the bud. The force which attracts the wandering defensive cells of the body to a site of infection is probably of the same nature as that which regulates the migration of neuroblasts.


Inferior Medullary Velum

When a section is made across the hindbrain of an embryo in the 6th week of development, the same parts are seen as in the spinal cord except that the roof plate has become enormously expanded to form the inferior medullary velum. The extent, shape and attachments of the roof plate are shown in Fig. 84 ; it is diamond shaped, its hind angle being continuous with the roof plate of the spinal cord, its front angle with the roof plate of the mid-brain, while its lateral angles mark the sites of the two lateral recesses of the 4th ventricle. Its upper lateral margin is attached to the border of that part of the alar lamina in which the cerebellum is to arise ; its lower lateral margin is attached to the rhombic lip, the dorsal border of the medullary part of the alar lamina. This border is folded outwards (Fig. 81). The shape of the roof plate, or inferior medullary velum, is altered by remarkable changes which set in during the 6th week (Fig. 84) ; growth changes cause the hind-brain to be folded, producing the pontine bend and bringing the cerebellar part of the hind-brain against the medullary. The inferior medullary velum becomes drawn out transversely. It is at this time that choroidal villi are produced on its ventricular surface, first in a transverse row extending from lateral recess to lateral recess and subsequently over its entire surface. At the same time secretion of cerebro-spinal fluid commences[2] ; the fluid percolates through the velum at three places — middle of the roof and at the lateral angles. At a later date (3rd month) the foramen of Magendie and the openings of the lateral recess appear at the points of percolation. The subarachnoid spaces begin to form at the sites of escape and from there extend.

As shown in Fig. 88, the velum is continuous with the cerebellum above and the roof of the central canal of the cord below. In the posterior margin of the cerebellar plates are developed : (1) the nodule, (2) the flocculus. (3) the peduncle of the flocculus between 1 and 2 (Figs. 89, 90). Hence the inferior medullary velum ends above in these structures. The obex and ligula, thickenings or ridges found on the margins of the 4th ventricle, mark the attachment of the roof plate or velum to the rhombic lip of the medullary plates. They represent the attached margin of the velum. The velum is also attached to the restiform body which is developed in the upper margin of the alar lamina. Over the opening of the central canal of the spinal cord into the 4th ventricle there is often a fold formed by the union of the alar laminae (see J. T. Wilson, Journ. Anat. and Physiol. 1906, vol. 40, p. 210).

Fig. 84 Showing the origin of the Inferior Medullary Velum from the roof plate of the Hind-Brain.

Fig. 84. Showing the origin of the Inferior Medullary Velum from the roof plate of the Hind-Brain.


The velum is to be regarded as a part of the neural tube, specially modified for the purpose of secreting the cerebro-spinal fluid which fills the central canal and subarachnoid systems. This fluid may help to support the central nervous mass in a mechanical sense, but its rapid secretion, its circulation and chemical composition point to some more important nutritive or regulatory influence on the neural centres. The ectodermal cells retain the primitive columnar type, and form an epithelial covering over inflections and processes of the pia mater which is derived from the mesodermal covering of the neural tube.

Cerebellum

At the beginning of the 2nd month the cerebellum[3] is still represented by simple right and left alar plates (Figs. 85 and 93) rudiment of cerebellum which show the usual triple stratification — an internal proliferating ependymal zone, a middle neuroblastic and an outer marginal meshwork. In the frog a plate-like cerebellum is retained (Fig. 86), for the amphibia have but an imperfect power for sustained co-ordination of their limbs during locomotion on land. By the end of the 2nd month (Fig. 87) there has been an active proliferation of neuroblasts in the cerebellar plates ; they fuse in the middle line to form the vermis or median lobe, and now bulge into the 4th ventricle, much as they do in the frog. What has happened may be best gathered from Fig. 87. The reception nucleus for the Vlllth nerve is developed in the rhombic lip near the lateral recess ; through the vestibular fibres of the Vllltli nerve this nucleus will receive impulses which make it the chief recipient of messages needed for the co-ordination of muscles. Elliot Smith regards the cerebellum as a product of the vestibular nucleus. Hence the proliferation of neuroblasts at the rhombic lip and their spread into the cerebellar plates. In the 3rd month neuroblasts invade the marginal zone of the plates and lay the basis of the molecular layer of the cortex. The cells of Purkinje — although not fully differentiated until after birth — take up their stations at the junction of the mantle and marginal zones.

Fig. 85 Lateral View of the Cephalic Part of the Neural Tube in a 5th week Human Embryo

Fig. 85. Lateral View of the Cephalic Part of the Neural Tube in a 5th week Human Embryo. (After His.)

Fig. 86 Median Section of the Cerebellum and 4th Ventricle of a Frog.

Fig. 86. Median Section of the Cerebellum and 4th Ventricle of a Frog.

At the time the cerebellar plates are being thus invaded in the 3rd month, in this way a cellular basis for the cortex being laid down, other cells, arising in the rhombic lip, invade the adjacent basal laminae — the parts which will become the pons (Fig. 87). There they lie in the path of fibres descending from the frontal cortex and thus bring the cerebellum into touch with the cerebrum. The restiform body begins to form in the second month, and by this means the cerebellum is placed in connection with the recipient nuclei of the cord and medulla. The dentate and other central cerebellar nuclei are isolated, the dentate nuclei being linked with the red nuclei of the mid-brain by the superior peduncles. In the differentiation of the cerebellum are to be seen numerous illustrations of the law of neurobiotaxis enunciated by Kappers.

Fig. 87 The Human Cerebellum at the end of the 2nd month of development.

Fig. 87. The Human Cerebellum at the end of the 2nd month of development. (After Streeter.) The arrows show the direction of the migration of the Pontine and Cerebellar Nuclei.

Differentiation of Lobes

At the end of the 3rd month (Fig. 88) the cerebellum has assumed a dumb-bell form — ^the lateral elevations representing the right and left lobes which are united by a median plate — the vermis. The cortex has already commenced to expand, as may be seen by the early appearance of transverse fissures on the vermis. It is at this period that the cerebellar plate becomes demarcated into anterior, middle and posterior primary lobes, these being separated by two transverse grooves or fissures — the first and second fissures (Elliot Smith). Since these three primary divisions are to be recognized in nearly all mammalian cerebelli, they must be of fundamental importance. Quickly succeeding these two primary fissures there appear two others, one which divides the median part of the posterior lobe — the post-nodular fissure — and the other the anterior lobe (Figs. 89, 90). The post-nodular fissure may appear in the human brain before the fissura secunda. Thus, at the end of the fourth month four fissures are seen to be developed in the human cerebellum (Fig. 89). The rapid growth of the cerebellum, with the pressure of the cerebrum above or in front, and the resistance of the occipital bone below or behind cause the plate-like form to be replaced by one which is wedge-shaped in section, with an upper and lower surface. The minor sulci and fissures of the cerebellum appear between the 5th and 7th months of foetal life.

Fig. 88 Diagram of the Cerebellum and of the Attachments of the Inferior Medullary Velum at the end of the 3rd month of development

Fig. 88. Diagram of the Cerebellum and of the Attachments of the Inferior Medullary Velum at the end of the 3rd month of development. (After Kollmann.)

Fig. 89 Diagrammatic Section of the Cerebellum of a Human Foetus early in the 4th month

Fig. 89. Diagrammatic Section of the Cerebellum of a Human Foetus early in the 4th month, showing the folding of the Cerebellar Plate. (After Kuithan and Elliot Smith.)


Parts derived from the Posterior Primary Lobe

(Figs. 89, 90, A, B). — From the median part arise the nodule and uvula separated by the post-nodular fissure. From the lateral parts arise the flocculus and paraflocculus, which represent the oldest of all the distinctive parts of the cerebellum, and the first to become differentiated in the human organ. The para-flocculus, part of which fills the subarcuate fossa in the temporal bone (p. 236), becomes reduced to a vestige in man and the anthropoids (Fig. 90, A).


Parts derived from the Anterior and Middle Primary Lobes

(Figs. 89, 90, A, B). — From the anterior primary lobe arise the lingula, central lobe, and alae, the culmen and the anterior crescentic lobes. The rest of the cerebellum, comprising by far its greater part, arises from the middle lobe. It represents an addition to the older and more primitive parts represented by the anterior and posterior lobes, and hence has been named the neocerebellum. The median part forms the pyramid and the clivus, separated by a deep fissure. The lateral parts undergo an enormous development in higher primates. In man the tonsillar and biventral lobes attain a very great size. The great development of the lateral parts of the middle primary lobe during the 5th and 6th months, leads to the formation of the great horizontal fissure (see Figs. 90, A and B).

Fig. 90  Left half of the Cerebellum of a Foetus of 5 months

Fig. 90, A. Left half of the Cerebellum of a Foetus of 5 months, seen on its inferior aspect. Only the middle and posterior primary lobes are exposed. The parts forming the posterior lobe are stippled. (After Elliot Smith.) B. — Right half of a typical Mammalian Cerebellum, spread out so as to show the anterior, middle and posterior primary lobes. The anterior and posterior lobea are stippled. The flssiu-es and parts are indicated by the terms used in human anatomy in order that the peculiar features of the human cerebellum may be made evident. (After Elliot Smith.)


The Superior Medullary Velum is part of the roof plate of the 4th ventricle which remains between the superior peduncles. The vestigial laminae which cover it form the lingula (Fig. 89).


Three points in connection with the development and comparative anatomy of the cerebellum are especially worthy of attention :

  1. It arises from the alar laminae, which are directly connected with afferent or sensory nerves only ; further, the nuclei in the mesencephalon, pons and medulla, with which it is connected, arise from the alar laminae.
  2. The part of the neural tube from which the cerebellum arises is the vestibular neuromere — the one to which the internal ear becomes closely linked.
  3. The cerebellum readies its greatest development in primates amongst mammals ; it is also greatly developed in swimming vertebrates. In primates, as in swimming mammals, the equilibrium of the body is finely adjusted. On embryological grounds alone we would infer that the cerebellum is part of a sensory mechanism. Clinical and experimental observations indicate that its main function is to co-ordinate the various muscles of the body in performing definite acts. It is therefore on the afferent nerve system arising from the muscles, joints and bones, that the cerebellum has been developed, but its position was determined by the nuclei of the vestibular nerves, cells of which invade the embryonic cerebellar plate.


Mid-Brain or Mesencephalon

By the end of the 3rd month the mid-brain is becoming overshadowed by the preponderating growth of the fore- and hind-brains, and by the 6th month is reduced to the peduncular body which unites cerebrum with cerebellum, its ventricle or canal becoming reduced to the aqueduct which unites the 4th ventricle to the 3rd. With the midbrain we reach the anterior limit of the primitive neural tube ; it lies over the terminal cephalic part of the notochord (Fig. 80) ; two cranial nerves (III and IV), corresponding to the anterior roots of spinal nerves, arise from it. A section across the mid-brain in the 4th week of development, reveals the same divisions as in the cord — lateral neural plates made up of basal and alar laminae, united by a roof plate and a floor plate. The same three zones arise — ependymal, mantle and marginal. In the 3rd month the quadrigeminal plate develops on the dorsal part of its alar laminae, much in the same way as the cerebellum arises within the alar laminae of the hind-brain. The neuroblasts invade the dorsal marginal zone, and evolve into a formation which may be described as a cortex. The quadrigeminal plate which thus arises on the dorsum of the mid-brain may be regarded as primary receptive centres for the nerve of sight (the optic tracts), and in birds this formation assumes great size and importance. The necessity of linking the receptive nuclei for sight with those for hearing is apparent ; hence we find the cochlear nuclei connected with the quadrigeminal formation by the lateral lemniscus. In the development of the mid-brain we see the quadrigeminal plate become divided into the inferior colliculus — in which the cochlear tract ends — and the superior collicuhcs, which receives fibres from the retina. Thus, in the main the mid-brain is connected with sight ; in the basal laminae arise the nuclei for the Ilird and IVth nerves — the chief source of motor supply for the muscles of the eye-ball (Fig. 93). From the mid-brain arise also sensory fibres of the Vth nerve which go to the orbit. They differ from all other sensory fibres in having their cell bodies implanted in the wall of the neural tube. From the 3rd month of development onwards the mid-brain becomes the highway of developing efferent nerve paths which unite the basal masses and cortex of the fore-brain with the nuclei in the pons, medulla and spinal cord, and of afferent or sensory paths which connect the nuclei of the cord, medulla and cerebellum with the basal masses of the fore-brain. The cerebral cortical paths develop in the marginal zone of the basal plates and form the crura cerebri, while the afferent paths — the median lemniscus; — develops in the mantle zone — the tegmentum. In this zone, too, appears the red nucleus, but as yet its neuroblasts have not been traced to their source.


The Three Neural Flexures

(see Figs. 80, 84, 85).— The pontine flexure, a convexity forwards of the pons, has already been mentioned ; it is the result of the elongation of the neural plates of the hind-brain due to the proliferation of the neuroblasts and the production of the cerebellar plates. The nuchal flexure is concave forwards, and occurs between the medulla and cord. The latter is compensatory and of but small import ; on the other hand, the anterior flexure, whereby, in the third week of foetal life, the fore-brain appears as a downward and forward development until it comes to lie on the ventral aspect of the cephalic end of the notochord, leads to a great alteration in the form and relationships of the foreand mid-brains, and is of great importance (Fig. 85). Even in the embryos of the lowest vertebrate types the expansion and bending of the anterior end of the neural tube is apparent. The mid-brain, by this flexure, comes to be, for a short time, the most anterior part of the neural canal ; the fore-brain is doubled back under the notochord. Round the projecting end of the notochord — projecting between the mid- and fore-brains —are developed the posterior clinoid processes and dorsum sellae. The dorsum sellae marks the position of the anterior flexure in the adult brain. The tentorium cerebelli is developed between the mid-brain and fore-brain, and lies at first at right angles to the axis of the mid-brain, but the subsequent great growth of the cerebrum forces it backwards arid downwards until it becomes a horizontal partition between the cerebellar and cerebral chambers of the skull.

Fig. 91 Section of the anterior part of the Roof of the Mid-Brain of a Cat, to sliow the subcommissural organ.

Fig. 91. Section of the anterior part of the Roof of the Mid-Brain of a Cat, to show the subcommissural organ. (Dendy and Nicholls.)

Subcommissural Organ

For some time it has been known that the ependyma on the roof of the mid-brain of lower vertebrates, immediately behind the posterior commissure (see Fig. 91), is modified to form a peculiar area of high columnar cells. The cells are related to a certain very large fibre (Reissner's fibre), which descends ventral to the central canal of the spinal cord in fishes and amphibians. Recently Dendy and NichoUs have shown that this ependymal structure, to which they have given the name of subcommissural organ, occurs in all vertebrates, including man. It is quite apparent in the human foetal brain, but is soon reduced to a vestige. The fibres are not nervous in nature. The function and significance of the structure are unknown.[4]


Constitution of the Mid- and Hind-Brain

We have traced the development of the neural tube in a forward direction, and have reached the point where the mid-brain passes into the fore-brain. On the roof the point of transition is marked by the posterior commissure (Fig. 91) ; below the floor the notochord ends (Fig. 80). We have reached the end of the neural tube proper ; the part in front — ^the fore-brain — appears to have arisen in connection with two great organs of sense — ^the nose and eye. We find that the neural tube, when it enters the region of the head, becomes greatly altered in its constitution. This is due, not only to the development of special parts such as the pons, the cerebellum, quadrigeminal plate and special nerve tracts which unite the cerebral and spinal centres, but especially to the fact that the structure of the head is older and more complex than that of the body. In the head region another element appears — a ventral mesodermic somite or branchiomere — in addition to the dorsal mesodermic somite seen in the trunk region (Fig. 92). The branchiomeres give rise to the gill arches, which are so apparent in the human embryo at the end of the first month. In the mid- and hind-brain special centres and nerves are developed in connection witli the gill arches. In the spinal cord there were two columns of motor nerves in the basal lamina, one for the somatic or voluntary muscles of the body, another for the visceral musculature — ^the splanchnic — but here a third or intermediate column is added — the motor cells for the muscles connected with the gills (Fig. 82). The branchial^or lateral somatic nerves are represented in the mid- and hindbrain by the motor or ventral root of the Vth nerve, by the motor part of the Vllth, by the parts of the IXth, Xth, Xlth, which supply striated muscles. The presence of branchial arches in the head region gives rise to a more complex arrangement of the nerve ganglia (Fig. 93). In the trunk region the neural crest gave origin to posterior root ganglia, the ganglia of the sympathetic chain (prevertebral), and other ganglia stationed in front of the spine. In the regions of the mid- and hind-brain the neural crest also is developed, but besides giving rise to ganglia (see Figs. 92, 93) representing the posterior root ganglion and sympathetic ganglia found in the region of the trunk, it also gives origin to a lateral mass of nerve cells, from which the sensory fibres to the gills are produced. Associated with this lateral mass are also cellular formations representing two rows of sense organs[5] — an upper, the organs of the lateral line ; a lower, the epibranchial sense organs. In man only vestiges of these sense organs appear. The ultimate fate of the epibranchial rudiments is not known for certain, but it is probable that some of their cells are included in the ganglia at the trunks of the Vllth, IXth and Xth nerves.

Fig. 92  Diagrammatic Section across the posterior region of the Head of Ammoecetes

Fig. 92. Diagrammatic Section across the posterior region of the Head of Ammoecetes — the immature form of the Lamprey — to show a Branchiomere and the ganglia derived from the Neural Crest of the Hind-Brain. (After Froriep.)


Segmental Arrangement of Cranial Nerves

We have seen that nine neuromeres can be recognized in the hind-brain at the 4th week of development (Fig. 80), and we may assign a double segmental origin to the midbrain. But when we look at the ganglia and nerves of an embryo in the 6th week of development (Fig. 93) it will be realized that it is impossible to assign a cranial or head segment to each of these. In the human embryo it is easy to see that the Vllth nerve enters the second or hyoid arch and may be regarded as the nerve of the hyoid segment — which may be reckoned the 3rd segment of the head, but the nerve of the segment arises from the 4:th neuromere of the hind-brain, while the nucleus of the Vlth apparently arises from the Vth. Meanwhile we regard both of these neuromeres as belonging to the third cranial segment. In this segment of the head, then, we have an approach to the full complement of nerve elements found in a typical cranial segment. The somatic motor fibres are represented by the Vlth nerve (to the external rectus) ; the lateral somatic motor or branchial, by the motor fibres of the Vllth or facial ; the splanchnic efferent or motor by the secretory fibres of the chorda-tympani of the Vllth ; the afferent or splanchnic sensory by the gustatory fibres of the Vllth (chorda tympani and great superficial petrosal) ; the somatic sensory fibres by the Vlllth or auditory nerve. The cochlear and vestibular ganglia represent a posterior root ganglion ; the submaxillary ganglion — a vagrant sympathetic ganglion. Thus the 4th neural segment has become associated with the hyoid (2nd visceral) arch, the eye and the ear.


In the other segments there have been great changes and reductions. As regards the nerves of the first cranial segment, only its somatic motor nerve — the Ilird nerve — remains ; its posterior root and ganglion are represented by the ophthalmic division of the Vth nerve. The ciliary ganglion represents the sympathetic ganglion of this segment ; the fibres from the Ilird to this ganglion, the efEerent or motor splanchnic fibres. In the ciliary ganglion there may also be motor splanchnic cells, carried out on the fibres of the Ilird nerve. The nerves of the second segment are represented by the I Vth or trochlear nerve (somatic motor), the nerves to the muscles of mastication (lateral somatic or branchial root), the somatic sensory by the maxillary and mandibular divisions of the Vth nerve. The sensory root of the Vth nerve has spread its dominion until it now forms connections with all the segments of the mid- and hind-brains, and even reaches the upper part of the spinal cord. There are no sensory somatic fibres in the nerves of the 4th, 5th, 6th and 7th cranial segments with the exception of the auricular branch of the vagus. The IXth or glossopharyngeal is the nerve of the 4th cranial segment and contains lateral somatic, efferent and afferent splanchnic fibres. The vagus and bulbar roots of the spinal accessory represent the splanchnic efferent and aff'erent nerves of the 5th, 6th and 7th segments — the most important segments in the neural tube, for they contain the nerve centres which dominate the heart, the lungs and the greater part of the alimentary canal. The somatic motor roots of the 5th, 6th and 7th cranial segments are represented by the fasciculi of origin of the Xllth nerve — the motor nerve of the tongue ; they arise from the 8th and 9th neuromeres. It will be thus seen that embryology and comparative anatomy supply a clue to the manner in which the cranial nerves are arranged. The basis of that arrangement is strictly a physiological one, but the specialization in certain segments, which has occurred in the course of evolution, has destroyed the original simplicity of their arrangement.[6] Further mention of the cranial nerves will be made in dealing with the nose, eye, ear, face and visceral arches.

Fig. 93 The Nerves and Ganglia of the Mid- and Hind-Brain of an Embryo at the end of the 6th week of development.

Fig. 93. The Nerves and Ganglia of the Mid- and Hind-Brain of an Embryo at the end of the 6th week of development. After George Streeter (1873-1948)


In the human embryo vestiges of posterior roots and ganglia may appear with the hinder hypoglossal fasciculi (Froriep's ganglion) ; we may infer that at one time the occipital segments had nerves with anterior and posterior somatic roots. Streeter also observed that the spinal rootlets of the Xlth nerve have vestigial ganglia (visceral sensory) on them when first formed (Fig. 92).



  1. See his more recent statement, Journ. of Nerv. and Mental Diseases, 1919, vol. 50, p. 1. Also Dr. Davidson Black, Jonrn. Comp. New. 1917, vol. 27, p. 467 ; vol. 28, p. 379.
  2. I have followed the account given by Dr. Lewis H. Weed for the developing pig. See Anat. Rec. 1916, vol. 10, p. 256.
  3. I have followed the accounts given of the cerebellum by Elliot Smith (See Cunningham's Text-Book of Anatomy, 1913) ; and by Streeter (see Keibel and Mall's Manual of Human Embryology, 1912). See also Dr. Sven Ingvar, Folia Neurobiologica, 1918.
  4. See Nicholls, Quart. Journ. Mic. Sc. 1912, vol. 58, p. 1.
  5. The formation here named lateral line " organ " is better termed the dorsolateral placode, and the epibranchial " organ " epibranchial placode.
  6. For segmentation of hind-brain see F. P. Johnson, Anat. Record, 1915, vol. 10, p. 209 ; J. C. Watt, Contrib. to Embryology, 1915, vol. 2, p. 1 ; H. L. Barniville reference, p. 47 ; Prof. D. Waterston, Journ. Anat. 1915, vol. 49, p. 90.



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Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures