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EMBRYOLOGY OF NERVOUS

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Originally appearing in Volume V19, Page 404 of the 1911 Encyclopedia Britannica.
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EMBRYOLOGY OF NERVOUS SYSTEM The development of the brain, spinal cord and organs of special sense (eye, ear, tongue), will be found in separate articles. Here that of the cranial and spinal nerves and the sympathetic system is dealt with. The thoracic spinal nerves are the most typical, and one of them is the best to begin with. In fig. 7, A the ganglion on the dorsal root (DR) is seen growing out from the neural crest, and the cells or neuroblasts of which it is composed become fusiform and grow in two directions as the ganglion recedes from the cord. Those which run toward the spinal cord are the axons, while those growing into the mesoderm are probably enlarged dendrites. The ventral roots (VR) rise as the axons of the large cells in the ventral horn of the grey matter, and meet the fibres of the dorsal root on the distal side of the ganglion (fig. 7, B). As the two roots join each divides into an anterior (ventral) and a posterior (dorsal) primary division (fig. 7, D), the latter growing into the dorsal segment of its muscle .plate and the skin of the back. The anterior primary division grows till it reaches the cardinal vein and dorsal limit of the coelom, and there forks into a somatic branch to the body wall (fig. 7, C, So), and a splanchnic or visceral branch (fig. 7, C, Vi) which joins the sympathetic and forms the white ramus communicans. The somatic branch grows round the body wall and gives off lateral and anterior branches (fig. 7, E). In the limb regions the anterior primary divisions of the nerves divide into anterior and posterior secondary divisions, which probably correspond to the anterior and lateral branches of the thoracic nerves (fig. 7, E and F). These unitewith neighbouring nerves to form plexuses, and divide again, but the anterior nerves keep to the ventral side of the limb and the posterior to the dorsal. The cranial nerves are developed in the same way as the spinal, so far as concerns the facts that the motor fibres are the axons of cells situated in the basal lamina of the mesencephalon and rhombencephalon (see BRAIN), and the sensory are the axons and den- there are two ventral roots to one dorsal. In the fishes and higher drites of cells situated in ganglia which have budded off from the vertebrates the dorsal and ventral roots unite, though in selachian brain. The evidence of comparative anatomy, however, shows that (shark) embryos F. M. Balfour says that the dorsal and ventral ,~ f-roots alternate (The Development of Elasmobranch Fishes, London, 1878). When limbs are developed, beginning with fishes, limb plexuses are formed. Where the limbs are suppressed rudimentary plexuses may persist, as in the snake, though usually they disappear. The cranial nerves are only represented by two pairs in Amphioxus. In the Cyclostomata, fishes and Amphibia, ten pairs of nerves are found, which in their distribution do not always agree with those of man. In the Amniota or reptiles, birds and mammals, the eleventh and twelfth nerves have been added. The researches of W. H. Gaskell (" On the structure, distribution and functions of the nerves which innervate the visceral and vascular systems," J. of Phys. vii. 1, 1886), O. S. Strong (" The cranial nerves of Amphibia," J. Morph. x. lot), J. B. Johnston (J. Comp. Neurol. xii. 2 and 87), and others, show that the cranial nerves are formed of at least five components: (I) Ventral motor, (2) Lateral motor, (3) Somatic sensory, (4) Visceral sensory, (5) Lateral line nerves. The ventral motor components are those which rise from cells situated close to the mid line, and probably correspond to the ventral roots of the spinal nerves. The nerves to the eye muscles (motor oculi, trochlearis and abducens) have this origin (see NERVE: Cranial), as also has the hypo-glossal, which doubtless is a cephalized spinal nerve. The lateral motor components rise from cells situated more laterally, and comprise the motor roots of the fifth (trigeminal), seventh (facial), and ninth, tenth and eleventh (glossopharyngeal, vagus and spinal accessory). These nerves supply muscles belonging to the branchial skeleton, instead of the muscles of the primitive cranium, of which the eye muscles are the remnants. The somatic sensory components supply the skin, and end in cells which, among the cyclostomes and fishes, form a considerable elevation in the rhombencephalon, known as the lobus trigemini (fig. 8, Nuc. V.). These components, in the lower forms, are found in the fifth, seventh and tenth nerves, but in mammals practically F, Formation of nerve trunks in relation only the fifth contains them. They correspond to the limb; dorsal and ventral to the dorsal roots of the spinal nerves. trunks corresponding to lateral and The splarchnic sensory or viscero sensory cm- anterior trunks in D and E. ponents end in the brain in the medullary cells known as the fasciculus communis in fishes, and fasciculus solitarius in mammals (see BRAIN, fig 4), as well as in the lobus trigemini and lobus vagi (fig. 8, Nuc. X.). They are found in the fifth, seventh, ninth, tenth and eleventh nerves, and supply visceral surfaces. In mammals the lingual and palatine
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