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 the ganglion on the dorsal root 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 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. As the two roots join each divides into an anterior (ventral) and a posterior (dorsal) primary division, 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 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 wall, and a splanchnic or visceral branch which joins the sym pathetic and forms the white ramus communicans. The branch grows round the body wall and gives off lateral and an terior branches.
rhombencephalon (see BRAIN), and the sensory are the axons and dendrites of cells situated in ganglia which have budded off from the brain. The evidence of comparative anatomy, however, shows that the cranial nerves cannot be directly homologized with the spinal, nor can the fact of there being twelve of them justify us in assuming that the head contains the rudiments of twelve fused or unsegmented somites.
The sympathetic system is developed from the posterior root ganglia of the spinal nerves, by cells which in man migrate a few at a time. A. M. Paterson, however, believes that the sympathetic
is developed, independently of the cerebrospinal system, in the mesoderm (Phil. Trans., B. clxxxi.).
The comparative anatomy of the brain and spinal cord is dealt with in the separate articles devoted to them.
In Amphioxus the dorsal and ventral roots of the spinal nerves do not unite with one another but alternate, a dorsal root on one side being opposite a ventral on the other. The dorsal roots are both sensory and motor, the ventral only motor. In the Cyclosto mata (Petromyzon) the arrangement is nearly the same, but in some regions there are two ventral roots to one dorsal. In the fishes and higher vertebrates the dorsal and ventral roots unite, though in selachian (shark) embryos the dorsal and ventral roots alternate (F. M. Balfour, The Development of Elasmobranch Fishes, London, 1878).
The cranial nerves are only represented by two pairs in Amphi oxus. 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 mam mals, the eleventh and twelfth nerves have been added. The cranial nerves are formed of at least five components : ventral motor, (2) lateral motor, (3) somatic sensory, (4) visceral sen sory, (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 hypoglossal, 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 belong ing to the branchial skeleton, instead of the muscles of the primi tive cranium, of which the eye muscles are the remnants.