358 THE PERIPHERAL NERVOUS SYSTEM 



learis, and thus in the adult it appears as though the saeculus and posterior 

 ampulla were supplied by the cochlear nerve. The cells of both the vestibular 

 and spiral ganglia are of the bipolar type. 



n. THE SOMATIC MOTOR NERVES 



The nerves of this group consisting of the three nerves to the eye muscles 

 and the n. hypoglossus are purely motor nerves, the fibers of which take origin 

 from the neuroblasts of the basal plate of the brain stem, near the midline. They 

 are regarded as the homologues of the ventral motor roots of the spinal cord, but 

 have lost their segmental arrangement and are otherwise modified. The nuclei 

 of origin of these nerves are shown in Fig. 345. 



12. N. Hypoglossus. This nerve is formed by the fusion of the ventral root 

 fibers of three to five precervical nerves. Its fibers take origin from neuroblasts 

 of the basal plate and emerge from the ventral wall of the myelencephalon in 

 several groups (Fig. 339). In embryos of the fourth week (7 mm.) the fibers 

 have converged ventrally to form the trunk of the nerve (Fig. 340). Later they 

 grow cranially lateral to the ganglion nodosum and eventually end in the muscle 

 fibers of the tongue (Fig. 341). The nerve in its development unites with the 

 cervical nerves to form the ansa hypoglossi. Its nucleus of origin is shown in 

 Fig. 345- 



That the hypoglossal is a composite nerve homologous with the ventral roots of the 

 spinal nerves is shown: (i) by the segmental origin of its fibers; (2) from the fact that its nucleus 

 of origin is a cranial continuation of the ventral gray column, or nucleus of origin for the ventral 

 spinal roots; (3) from the fact that in mammalian embryos (pig, sheep, cat, etc.) rudimentary 

 dorsal ganglia are developed, one of which at least (Froriep's ganglion) sends a dorsal root to the 

 hypoglossal. In human embryos Froriep's ganglion may be present as a rudimentary structure 

 (Figs. 341 and 344), or it may be absent and the ganglion of the first cervical nerve may also 

 degenerate and disappear. In pig embryos the writer has found two and three accessory 

 ganglia (including Froriep's) from which dorsal roots extended to the root fascicles of the hypo- 

 glossal nerve (Fig. 116). 



3. The Oculomotor Nerve, as we have seen, takes origin from neuroblasts 

 in the basal plate of the mesencephalon (Fig. 321 B). The fibers emerge as 

 small fascicles on the ventral surface of the mid-brain in the concavity due to the 

 cephalic flexure (Figs. 341 and 345). The fascicles converge, form the trunk of 

 the nerve and end in the premuscle masses of the eye. The nerve eventually 

 supplies all of the extrinsic muscles of the eye save the superior oblique and ex- 

 ternal rectus. A branch is also supplied to the ciliary ganglion. In the chick 

 embryo bipolar cells migrate along the fibers of the oculomotor nerve to take part 



