360 THE PERIPHERAL NERVOUS SYSTEM 



fibers form the cochlear division of the acoustic nerve. This is distinctly sepa- 

 rated from the central fibers of the vestibular ganghon which constitute the ves- 

 tibular division of the acoustic nerve, the fibers of which are not auditory in func- 

 tion. The pars inferior of the vestibular ganghon becomes closely connected 

 with the n. cochlearis, and thus in the adult it appears as though the sacculus and 

 posterior ampulla were suppHed by the cochlear nerve. 



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 midhne. 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. 364. 



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. 357). In embryos of five weeks (7 mm.) the fibers have 

 converged ventrally to form the trunk of the nerve (Fig. 358). Later they grow 

 cranially, lateral to the ganghon nodosum, and eventually end in the muscle 

 fibers of the tongue (Fig. 359) . The nerve in its development unites with the first 

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

 Fig. 364. 



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

 spinal nerves is shoWn: (1) 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 mammahan embryos (pig, sheep, cat, etc.) 

 rudimentary dorsal ganglia are developed, one of which at least (Froriep's ganghon) sends a 

 dorsal root to the hypoglossal. In human embryos Froriep's ganglion may be present as a 

 rudimentary structure (Figs. 359 and 363), or it may be absent and the ganghon of the first 

 cervical nerve may also degenerate and disappear. In pig embryos Prentiss (Jour. Comp. 

 Neurol., vol. 20, 1910) has found two and three accessory ganglia (including Froriep's) from 

 which dorsal roots extended to the root fascicles of the hypoglossal nerve (Fig. 121). 



3. The Oculomotor Nerve originates from neuroblasts in the basal plate of 

 the mesencephalon (Fig. 339 B). The fibers emerge as small fascicles on the 

 ventral surface of the mid-brain in the concavity due to the cephalic flexure (Figs. 

 359 and 364). 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 



