360 THE PERIPHERAL NERVOUS SYSTEM 



The greater part of the pars inferior is, however, differentiated into the 

 spiral ganglion, the peripheral fibers of which innervate the hair cells of 

 the spiral organ (of Corti) in the cochlea. The spiral ganglion appears- in 

 9 mm. embryos and conforms to the spiral turns of the cochlea, hence its 

 name. Its central nerve fibers form the cochlear division of the acoustic 

 nerve. This is distinctly separated from the central fibers of the vestibular 

 ganglion which constitute the vestibular division of the acoustic nerve, the 

 fibers of which are equilibratory in function. The pars inferior of the 

 vestibular ganglion becomes closely .connected with the n. cochlearis, and 

 thus in the adult it appears as though the sacculus and posterior ampulla 

 were supplied by the cochlear nerve. 



n. THE SOMATIC MOTOR NERVE 



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 they have lost their segmental arrange- 

 ment and are otherwise modified. The nuclei of origin of these nerves are 

 shown in Fig. 364. 



12. The Hypoglossal Nerve is formed by the fusion of the ventral root 

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

 blasts of the basal plate and emerge from the ventral wall of the myelence- 

 phalon in several groups (Figs. 357 and 364). 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 ganglion 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. 



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 pres- 

 ent as a rudimentary structure (Figs. 359 and 363), or it may be absent and the ganglion of 

 the first cervical nerve may also degenerate and disappear. In pig embryos Prentiss (1910) 

 has found one to four accessory ganglia (including Froriep's) from which dorsal roots ex- 

 tend 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 



