214 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



springs from the bulb by two roots: the external and the 

 internal. The former has been shown to supply, mainly at 

 least, the cochlear portion of the internal ear, and is, there- 

 fore, the auditory nerve proper. This division is spoken of 

 as the cochlear branch. The internal root supplies mainly 

 the vestibular branch of the internal ear, and is, therefore, 

 spoken of as the vestibular branch (see Fig. 94). It seems cer- 

 tain that the latter is not an auditory nerve, but is concerned 

 with peculiar sensations from the semicircular canals and vestibule 

 that have an important influence on muscular' activity, especially 

 in complex movements. The central course of these two roots is 

 quite as distinct as their peripheral distribution, a fact that bears 

 out the supposition that they mediate different functions. The 

 vestibular branch ends in the nucleus of Deiters, the nucleus of 

 Bechterew, and the nucleus fastigii of the cerebellum. Through 

 these nuclei reflex connections are made with the motor centers 

 of the cord and midbrain, and probably also with the cerebellum. 

 The path is not known to be continued forward to the cerebrum. 

 The central course of the cochlear branch is indicated schematically 



in Figs. 94 and 95. The 

 fibers constituting this 

 branch arise from nerve 

 cells in the modiolus of 

 the cochlea, the spiral 

 ganglion. These cells, 

 like those in the poste- 

 rior root ganglia, are bi- 

 polar. One axon passes 

 peripherally to end 

 around the sense cells 

 of the cochlea, at which 

 point the sound waves 

 arouse the nerve im- 

 pulses. The other axon 

 passes toward the pons, 

 forming one of the fibers 

 of the cochlear branch. 

 On entering the pons 

 these cochlear branches 

 end in two nuclei, one 

 lying ventral to the res- 

 tiform body and known 

 as the ventral or acces- 

 sory nucleus (V.n., Fig. 



95), and one dorsally, known as the dorsal nucleus or the tuber- 

 culum acusticum (D.n.). From these nuclei the path is continued 



if. 



Fig. 96. Diagram to show central course of 

 auditory fibers (modified from Van Gehuchten): 

 D.n., Dorsal nucleus giving rise to the fibers that 

 form the medullary strise (a.s.); V.n., the ventral 

 nucleus, giving origin to the fibers of the corpus 

 trapezoideum (c.tr.); s.o., superior olivary nucleus; 

 l.f., lateral lemniscus; n.s., nucleus of the lateral 

 lemniscus; t.g.i.,, the inferior colliculus. 



