202 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



in Figs. 88 and 89. The fibers constituting this branch arise from 

 nerve cells in the modiolus of the cochlea, the spiral ganglion. 

 These cells, like those in the posterior root ganglia, are bipolar. 

 One axon passes peripherally to end around the sense cells of the 

 cochlea, at which point the sound waves arouse the nerve impulses. 

 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 restiform 

 body and known as the ventral or accessory nucleus (V.n., Fig. 89), 

 and one dorsally, known as the dorsal nucleus or the tuberculum 

 acusticum (D.n.). From these nuclei the path is continued by sec- 

 ondary sensory neurons, and its further course toward the brain is 

 still a matter of much uncertainty in regard to many of the de- 

 tails.* The general course of the fibers, however, is known. Those 

 axons that arise from the accessory nucleus pass mainly to the oppo- 

 site side by slightly different routes (Fig. 89). Some strike directly 

 across toward the vental side of the pons, forming a conspicuous 

 band of transverse fibers that has long been known as the corpus 



trapezoideum ; others 

 pass dorsally around 

 the restiform body and 

 then course downward 

 through the tegmental 

 region to enter the cor- 

 pus trapezoideum. The 

 fibers of this cross band 

 end, according to some 

 observers, in certain nu- 

 clei of gray matter on the 

 opposite side of the pons, 

 especially in the superior 

 olivary body and the 

 trapezoidal nucleus, and 

 thence the path forward 

 is continued by a third 

 neuron. Certainly from 

 the level of the superior 

 olivary body the audi- 

 tory fibers enter a dis- 

 tinct tract long known 

 to the anatomist and des- 

 ignated as the lateral 

 fillet or lateral lemniscus. 



Authors differ as to whether the auditory fibers of this tract arise 



from nerve cells in the superior olivary and neighboring nuclei, or 



* For literature see Van Gehuchten, " Le Nevraxe/' 4, 253, 1903. 



Fig. 89. Diagram to show central course of 

 auditory fibers (modified from Van Gehuchten) : 

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

 form the auditory striae (..); V.n., the ventral nu- 

 cleus, giving origin to the fibers of the corpus trape- 

 zoideum (c.tr.); s.o., superior olivary nucleus; l.f., 

 lateral fillet; n.s., nucleus of the lateral fillet; t.g.x., 

 the inferior colliculus. 



