CENTRAL NERVOUS SYSTEM. 679 



tory, the second or optic, and the first or olfactory ; and these require special 

 comment. 



Eighth Nerve, Hearing. The eighth nerve goes to the ear. The gan- 

 glion-cells appear in two groups, the accessory ganglion Gl.Ac. and the spiral 

 ganglion of the cochlea. This latter is definitely associated with the cochlear 

 branch of the auditory nerve which has to do with the organ of Corti. The 

 other branch of the auditory nerve, the vestibular, is associated with the semi- 

 circular canals, the functions of which are not auditory, but concerned with the 

 maintenance of equilibrium (see Fig. 180). 



The branch for the semicircular canals and that for the cochlea have dif- 

 ferent central connections. 1 The auditory fibres proper arising from the cells 

 of the spiral ganglion in the cochlea and from those of the anterior auditory 

 nucleus (Gl. ac.), first connect with the cells of the tuberculum acusticum 

 (T.A.), and are thence continued by the striae acusticae (St. med.) into the 

 lemniscus of the opposite side ; through this with the posterior quadrigemi- 

 num and the internal geniculate body of that side, probably the thalamus also, 

 and thence by the internal capsule toward its occipital end, with the cortex 

 of the more occipital portions of the first and second temporal convolutions. 



This path is indicated by comparative anatomy (Spitzka), by experimental 

 degeneration practised on animals (von Monakow), and by pathological observa- 

 tions on man where the pathway has become injured or diseased in one of its 

 parts. 



By the two latter forms of evidence it appears that the portion of the cere- 

 bral cortex is also associated with the lateral nucleus of the thalamus of the 

 same side, for injury to the cortex causes atrophy of this part of the 

 thalamus. 



Second Nerve, Optic. As has long been recognized, the optic nerve, so 

 called, is a cerebral tract morphologically equivalent to such tracts as connect 

 any portion of the cerebral cortex with a primary centre, the retina being in 

 part the representative of the cerebrum, and the pulvinares, the quadrigemina, 

 and geniculata externa being the primary centres. 



At the chiasma where the two optic nerves come together their fibres inter- 

 mingle, and then emerge as the optic tracts, which contain not only the fibres 

 connected with the retina, but others added from the superposed parts of the 

 brain. 



In the higher mammals it was shown by von Gudden 2 that in the chiasma 

 the majority of the fibres forming one optic nerve pass to the tract of the 

 opposite side, but that a portion of the fibres remain in the tract of the same 

 side. 



This was inferred because removal of one optic bulb caused in young 

 rabbits a degeneration in the associated optic nerve and also in both optic 

 tracts most marked, however, in the tract of the side opposite to the lesion. 



1 Onufrowicz : " Exper. Beitrag zur Kenntniss des centralen Ursprunges des Nervus acus- 

 ticus," Inaug. Diss., 1885. 



2 von Gudden : Gesammelte und hinterlasserie Abhandlungen, Wiesbaden, 1889. 



