1008 ORIGIN OF AUDITORY IMPULSES. [Book hi. 



brane of Reissner, the endolymph of the canalis cochlearis, and 

 the basilar membrane to the scala tympani, and so reach the fenes- 

 tra rotunda. The bulk of the vibrations ascending the scala 

 vestibuli thus reach the scala tympani by crossing the canalis 

 cochlearis, and in so crossing affect in some way or other 

 the auditory epithelium of the organ of Corti ; it is probably 

 only a remnant which at the summit of the spiral passes di- 

 rectly from the one scala to the other. The features of the 

 basilar membrane point to its being readily thrown into vibra- 

 tions, and we may conclude that the vibrations started at the 

 fenestra ovalis and transmitted from the scala vestibuli to the 

 scala tympani throw the basilar membrane into corresponding 

 vibrations. By the vibrations of the basilar membrane, or by a 

 more direct action of the vibrations of the endolymph, the au- 

 ditory epithelium is so affected as to give rise to auditory im- 

 pulses. 



We now come upon matters of no little difficulty. We have 

 seen reason to think that the eighth nerve serves as the chan- 



n.aud 



Up mb fg sp 



i i 



— r.ah r.as chl' cbl chl 



Fig. 178. The Membranous Labyrinth (of the Right Ear) as seen 



FROM ABOVE, MAGNIFIED SIX TIMES. ( After RetziuS. ) 



The bony envelope has been wholly removed from the vestibular division, 

 but only in part broken through in the cochlear division. 



chl the cochlea, chl' the first part of the basal whorl, chl" the summit. To 

 the right, where the bony wall has been broken through, are seen : l.sp the 

 spirallamina, m.b the basilar membrane, Ig.sp the spiral ligament. 



n aud the auditory nerve, lying alongside of which is seen VII, the seventh, 

 facial nerve. 



m.s macula of the saccule, m. u macula of the utricle, cr.p the crista of the 

 posterior semicircular canal, with r.a.p the branch of the auditory nerve dis- 

 tributed to it, cr.s crista of the superior canal with r.a.s its nerve, a.h ampulla 

 and cr.h crista of the horizontal canal, with r.ah its nerve. 



x the conjoined posterior and superior canals, d.e ductus endolymphaticus, 

 with c.u.s its junction with the utricle. 



