CH. LIV.] 



THE AUDITORY NERVE 



743 



saccule. The accompanying diagram (fig. 545) shows the relation- 

 ship of all these parts to one another. 



Auditory Nerve. All the organs now described are provided 

 for the appropriate exposure of the filaments of the auditory nerve 

 to vibrations. It enters the bony canal (the meatus auditories 

 internus), with the facial nerve and the nervus intermedius, and, 

 traversing the bone, enters tfie labyrinth at the angle between the 

 base of the cochlea and the vestibule, 

 in two divisions; one for the vestibule 

 and semicircular canals, and the other for 

 the cochlea. 



There are two branches for the vesti- 

 bule, one, superior, distributed to the 

 utricle and to the superior and hori- 

 zontal semicircular canals, and the other, 

 inferior, which arises from the cochlear 

 nerve, ends in the saccule and posterior 

 semicircular canal. There can, however, 

 be little doubt that the inferior nerve, 

 although it is contained for some distance 

 in the sheath of the cochlear nerve, is 

 really composed of vestibular fibres. The 

 terminations of the nerve in the saccule, 

 utricle, and semicircular canals have been 

 already described in page 707 ; so we can 

 pass at once to the cochlea. 



This is best seen in vertical section ; 

 the cavity is divided partly by bone (the 

 spiral lamina), partly by membrane (the 

 basilar membrane), into two spiral scalae, the seal a tympani and scala 

 vestibuli (fig. 546). The basilar membrane increases in breadth from the 

 base towards the apex of the cochlea. It contains fibres (about 

 24,000 in all) embedded in a homogeneous matrix, and running radially 

 straight from the spiral lamina to the spiral ligament, where its other 

 end is again attached to the bone. At the apex of the cochlea, the 

 lamina ends in a small harrmlus, the inner and concave part of which 

 being detached from the summit of the modiolus, leaves a small 

 aperture named the helicotrema, by which the two scalse, separated in 

 all the rest of their length, communicate. 



Besides the scala vestibuli and scala tympani, there is a third 

 space between them, called scala media or canal of the cochlea (CC, 

 fig. 547). In section it is triangular, its external wall being formed 

 by the wall of the cochlea, its upper wall (separating it from tho 

 scala vestibuli) by the membrane of Eeissner, and its lower wall 

 (separating it from the scala tympani) by the basilar membrane, 



C.R. 



FIG. 545. Diagram of the right mem- 

 branous labyrinth. U, Utricle, into 

 which the three semicircular canals 

 open ; 8, saccule, communicating 

 with the cochlea (C) by C.R., the 

 canalis reuniens, and with the utricle 

 by a canal having on it an enlarge- 

 ment, the saccus endolymphaticus 

 (S.E.). The black shading repre- 

 sents the places of termination of 

 the auditory nerve, namely, in the 

 maculae of the utricle and saccule ; 

 the cristae in the ampullary ends of 

 the three semicircular canals; and 

 in the whole length of the canal of 

 the cochlea. (After Schafer.) 



