HEARING. [CH. LIII. 



shaped like a snail-shell, is situated in front of the vestibule, its 

 base resting on the bottom of the internal meatus, where some 

 apertures transmit to it the cochlear filaments of the auditory 

 nerve. In its axis, the cochlea is traversed by a conical column, 

 the modiolw, around which a spiral canal winds with two turns 

 and a half from the base to the apex. At the apex of the cochlea 

 the canal is closed ; at the base it presents three openings, of 

 which one, already mentioned, communicates with the vestibule ; 

 another, called fenestra rotunda, is separated by a membrane from 

 the cavity of the tympanum ; the third is the orifice of the 

 aquceductus cochlece, a canal leading to the jugular fossa of the 

 petrous bone. The spiral canal is divided into two passages, or 

 scalce (staircases), by a partition formed partly of bone, the 

 lamina spiralis, connected with the modiolus, and partly of a 

 membrane called the basilar membrane. 



The Membranous Labyrinth. Tiie membranous labyrinth 



C.R. 



Fig. 543. Diagram of the right membranous labyrinth. U, utricle, into which the three 

 semicircular canals open ; S, saccule, communicating with the cochlea (C) by C.R., 

 the canalis reumens, and with the utricle by a canal having on it an enlargement, the 

 saccus endplymphaticus (S.E.). The black shading represents the places of termination 

 of the auditory nerve, namely, in the maculse of the utricle and saccule ; the cristse in 

 the ampullary ends of the three semicircular canals ; and in the whole length of the 

 canal of the cochlea. (After Schafer.) 



corresponds generally with the form of the osseous labyrinth, so far 

 as regards the vestibule and semicircular canals, but is separated 

 from the walls of these parts by perilymph, except where the nerves 

 enter into connection within it. The labyrinth is a closed membrane 

 containing endolymph, which is of much the same composition as 

 perilymph, but contains less solid matter. It is somewhat viscid, 

 as is the perilymph, and it is secreted by the epithelium lining its 

 cavity ; all the sonorous vibrations impressing the auditory nerves 

 in these parts of the internal ear, are conducted through fluid to 

 a membrane suspended in and containing fluid. In the cochlea, 

 the membranous labyrinth completes the septum between the 



