THE SENSES 



with the cochlea and semicircular canals, and on its internal 

 aspect are openings for the entrance of some of the branches 

 of the auditory nerve. The cochlea, shaped like a snail shell, 

 runs off from the front of the vestibule, winds about two 

 and a half times around a cone-shaped central axis the 

 modiolus and ends in a blind apex. The canal of the coch- 

 lea is partially separated into two compartments by a bony 

 plate, the lamina spiralis. 



The basilar membrane completes the septum and divides 

 the lumen of the cochlea into two canals, the scala tympani 

 and the scala vestibuli, corresponding in name to the tym- 

 panic and vestibular openings of the cochlea. The semicircu- 

 lar canals, three in number superior, external and posterior 

 describe arches from the posterior aspect of the vestibule, 

 communicating by both their extremities with that cavity. 



The membranous labyrinth consists of a special mem- 

 brane lying inside the bony labyrinth and corresponding in 

 general outline to the walls of the cavity. It is, however, sep- 

 arated from the walls by perilymph, and encloses a similar 

 fluid, the endolymph. It covers the sides of the lamina spir- 

 alis in the cochlea and completes the septum, besides follow- 

 ing the wall proper ; and on one side it sends a distinct pro- 

 cess from the tip of the lamina spiralis to the wall of the ca- 

 nal, so that there are in reality three divisions of the lumen of 

 the cochlea. This process is the membrane of Reissner, and 

 the third canal is the scala media the true membranous coch- 

 lea. (See Fig. 91.) 



Termination of Auditory Nerve. The membranous laby- 

 rinth, containing and being suspended in fluid, receives the 

 terminal filaments of the eighth nerve as well as all the so- 

 norous vibrations intended for that nerve. When the audi- 

 tory nerve has reached the base of the internal auditory 

 meatus it enters the internal ear by two divisions, one for the 

 vestibule and semicircular canals and the other for the coch- 

 lea. The vestibular portion again subdivides, sending one 

 branch to the utricle and superior and horizontal semicircular 



