236 THE POCKET ANATOMY 



a macula and a collection of otoliths. Below there is a small 

 canal, canalis reuniens, which connects it with the membranous 

 canal of the cochlea. 



The membranous semicircular canals are about one-third 

 the size of the osseous ones, except at the ampullae, where 

 they dilate to nearly fill the bony canal. Each membranous 

 canal is free on the convex surface, the concavity being fixed 

 to the osseous canal. On the part of the internal surface of 

 the ampulla, corresponding to the attachment to the bony 

 canal, is a transverse projection, the crista acustica, in which 

 some filaments of the auditory nerve end. 



The membranous cochlea consists of a membranous tube 

 divided into three parts the scala tympani, scala vestibuli, 

 and canal of the cochlea (ductus cochlearis), which latter 

 contains the organ of Corti, to which the auditory nerve is 

 chiefly distributed. 



The lamina spiralis ossea partly divides the spiral canal 

 into the scala tympani and scala vestibuli, the latter being 

 superior. The septum is completed by the membrana basilaris, 

 which is attached to the outer free edge of the lamina spiralis 

 ossea, and passed outwards to the wall of the cochlea, where 

 it is fixed to a thickening of the periosteum called the spiral 

 ligament. 



The canal of the cochlea contains the organ of Corti. It 

 lies on the outer part of the scala vestibuli, and is separated 

 from it by the membrane of Reissner, which passes from the 

 lamina spiralis upwards and outwards to the roof of the scala. 

 Thus a triangular piece is cut off, bounded internally by the 

 membrane of Reissner, externally and above by the outer 

 wall of the cochlea, and below by the basilar membrane. 

 The canal of the cochlea is connected below with the saccule 

 by the canalis reuniens, and above it terminates in a blind cone- 

 shaped extremity, partly bounding the helicotrema and fixed 

 to the cupola. The part of the lamina spiralis within the 

 canal of the cochlea becomes thickened, and is called the 

 limbus, and terminates in a concave border, the sulcus spiralis. 

 The basilar membrane is attached to the lower margin of the 

 sulcus spiralis. 



The Auditory Nerve. The auditory nerve divides in the in- 

 ternal auditory meatus into two branches, which perforate 

 the cribriform plate, and are distributed to the cochlea and 

 the vestibule. 



The superior or vestibular branch supplies : 



1. The utricle. 



2. The ampulla of the superior semicircular canal. 



3. The ampulla of the horizontal semicircular canal. 



