SENSE OF HEARING. 229 



The cochlea forms the anterior part of the internal ear. It is a gradually 

 tapering canal, about 3 \ inches in length, which winds spirally around a 

 central axis, the modiolus, two and one half times. The interior of the cochlea 

 is partly divided into two passages by a thin plate of bone, the lamina osseous 

 spiralis, which projects from the central axis two thirds of the way across the 

 canal. These passages are termed the scala vestibuli and the scala tympani, 

 from their communication with the vestibule and tympanum. The scala 

 tympani communicates with the middle ear through the foramen rotundum, 

 which, in the natural condition, is closed by the second membrana tympani ; 

 superiorly they are united by an opening, the helicotrema. 



The whole interior of the labyrinth, the vestibule, the semicircular canals, 

 and the scala of the cochlea, contains a clear, limpid fluid, the perilymph 

 secreted by the periosteum lining the osseous walls. 



The membranous labyrinth corresponds to the osseous labyrinth with 

 respect to form, though it is somewhat smaller in size. 



The vestibular portion consists of two small sacs, the utricle and the saccula, 



The semicircular canals communicate with the utricle in the same manner 

 as the bony canals communicate with the vestibule. The saccule communi- 

 cates with the membranous cochlea by the canalis reuniens. In the interior 

 of the utricle and saccule, at the entrance of the auditory nerve, are small 

 masses of carbonate of lime crystals, constituting the otoliths. Their function 

 is unknown. 



The membranous cochlea is a closed tube, commencing by a blind extremity 

 at the first turn of the cochlea, and terminating at its apex by a blind extremity 

 also. It is situated between the edge of the osseous lamina spiralis and the 

 outer wall of the bony cochlea, and follows it in its turns around the 

 modiolus. 



A transverse section of the cochlea shows that it is divided into two por- 

 tions by the osseous lamina and the basilar membrane: 



1 . The scala vestibuli, bounded by the periosteum and membrane of Reissner. 



2. The scala tympania, occupying the inferior portion, and bounded above by 

 the septum, composed of the osseous lamina and the membrana basilaris. 

 The true membranous canal is situated between the membrane of Reissner 



and the basilar membrane. It is triangular in shape, but is partly divided 

 into a triangular portion and a quadrilateral portion by the tectorial 

 membrane. 



The organ of Corti is situated in the quadrilateral portion of the canal, and 

 consists of pillars of rods of the consistence of cartilage. They are arranged 

 in two rows the one internal, the other external; these rods rest upon the 

 basilar membrane; their bases are separated from one another, but their 



