256 HUMAN PHYSIOLOGY. 



circular 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 saccule. 



The semicircular canals communicate with the utricle in the same 

 manner as the bony canals communicate with the vestibule. The 

 saccule communicates 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 crys- 

 tals, 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 portions 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 upper extremities are united, forming an 

 arcade. In the internal row it is estimated there are about 3,500 

 and in the external row about 5,200 of these rods. 



On the inner side of the internal row is a single layer of elongated 

 hair-cells ; on the outer surface of the external row are three such 

 layers of hair-cells. Nothing definite is known as to their function. 



