THE SENSE OF HEARING. 147 



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 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 

 portions by the osseous lamina and the basilar membrane : I. The scala 

 vestibuli, bounded by the periosteum and membrane of Reissner. 2. The 

 scala tympani^ 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 Reiss- 

 ner 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 each other, but 

 their upper extremities are united, forming an arcade. In the internal row 

 it is estimated there are about 3500, and in the external row about 5200 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. 



The Endolymph occupies the interior of the utricle, saccule, membranous 

 canals, and bathes the strictures in the interior of the membranous cochlea, 

 throughout its entire extent. 



The Auditory Nerve at the bottom of the internal auditory meatus 

 divides into (i) a vestibular branch, which is distributed to the utricle and 

 semicircular canals ; (2) a cochlear branch, which passes into the central 

 axis at its base, and ascends to its apex ; as it ascends, fibres are given off, 

 which pass between the plates of the osseous lamina, to be ultimately con- 

 nected with the organ of Corti. 



The Function of the semicircular canals appears to be to assist in main- 

 taining the equilibrium of the body ; destruction of the vertical canal is 



