THE EAR 



883 



divides it) is continued into a projection from the periosteum of the outer wall 

 (spiral ligament) by means of a distinct membrane, membrana basilaris, thus 

 completely roofing in the scala tymi)ani which before connnunicated with the scala 

 vestibuli. In addition, a delicate membrane, the membrane of Reissner, 

 stretches from the upper surface of the lamina internal to its free edge to the 

 periosteum of the outer wall, shutting off a small triangular space (scala media) 

 from the scala vestibuli ; the sides of this space are bounded by the tw^o membranes 

 (basilar and Reissner), and its base which lies externally is formed by the osseous 

 wall with its periosteal lining. This canal is lined with epithelium, winds through 

 the whole length of the cochlea, and is commonly known as the ductus cochlearis, 



Fig. 506. — Enlarged View of Longitudinal Section of the First Turn of the 

 Cochlea, showing the Positions and Boundaries of the Three Scal^. 



MEM3RANE OF REISSNER — 

 SCALA MEDIA -^ 



MEMBRANA BASILARIS 



SCALA VESTIBULI 



SPIRAL GAXGLIOX 



SCALA TYMPANl 



or membranous canal of the cochlea. It ends blindly by pointed extremities, 

 one situated at the apex of the cochlea and the other at its base, the latter lies in 

 relation with the floor of the vestibule and receives the small canal already 

 described, the canalis reuniens, which proceeds from the saccule. The canal of 

 the cochlea and the saccule are thus brought into communication. 



The Auditory Nerve divides into two portions before it reaches the fundus 

 of the internal auditory meatus. 



The superior division consists of three branches which pass to the crista 

 vestibuli, through special openings, to be distributed to the utricle and the ampul- 

 lary enlargements of the superior and external semicircular canals. 



The inferior division supplies the cochlea through the foramina of the tractus 

 foraminulentus; also a Ijranch to the saccule, the filaments of whicli pass through 

 the openings of the fovea hemispherica; and a longer branch to the posterior 

 semicircular canal, which reaches it by a special channel, the foramen singulare, 

 situated behind the foramina which transmit the filaments to the saccule. 



Vessels. — The internal auditory branch of the basilar artery, which accom- 

 panies the auditory nerve, supplies both the vestibule and cochlea, with their 

 membranous contents. The veins correspond and open into the inferior petrosal 

 sinus, which also receives the minute veins which traverse the aqueductus 

 vestibuli. 



