678 THE SENSE OF HEARING. 



turns and a half from the base to the apex. At the apex 

 of the cochlea the canal is closed ; at the base, it presents 



three openings, of which 

 one, already mentioned, 

 communicates with the ves- 

 tibule ; another, called fe- 

 nestra rotunda, is separated 

 by a membrane from the 

 cavity of the tympanum ; 

 the third is the orifice of 

 the aqu&ductus cochlece, a 

 canal leading to the jugular fossa of the petrous bone, and 

 corresponding, at least in obscurity of purpose and origin, 

 to the aquseductus vestibuli. The spiral canal is divided into 

 two passages, or scalseby a partition of bone and membrane, 

 the lamina spiralis. The osseous part or zone of this lamina 

 is connected with the modiolus; the membranous part, 

 with a muscular zone, according to Todd and Bowman, 

 forming its outer margin, is attached to the outer wall of 

 the canal. Commencing at the base of the cochlea, be- 

 tween its vestibular and tympanic openings, they form a 

 partition between these apertures ; the two scalse are, 

 therefore, in correspondence with this arrangement, named 

 scala vestiluli and scala tympani. At the apex of the cochlea, 

 the lamina spiralis ends in a small hamulus, the inner and 

 concave part of which, being detached from the summit of 

 the modiolus, leaves a small aperture named helicotrema, by 

 which the two scalee, separated in all the rest of their 

 length, communicate. 



Besides the scala vestiluli and scala tympani, there is a 

 third space between them, in the substance of the lamina 

 spiralis, called the scala media, and in this are some pecu- 



* Fig. 193. View of the osseous cochlea divided through the middle 

 (from Arnold). . I, central canal of the modiolus; 2, lamina spiralis 

 ossea ; 3, scala tympani ; 4, scala vestihuli ; 5, porous substance of the 

 modiolus near one of the sections of the canalis spiralis modioli. 



