THE COCHLEAE PASSAGES. 819 



fibrous structure (a), which ends in wedge-shaped teeth near the 

 margin of the bony plate. 



Between the teeth and the underlying bone is a channel (6) Spiral 

 which is called sulcus spiralis : its edges are named vestibular (a) s roove - 

 and tympanic (c). 



The membranous part of the septum (membrana basilaris ; Mem- 

 fig. 304, m b) reaches from the lower (tympanic) jedge (c) of the pt us 

 lamina spiralis to the outer wall of the cochlear tube, where it is includes 

 fixed by a fibrous band ligamentum spirale ( 4 ). Its width varies, Jj^brane 

 for near the base of the cochlea it forms half of the partition across and spiral 

 the tube ; but at the apex, where the lamina spiralis is wanting, it Kg* 1 3111 " 

 constructs the septum altogether. 



SCAL.E OF THE COCHLEA (fig. 303). The tube of the cochlea is Scaia* of 

 divided by the septum into two primary passages, of which one is tube? 0111 ** 

 the scala tympani (s ), and the other scala vestibuli (s v) ; but the 

 latter is rendered smaller by a third canal being cut off from it by 

 membrane. 



The passages are placed one above another, the scala vestibuli position ; 

 (s v) being nearest the apex of the cochlea. Above, they com- ex tent; 

 municate through the aperture named helicotrema (/). Below, j oined 

 they end differently, as the names express: the scala vestibuli above ; 

 opens into the front of the vestibule (fig. 301, g) ; but the scala se p a rate 

 tympani is shut out from the vestibular cavity by the lamina spiralis below 

 of the septum cochleae, and is closed below by the membrane of the 

 fenestra rotunda, though in the dry bone it opens into the 

 tympanum. 



Each has certain peculiarities. The vestibular scala extends into they differ 

 the apex of the cochlea ; while the tympanic scala is largest near 

 the base. Connected with the last is the small aqueduct of the 

 cochlea, which begins at an opening close to the lower end of the open j ng j n 

 scala, and ends at the lower border of the petrous portion of the lower; 

 temporal bone : it transmits a small vein from the cochlea. 



The scala are clothed with a thin fibrous membrane, continuous Hnine 

 with that in the vestibule : in the scala tympani it helps to close membrane, 

 the fenestra rotunda, forming the inner layer of the secondary 

 membrane of the tympanum, and joins the librous process in the and con- 

 aqueduct of the cochlea. The perilymph fills both scalae. 



CANAL OF THE COCHLEA. In the upper division of the cochlear Cochlear 

 tube a fine membrane (fig. 304, T ) extends obliquely across from the c 311 * 118 

 upper surface of the lamina spiralis, at the inner border of the between 

 limbus, to the outer wall of the cavity a little above the spiral membrane 

 ligament. This is called the membrane of Reissner, and separates a and mem . 

 small cavity named the canal or duct of the cochlea (c c) from the scala brane of 

 vestibuli (s v). The canal thus formed extends from apex to base of 

 the cochlea, and contains a fluid endolymph. Above, it reaches into 

 the cupola and is closed. Below, it is connected by a very small tube duct from 

 (canalis reuniem ; fig. 305, c) with the saccule in the vestibule. 

 Within the canal of the cochlea, resting on the basilar membrane, is 

 the complicated structure known as the organ of Corti (fig. 304), in 

 which the cochlear branches of the auditory nerve end. Corti. 



3G 2 



