652 



THE EAR 



semicircular canals just described in consisting of a tube, lined by 

 epithelium and containing endolymph, partly surrounded by a clear 

 space containing perilymph, but it differs from them materially both 

 in shape and in the modifications presented by its epithelial lining. 

 In macerated specimens, the two parts into which the osseous tube 

 of the cochlea is divided are, it will be remembered, only imperfectly 

 separated by the osseous spiral lamina which projects from the 

 columella ; but in the fresh specimen the tube is separated completely 

 into three distinct parts by means of two membranes, which extend 

 along its Avhole length (figs. 465, 466). In the first place the lamina 

 spiralis is directly prolonged by a comparatively strong, well-marked 

 membrane, the basilar membrane (fig. 467, b), which stretches straight 

 across to the outer wall of the cochlea, and is here connected to an 

 inward projection of the lining periosteum and sub-periosteal tissue 

 known as the spiral ligament (Isp). The basilar membrane thus 

 helps to complete the upper* limit of the scala tympani (ST), but 

 does not enter into the lower boundary of the scala vestibuli, for a 

 second, much more delicate membrane, known as the membrane of 

 Eeissner (R) passes from the upper part of the lamina a little distance 



from its end, and stretches 

 Fig. 467. obliquely upwards and out- 



wards, also to become con- 

 nected with the lining peri- 

 osteum ; neither of the 

 lines of insertion of this 

 membrane are prominently 

 marked. The oblique di- 

 rection of the membrane of 

 Ileissner causes a triangu- 

 lar space to be shut off be- 

 tween it and the basilar 

 membrane, which is bound- 

 ed externally by the outer 

 osseous wall of the cochlea 

 lined by periosteum : and 

 this space, extending 

 throughout the whole 

 length of the osseous 

 tube, and lined throughout 

 by an epithelium variously 

 modified in different parts, 

 is known distinctively as 

 the canal of the cochlea 

 canalis membranaceus, or ductus cochlearis (figs. 465, 467, CC, fig, 469, 

 DC). It terminates in a blind pointed extremity at the apex, and 

 another at the base. That at the apex, extending beyond the hamulus, 

 is fixed to the wall of the cupola, and partly bounds the helicotrema ; 

 that at the base fits into the ano-le at the commencement of the osseous 



Fig. 467. — Sectiox thkough one of the Coils 

 OF THE Cochlea, LiAiiRAMiiATic (altered from 

 Henle). f 



S T, scala tympani ; S Y, scaia restihuli ; C C, 

 canalis coclilese ; R, membrane of Eeissner forming 

 its vestibular wall ; I s o, lamina spiralis ossea ; 

 lis, limbus laminse spiralis ; s s, sulcus spiralis ; 

 n c, cochlear nerve ; g s, ganglion spirale ; t, mem- 

 brana tectoria ; b, membrana basilaris ; C o, rods 

 of Corti ; I s p, ligamentum spirale. 



* To avoid repetition it may here be stated that for convenience sake the cochlea 

 is considered in the present description as having its larger part or base lower- 

 most, and the domed extremity uppermost, although of course this is far from being the 

 relative position of the parts whilst within the body. Moreover, parts nearer the 

 columella are spoken of as inner ; parts nearer the external wall as outer. 



