116 



THE EAU. 



When the limbus is viewed from above, the vestibular edge is seen to present a 

 succession of tooth-like projections (fig. 132, Or), about 7,000 in number alto- 

 gether, which give it a jagged aspect. These projections are continued as flattened 

 eminences a short distance on the upper surface of the limbus, which is, therefore, 

 not smooth, at least near the edge, but marked in this way with eminences and 

 intervening furrows. Nearer the line of origin of the membrane of Reissner, it 

 becomes smoother, and here, too, its epithelial covering, which is directly continuous 

 with that of the under surface of Reissner's membrane, is evenly distributed ; 

 whereas at the crest itself the epithelial cells are columnar in the furrows, but 

 flattened out over the teeth, so as to be invisible here with ordinary methods of 

 preparation. Their outlines can, however, according to G. Retzius, be brought to 

 view by the employment of the silver method. Immediately below the overhanging 

 projections, the epithelium again forms a well-defined layer of cubical or short 

 columnar cells which lines the spiral groove, and is continuous externally with the 

 specialized cells, presently to be described as forming the organ of Corti. 



The basilar membrane stretches, as before mentioned, straight between the 

 osseous lamina and the spiral ligament, and separates the canal of the cochlea from the 



Fig. 132. SEMI-DIAGRAMMATIC VIEW OP PART OP THE BASILAR MEMBRANE AND TUNNEL OF CORTI OF 



THE RABBIT, FROM ABOVE AND THE SIDE. Much magnified. (E.A.S.) 



I, limbus ; Cr, labium vestibulare or crest of limbus with tooth-like projections ; b b, basilar mem- 

 brane ; sp.l, spiral lamina with, p, perforations for transmission of nerve fibres. In the lower half of 

 the part of the spiral lamina here represented the nerve fibres are left, and are supposed to be seen 

 through the upper layer of that lamina, converging to three of the perforations ; below, in the section of 

 the lamina, they are shown occupying a canal, or cleft, in the osseous substance ; i.r, fifteen of the 

 inner rods of Corti ; h.i, their flattened heads seen from above ; e.r, nine outer rods of Corti ; h.e, their 

 heads, with the phalangeal processes extending outward from them and forming, with the two rows of 

 phalanges, the lamina reticularis, I.r. On the left of the figure the connective tissue fibres and nuclei 

 of the undermost layer of the ba,silar membrane are seen through the upper layers. Portions of the 

 basilar processes of the outer hair-cells remain attached here and there to the membrane at this part. 



scala tympani. It increases in breadth, at first rapidly but afterwards more gradu- 

 ally, from the base to the apex of the cochlea, while the breadth of the osseous spiral 

 lamina diminishes. At the lowest part of the cochlea, where this membrane occupies 

 the naiTow cleft between the lamina spiralis ossea and the lamina secundaria, the 

 breadth is only about 0*041 mm. ; but towards the apex of the cochlea it increases 

 at the expense of the bony lamina, until, near the helicotrema, the membranous part 

 is left almost unsupported by any plate of bone, measuring as much as 0'495 mm., 

 or about twelve times as much as at the base (Hensen). The average measurements 

 given by Retzius are, for the first or basal turn 0'21 mm. ; for the middle turn 

 0'34 mm. ; and for the apical turn 0*36 mm. Its total breadth averages, according 



