THE MEMBRANOUS COCHLEA. 131 



may be termed respectively the basal, middle, and apical, the last-named being 

 incomplete. The total length of the tube is about 35 mm. In structure it 

 resembles the membranous 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 both in shape and in the modifica- 

 tions 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 whole length 

 (figs. 127 to 129). In the first place the lamina spiralis is directly prolonged by a 

 comparatively strong, well-marked membrane, the basilar membrane, which stretches 

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



-cc 



Fig. 127. LEFT COCHLEA OF A CHILD SOME WEEKS OLD (Reichert). 



The drawing was taken from a specimen which had been preserved in alcohol, and was afterwards 

 dried ; a section is made so as to show the lamina spiralis, scalse, and cochlear canal in each of the 

 three coils : the membranous spiral lamina is preserved, but the appearances connected with the organ 

 of Corti, &c., have been lost from drying, f r, fossula of the fenestra rotunda. The secondary 

 membrane of the tympanum is seen at the bottom of the fossula attached below to a bony projection 

 of the lower wall (crista semilunaris, to the right of d) ; s t, scala tympani ; s v, scala vestibuli. In 

 the lowermost turn the scala tympani is seen to be the larger : in the higher turns the proportions are 

 reversed ; I s, lamina spiralis ; h, hamulus ; c c, canalis cochleae ; d, opening of the aqueductus 

 cochleae. 



Fig. 128. VERTICAL SECTION OP THE COCHLEA OP A FCETAL CALF (Kb'lliker). 2 



In this specimen the external wall was ossified, but the modiolus and spiral lamina was still 

 cartilaginous ; the section shows in each part of the cochlear tube the two scalse with the intermediate 

 canalis cochleae and lamina spiralis. 



projection of the lining periosteum and sub-periosteal tissue known as the spiral 

 ligament. The basilar membrane thus helps to complete the upper * limit of 

 the scala tympani but does not, properly speaking, enter into the lower boundary 

 of the scala vestibuli, for a second, much more delicate membrane, known as 

 the membrane of Reissner, passes from the upper part of the lamina a little 

 distance from its end, and stretches obliquely upwards and outwards, also to 

 become connected with the lining periosteum. The oblique direction of the 

 membrane of Eeissner causes a triangular space to be shut off between it and 

 the basilar membrane, which is bounded 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 , 



1 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 lowermost, 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. 



VOL. III. PT. 3. I 



