CHAP, iv.] HEARING. 205 



along the whole length of the tube, and partially dividing it length- 

 ways into two, while the canalis cochlearis would appear as a smaller 

 tube, of triangular section, slid into the larger tube in such a way 

 that the apex of the triangle all the way along touched the edge of 

 the lamina spiralis, and the base all the way along was adherent 

 to the opposite side of the main tube, thus separating completely 

 the scala vestibuli on the one side from the scala tyrnpani oa the 

 other. Only at the pointed end of the main tube would the canalis 

 cochlearis be wanting, and here the two scala3 would run into each 

 other. 



The third great feature of the cochlea is that the auditory 

 nerve is connected with the canalis cochlearis, not in a circum- 

 scribed patch, macula, or ridge, crista, but along its whole length ; 

 and there is accordingly an area of modified auditory epithelium 

 along the whole length of the canal from close to the bottom of 

 the undermost whorl to the tip, or nearly to the very tip of the 

 topmost whorl. 



The three sides of the canalis cochlearis differ markedly in 

 structure and appearance. We shall study the details of these 

 later on ; meanwhile we may say that the wall which separates the 

 canal from the scala vestibuli is a thin membrane (Fig. 177 m. R.), 

 known as the membrane of Reissner, the epithelium on which lining 

 the canal is of a simple character ; that the epithelium which lines 

 the base of the triangle, firmly attached to the bony wall, is also 

 simple in character : but that a part of the epithelium lining the 

 wall which separates the canalis cochlearis from the scala tympani 

 is, along the whole length of the spiral, modified auditory epithelium 

 and is known as the organ of Corti (Fig. 177, Org. C.}. 



The auditory, cochlear nerve, leaving the meatus internus, 

 passes up the axis of the cochlea. As it ascends it gives off 

 fibres passing outwards in a spiral manner into the lamina spiralis 

 (cf. Fig. 175), which, thick towards the central axis, thins out 

 towards its attachment to the canalis cochlearis. As these fibres 

 traverse the lamina on their way outwards, numerous bipolar nerve- 

 cells appear on their course, thus forming along the whole length 

 of the spiral a continuous spiral ganglion, the ganglion spirale 

 (Fig. 177, Gg. sp.). Having passed this ganglion and having reached 

 the edge of the lamina spiralis, the fibres pass into and become 

 connected, in a manner presently to be described, with the 

 auditory epithelium of the organ of Corti. 



We may now turn to the minute structure of the membranous 

 labyrinth and its auditory epithelium, but before doing so it will 

 be well to recall the position in relation to the tympanic cavity 

 of the several parts of the internal ear which we have just described. 

 The whole internal ear lies to the median side of and forms in part 

 the median wall of the tympanic cavity. Nearly opposite the 

 middle of the tympanic ring and membrane, the median wall 

 of the tympanum is marked by an elevation (the promontorium), 



