THE SENSES. 183 



The cochlea (6, 7, 8, Figs. 359 and 360), a small organ, shaped like 

 a common snail-shell, is seated in front of the vestibule, its base rest- 

 ing on the bottom of the internal meatus, where some apertures transmit 

 to it the cochlear filaments of the auditory nerve. In its axis, the cochlea 

 is traversed by a conical column, the modiolus, around which a spiral 

 canal winds with about two turns and a half from the base to the apex. 

 At the apex of the cochlea the canal is closed; at the base it presents 

 three openings, of which one, already mentioned, communicates with the 

 vestibule; another called fenestra rotunda, is separated by a membrane 

 from the cavity of the tympanum; the third is the orifice of the aquce- 

 duclus cochlea, a canal leading to the jugular fossa of the petrous bone, 

 and corresponding, at least in obscurity of purpose and origin, to the 

 aquseductus vestibuli. The spiral canal is divided into two passages, or 



FIG. 361. FIG. 362. 



Fie. 361. View of the osseous cochlea divided through the middle. 1, central canal of the modio- 

 lus; 2, lamina spiralis ossea; 3, scala tympani; 4. scala vestibuli ; 5, porous substance of the modiolus 

 near one of the sections of the canalis spiralis modioli. 5 . (Arnold.) 



FIG. 362. Section through one of the coils of the cochlea (diagrammatic). S T, scala tympani ; 8 V, 

 scala vestibuli; C C, canalis cochleae or canalis membranaceus; R, membrane of Reissner; I s o, 

 lamina spiralis ossea; 1 1 s, limbus laminae spiralis; s s, sulcus spiralis; n c, cochlear nerve; g s, gang- 

 lion spirale; , membrana tectoria (below the membrana tectoria is the lamina reticularis); 6, mem- 

 brana basilaris; Co, rods of Corti; Isp, ligamentum spirale. (From Quain's Anatomy.) 



scalae, by a partition of bone and membrane, the lamina spiralis. The 

 osseous part or zone of this lamina is connected with the modiolus; the 

 membranous part, with a muscular zone, according to Todd and Bowman, 

 forming its outer margin, is attached to the outer wall of the canal. 

 Commencing at the base of the cochlea, between its vestibular and tym- 

 panic openings, they form a partition between these apertures; the two scalae 

 are, therefore, in correspondence with this arrangement, named scala 

 vestibuli and scala tympani (Fig. 361). At the apex of the cochlea, the 

 lamina spiralis ends in a small hamulus, the inner and concave part of 

 which, being detached from the summit of the modiolus, leaves a small 

 aperture named helicotrema, by which the two scalae, separated in all 

 the rest of their length, communicate. 



Besides the ' 'scala vestibuli" and "scala tympani/' there is a third 

 space between them, called scala media or canalis membranaceus (CO, Fig 

 362). In section it is triangular, its external wall being formed by the 

 wall of the cochlea, its upper wall (separating it from the scala vestibuli) 

 by the membrane of Reissner, and its lower wall (separating it from the 

 scala tympani) by the basilar membrane, these two meeting at the outer 



