720 



HEARING. 



[CH. LIII. 



Fig. 54=}. View of the osseous cochlea 

 divided through the middle, i, cen- 

 tral canal of the modiolus; 2, lamina 

 spiralis ossea ; 3, scala tympani ; 4, 

 scala vestibuli ; 5, porous substance 

 of the modiolus near one of the sec- 

 tions of the canalis spiralis modioli. 

 f. (Arnold.) 



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

 small aperture named the helicotrema, by which the two scalse, 

 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 canal of the cochlea 

 (CC, fig. 545). In section it is 

 triangular, its extemal wall being 

 formed by the wall of the co- 

 chlea, 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 edge of the bony 

 lamina spiralis. Following the 

 turns of the cochlea to its apex, 



the scala media there terminates blindly ; while towards the base 

 of the cochlea it is also closed with the exception of a very 

 narrow passage (canalis reuniens) uniting it with the saccule. 



The scala media (like 

 the rest of the mem- 

 branous labyrinth) 

 contains endolymph. 

 Organ of Corti. 

 Upon the basilar mem- 

 brane are arranged 

 cells of various shapes. 

 About midway be- 

 tween the outer edge 

 of the lamina spiralis 

 and the outer wall of 

 the cochlea are situ- 

 ated the rods of Corti. 

 Viewed sideways, they 

 are seen to consist of 

 an external and in- 

 ternal pillar, each ris- 

 ing from an expanded 

 foot or base attached 



to the basilar membrane (o, n, fig. 546). They slant inwards 

 towards each other, and each ends in a swelling termed the head ; 

 the head of the inner pillar overlies that of the outer (fig. 546). 



Fig. 545. Section through one of the coils of the cochlea 

 (diagrammatic). ST, scala tympani ; SV, scala ves- 

 tibuli ; CC, canalis cochleae or canalis membranaceus ; 

 /? , membrane of Reissner ; Iso, lamina spiralis ossea ; 

 Us, limbus laminae spiralis ; ss, sulcus spiralis ; nc, 

 cochlear nerve ; gs, ganglion spirale ; t, membrana 

 tectoria (below the membrana tectoria is the lamina 

 reticularis) ; ft, membrana basilaris ; Co, rods of Corti ; 

 Isp, ligamentum spirale. (Quain.) 



