572 HANDBOOK OF PHYSIOLOGY. 



aquceduclus cochlece, a canal leading to the jugular fossa of the petrous 

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

 the aquaeductus vestibuli. The spiral canal is divided into two passages, 

 or scalas, 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, forming its outer margin, 

 is attached to the outer wall of the canal. Commencing at the base of 

 the cochlea, between its vestibular and tympanic openings, they form a 

 partition between these apertures; the two scalae are, therefore, in cor- 

 respondence with this arrangement, named scala vestibuli and scala 

 tympani (Fig. 386). At the apex of the cochlea, the lamina spiralis 

 nds in a small hamulus, the inner and concave part of which, being de- 

 tached from .the summit of the modiolus, leaves a small aperture named 

 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 canalis membranaceus (CO. Fig. 



FIG. 386. FIG. 387. 



FIG. 386. View of the osseous cochlea divided through the middle. 1, central canal of the 

 modiolus; 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. 5/1. (Arnold.) 



FIG. 387. Section through one of the coils of the cochlea (diagrammatic). S T, scala tym- 

 pani; 8 V, scala vestibuli; C C. canalis cochleae or canalis membranaceus; R, membrane of Reiss- 

 ner; I s o, lamina spiralis ossea; I I s, limbus laminae spiralis; s s, sulcus spiralis; n c, cochlear 

 nerve; g s, ganglion spirale; t, membrana tectoria (below the membrana tectoria is the lamina 

 reticularis) ; 6, membrana basilaris; Co, rods of Corti; I sp, ligamentum spirale. (Quain.) 



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

 wall of the cochlea, its upper wall (separating it from the scala vesti- 

 buli) by the membrane of Eeissner, 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 cjosed with the exception of a 

 very narrow passage (canalis reunions) uniting it with the sacculus. The 

 scala media (like the rest of the membranous labyrinth) contains endo- 

 lymph. 



Organ of Corti. Upon the basilar membrane are arranged cells of 

 various shapes. 



About midway between the outer edge of the lamina spiralis and the 

 outer wall of the cochlea are situated the rods of Corti. Viewed side- 



