-540 THE HUMAN BODY. 



wound two and a half times (from left to right in the right 

 ear and vice versa) around a central bony axis, the modiolus. 

 From the axis a shelf, the lamina spiralis, projects and par- 

 tially subdivides the tube, extending farthest across in its 

 lower coils. Attached to the outer edge of this bony 

 plate is the membranous cochlea (scala media), a tube 

 triangular in cross-section and attached by its base to 

 the outer side of the bony cochlear spiral. The spiral 

 lamina and the membranous cochlea thus subdivide the 

 cavity of the bony tube (Fig. 147) into an upper portion, 

 the scala vestibuli, S V, and a lower, the scala tympani, ST. 

 JBetween these lie the lamina spiralis (Iso) and the mem- 



f +Q. 147. Section of one coil of the cochlea, magnified. /SF", scala vestibuli' 

 R, membrane of Reissner; CO. membranous cochlea (scala media); Us, limbua 

 iamince spiralis, t, tectorial membrane; ST, scala tympani; Iso, spiral lamina; 

 Co. rods of Corti; 6, basilar membrane. 



branous cochlea (CO), the latter being bounded above by 

 the membrane of Reissner (R) and below by the basilar 

 membrane (#). The free edge of the lamina spiralis is 

 thickened and covered with connective tissue which is hol- 

 lowed out so as to form a spiral groove (the sulcus spiralis, 

 ss) along the whole length of the membranous cochlea. 

 The latter does not extend to the tip of the bony cochlea; 

 above its apex the scala vestibuli and scala tympani com- 

 municate; both are filled with perilymph, and the former 

 communicates below with the perilymph cavity of the ves- 

 tibule, while the scala tympani abuts below on the round 

 foramen, which, as has already been pointed out, is closed 

 by a membrane. The membranous cochlea contains cer- 



