574 



PHYSIOLOGY 



scala tympani. The increased pressure thus communicated to the 

 scala tympani causes a bulging of the membrane closing the foramen 

 rotundum. Movement inwards of the stapes causes therefore a move- 

 ment outwards of this membrane and vice versa, and the wave of pres- 

 sure in passing from one aperture to the other must be communicated 

 to the scala media with all the sensitive structures which it contains. 



The scala media is triangular in cross-section, having its apex 

 at the spiral lamina and its base at the outer wall of the cochlea. It 



s.v 





n 



SJ3.1 



FIG. 249. Vertical section of the first turn of the human cochlea. (G. RETZIUS.) 



s.v, scala vestibuli ; s.t, scala tympani ; d.c, canal or duct of the cochlea ; sp.l, 

 spiral lamina ; n, nerve fibres ; l.sp, spiral ligament ; str.v, stria vascularis ; s.sp, 

 spiral sulcus ; R, section of Reissner's membrane ; I, limbus laminae spiralis ; m.t, 

 membrana tectoria ; tC, tunnel of Corti ; b.m, basilar membrane ; h.i, h.e, internal 

 and external hair-cells. 



is separated by the membrane of Reissner from the scala vestibuli and 

 by the basilar membrane from the scala tympani. The basilar mem- 

 brane is composed of a number of elastic fibres, which pass in a radial 

 direction from the spiral lamina to the spiral crest on the outer wall 

 of the cochlea. The length of these fibres increases from 0-041 mm. at 

 the base of the cochlea to 0-495 mm. at the helicotrema. 



The end-organ of the auditory nerve is represented by the organ 

 of Corti, which rests on the basilar membrane (Fig. 250). It consists 

 of a double row of stiff cells, the inner and outer rods of Corti, which run 

 throughout the whole length of the scala media and are surrounded by 

 sense epithelium, the hair-cells. On the inner side of the rods of Corti 



