304 COMPARATIVE ANATOMY 



part is connected the bony cochlea, the axis of which lessens in size 

 from base to apex (Fig. 222), and round it a bony lamella (lamina 

 spiralis ossca) winds in a spiral manner; this extends into the 

 cavity of the coils of the cochlea without quite reaching as far as 

 the opposite wall, being continued outwards by two laterally- 

 diverging lamellae, mentioned above as the basilar membrane and 

 membrane of Reissner ; these lie at an angle to one another and 

 correspond to the inner walls of the membranous cochlea (ductus 

 cochlearis or scala media), which is approximately triangular in 

 transverse section. The outer wall abuts against a portion of the 

 peripheral part of the bony cochlea. It is apparent therefore that 

 the scala media does not by any means fill up the lumen of the 

 bony cochlea, but that a cavity is left on either side of it. corre- 



FIG. 222. BONY COCHLEA OF MAN. (After A. Ecker.) 



A, axis; H, hamulus ; Lso, Lso 1 , lamina spiralis ossea, the free edge of which, 



perforated by the fibres of the auditory nerve, is visible at f. 



FIG. 223. DIAGRAMMATIC TRANSVERSE SECTION OF THE COCHLEA OF A 



MAMMAL. 



B, membrana basilaris, on which the neuro-epithelium lies ; C, membrane of 



Corti ; KS, bony cochlea ; L, limbus laminae spiralis ; Lo, Lo l , the two 

 layers of the lamina spiralis ossea, between which at N the auditory nerve 

 (together with the ganglion, left of L) is seen ; Ls, ligamentum spirale ; 7?, 

 Reissner's membrane ; Sm, scala media (membranous cochlea) ; St, scala 

 tympani ; Sv, scala vestibula. 



spending to those already met with in the auditory organ of Birds 

 and known as the scala ^estibuli and scala tympani (Figs. 215 and 

 223). Both of these are continuous with the perilymphatic space, 

 and, following the direction of the scala media, open into one 

 another at the blind end of the latter. The scala vestibuli is 

 shut off from the tympanic cavity by the membrane of the fenestra 

 vestibuli, to which the stapes is applied externally ; the scala tym- 

 pani is closed by the membrane of. the fenestra cochleae. 



On the floor of the bony cochlea, not far from the fenestra 

 cochleae, is an opening leading into a narrow ductus perilymphaticus 

 or aqueductus cochleae (Fig. 215). 



The fibres of the auditory nerve running along the axis of the 

 bony cochlea extend in their course laterally outwards, between 



