i>3i> COMPARATIVE ANATOMY 



membrane and membrane of Reissner ; tbese lie at an angle to one 

 another and correspond to the inner walls of the membranous 

 cochlea 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 (the region between Ls and 

 the peripheral end of R in Fig. 186). It is apparent therefore that 



FIG. 185. BONY 'COCHLEA OF MAX. (After A. Ecker.) 



A, axis ; Lso, Lso 1 , lamina spiralis ossea, the free edge of which, perforated by the 

 fibres of the auditory nerve, is visible at f ; H, hamulus. 



FIG. 186. DIAGRAMMATIC TRANSVERSE SECTION OF THE COCHLEA OF A 



MAMMAL. 



KS, bony cochlea ; Lo, Lo l , the two layers of the lamina spiralis ossea, between 

 which at JVthe auditory nerve (together with the ganglion, left of L) is seen ; L, 

 limbus laminae spiralis ; B, membrana basilaris, on which the neuro-epithe- 

 lium lies ; R, Reissner's membrane ; Sv, scala vestibula ; St, scali tympani ; 

 8m, scala media (membranous cochlea) ; C, membrane of Corti ; Ls, liga- 

 mentum spirale. 



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, corresponding 

 to those we have already met with in the auditory organ of Birds 

 and known as the scala vestibuli and scala tympani (Figs. 179 and 

 186). 



Both of these are continuous with the perilymphatic cavity, 

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

 another at the blind end of the latter (Fig. 179). The scala 

 vestibuli is shut off from the tympanic cavity by the membrane 

 of the fenestra ovalis, to which the stapes is applied externally ; 

 the scala tympani is closed by the membrane of the fenestra 

 rotunda. 



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

 rotunda, is an opening leading into a narrow canal, the ductus 

 perilymphatictySt which serves as a communication between the 

 perilymphatic cavity and the peripheral lymphatic trunks of the 

 head (Fig. 179). 1 



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

 cochlea extend in their course laterally outwards, between the two plates 



1 A ductus perilymphaticus can be plainly made out from Reptiles onwards. 



