AUDITORY ORGAN 



305 



the two plates of the lamina spiralis ossea (Figs. 223, 224). On 

 the free border of the latter they pass out, and break up into 

 terminal fibrill on the inner surface of the basilar membrane. 

 The fibrillaB extend to the sensory or auditory cells, and these are 

 stretched as in a frame between the firm supporting and isolating 

 cells or bacilli. From the surface of the bacilli a resistant net-like 

 membrane (mcmbrana reticulnris) extends laterally, and through 

 the meshes of the latter the hairs of the auditory-cells project. 

 The auditory cells are covered by a thick and firm membrane 



Fio. 224. THE ORGAN OF CORTI. (After La vdowsky.) 



B, B, basilar membrane ; Ba, Ba, bacilli, or supporting cells; 6', membrane of 

 Corti ; Lo, Lo l , the two plates of the lamina spiralis ossea ; Ls, ligamentum 

 spirale, passing into the basilar membrane ; Mr., meinbrana reticularis ; X, 

 auditory nerve with ganglion ; JV 1 , JV-, the nerve branching up into fibrilla- 

 and passing to the auditory cells (G, (V) ; A', membrane of Reissner ; X<, 

 scala media. 



-the membrana tectoria s. Corti which perhaps acts as a 

 damper, and which arises from the vestibular lip of the lamina 

 spiralis ossea. The whole extent of the basilar membrane consists 

 of clear, thread-like and very elastic fibres, of which about 16,000 

 to 20,000 can be made out in Man. 



A true pinna or auricula (Fig. 225), attached to the border of 

 the external auditory meatus and projecting freely from the 

 head, appears for the first time in Mammals. It is present 

 in Monotremes, and, more especially in Echidna, is in organic 

 connection with the upper end of the hyoid 1 by means of the 



1 This fact appears to indicate a probable genetic relation between the 

 auricula and the visceral arches. 



