AUDITORY ORGAN 



233 



of the lamina spiralis ossea (Figs. 186, J87). On the free burder of the 

 latter they pass out, and break up into terminal fibrillse on the inner surface 

 of the basUar membrane. 



The fibrillse 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 (membrana 

 reticularis) extends laterally, and through the meshes of the latter the hairs 

 of the auditory cells project. The number of the outer hair-cells may be 

 estimated at about 12,000. The auditory cells are covered by a thick and 



.Jf- 



Fig. 187. — The Orc.vn of Corti. (After Lavdowsky.) 



Lo, Lo\ the two plates of the lamina spiralis ossea ; X, auditory nerve witli 

 ganglion ; ^V^ N-, the nerve branching up into fibrillie and passing to the 

 auditory cells (G, G) ; £a, Ba, bacilli, or supporting cells ; i!/., membrana 

 reticularis ; C, membrane of Corti ; Ls, ligamentum spirals, passing into the 

 basilar membrane; Sm, scala media ; tt, membrane of Reissner ; B, iJ, basilar 

 membrane. 



firm membrane — the membrana tectoria o. Corti — which perhaps acts as a 

 damper, and which arises from the labium vestibulare of the lamina spiralis 

 ossea. The whole extent of the basilar membrane consists of clear thread- 

 like and very elastic libres, of which about 16,000 to 20,000 can be made out 

 in Man. 



A true pinna or auricula (Fig. 188), attached to the border of 

 the external auditory meatus and projecting freely from the head, 

 occurs in Mammals only (comp. p. 229). It is supported by 

 cartilage, and the intrinsic and extrinsic muscles in connection 

 with it are supplied by the facial nerve. 



The piiuia arises from a series of rounded eminences on the tirst 

 and second visceral arches, around the hyoid (spiracular) cleft, the lower 

 part of which closes up, while the upper part gives rise to the external 



