206 



COMPARATIVE ANATOMY. 



HISTOLOGY OP THE MAMMALIAN COCHLEA. 



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

 extend in their course laterally outwards, and come to lie between the two 

 plates of the lamina spiralis ossea (Fig. 168, Lso, Lso\ Figs. 169, 170, N, 

 between Lo and Lo 1 ). On the free border of the latter, these pass out, and 

 break up into terminal fibrillse on the inner surface of the basilar membrane 

 (Fig. 170, N, N 1 , N*). 



Lo 



Lo 



FIG. 170. THE ORGAN OF COKTI. (After Lavdowsky.) 



Lo, Lo 1 , the two plates of the lamina spiralis ossea ; A r , auditory nerve with ganglion ; 

 N 1 , N'-', the nerve branching up into fibrillne and passing to the auditory cells 

 (G, G) ; Ba, Ba, bacilli, or supporting cells ; Mz, membrana reticularis ; 0, mem- 

 brane of Corti ; Ls, ligamentum spirale, passing into the basilar membrane ; 

 Sm, scala media ; It, membrane of Reissner ; B, B, basilar membrane. 



The fibrillae extend to the sensory or auditory cells (G, G), and these are 

 stretched as in a frame between the firm supporting and isolating cells or 

 bacilli (Ba, Ba). From the surface of the bacilli a resistant net-like mem- 

 brane (membrana reticularis) extends laterally, and through the meshes of the 

 latter the hairs of the auditory cells project (Fig. 170, Mz}. The number of 

 the outer hair-cells may be estimated at about 12,000. The auditory cells 

 are covered by a thick and firm membrane the membrana tectoria s. Corti 

 (Fig. 170, O) which probably 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 fibres, of which about 

 16,000 to 20,000 can be made out in Man. These are capable of vibrating 

 freely, and, as their length varies definitely in different regions of the cochlea, 

 they might be looked upon (were it not that they are absent from Birds) as 

 forming a sort of keyboard or harp, that is, as a definite apparatus of strings 

 capable of analysing the different waves of sound, the vibrations of which are 

 communicated to the auditory cells, and thence by means of the nerves to 

 the brain. 



