HEARING 



64ft 



ally to the inner rods is a layer of columnar cells of the same height 

 as the rods, with about fifteen to twenty short, stiff hair-like processes 

 arising in crescentic manner from the surface. These are known as 

 the inner hair cells. 



Externally to the outer rods there are also hair cells the outer 

 hair cells. There are generally three or four rows of these, each cell 

 being supported outside by a supporting cell, known as Deiters' cell. 

 These cells are broad at their base of attachment to the basilar mem- 

 brane, and pass as narrow processes to be attached to a fenestrated 

 membrane, or membrana reticularis, which arises as a sort of lattice- 

 work from the upper portions of the rods, and serves to support the 

 free hairs of the hair cells. A similar membrane supports the inner 

 hair cells. Outside the hair cells are several rows of columnar sup- 

 porting cells devoid of hairs, which become continuous with a layer 

 of cells lining the whole of the cochlear canal. Rising from a con- 

 nective-tissue structure on the spiral lamina there arches over the 

 whole organ a homogeneous membrane the membrane tectoria. 



Uli. I 



FIG. 379. SECTION OF ORGAN OF CORTI OF A YOUNG GUINEA-PIG. 

 (Redrawn from Dahlgren and Kepner.) 



tectoria; S.S.C., cells lining sulcus spiralin, 



Situated in the spiral lamina is the spiral ganglion, from which 

 the fibres of the nerve of hearing arise. Processes from these nerve 

 cells pass along to and around the hair cells, the bases of which do 

 not touch the basilar membrane. The central connections of these 

 nerves go to the acoustic nuclei in the pons (see p. 609). 



The part of the membranous labyrinth corresponding to the 

 vestibule consists of two membranous sacs the saccule and the utricle 

 connected by a small canal. From the saccule arises the cochlear 

 canal. In connection with the utricle are three membranous semi- 

 circular canals, which lie within three corresponding bony semicircular 

 canals. These latter open into the posterior and superior aspect of 

 the vestibule. This apparatus probably has no connection with 

 hearing, and is dealt with later (see p. 654). 



