THE COCHLEA AND THE ORGAN OF CORTI 



705 



Organ of Corti. The basilar membrane supports cells of several types. 

 About midway between the outer edge of the lamina spiralis and the outer 

 wall of the cochlea are situated the rods of Corti. Viewed sideways, they are 

 seen to consist of an external and internal pillar, each rising from an ex- 

 panded foot or base on the basilar membrane, figure 438. They slant in- 

 ward toward each other, and each ends in a swelling termed the head, the 

 head of the inner pillar overlying that of the outer, figure 438. Each pair of 

 pillars forms, as it were, a pointed roof arching over a space, and by a suc- 

 cession of them a little tunnel is formed. It has been estimated that there 



FIG. 437. Semidiagrammatic Section of a Cochlear Whorl. (After Heitzmann.) 



are about four thousand of these pairs of rods of Corti between the base of 

 the cochlea and its apex, in man 3,848 outer and 5,590 inner rods (Retzius). 

 They are found progressively to increase in length, and become more 

 oblique; in other words, the tunnel becomes wider, but diminishes in height 

 as we approach the apex of the cochlea. 



Leaning against the rods of Corti and apparently supported by them 

 are sensory cells or hair cells. The hair cells are in two series, the inner and 

 the outer hair cells. The former consist of a single layer, the latter of three 

 or four layers, figure 438. There are two additional types of supporting cells, 

 the cells of Deiters and of Hensen. The whole structure when viewed from 

 above bears a remarkable resemblance to the keyboard of a piano. 



The cochlear division of the auditory nerve enters the base of the modiolus 

 and sends a spiral whorl of fibers out under the spiral lamina. The gan- 

 glionic cells of the cochlear division of the auditory nerve are located in the 



45 



