HEARING 



415 



hairs. On the outer side of the outer rod are three or four 

 hair-cells, each with a cloudy outer segment containing the 

 nucleus, a granular middle segment, and a stiffish stalk, which 

 attaches it to the basilar membrane. Between the hair-cells 

 are supporting cells, thicker below, tapering above, containing 

 in their substance a firm fibre. Still farther to the outer side 

 are epithelial cells, of no special interest. The purpose of the 

 rods of Corti and the supporting cells is to give attachment 

 and support to a reticulated membrane of exquisite delicacy, 

 through the oblong apertures of which the hairs of the hair- 

 cells project into the endolymph. The spiral lamina is 



FIG. 40. ORGAN OF CORTI. 



The spiral lamina, on the left of the drawing, gives attachment to the membrane of Corti, 

 which stretches to the opposite wall. Below the membrane is a bloodvessel which runs 

 its whole length beneath the tunnel of Corti. The tunnel is formed by pillars the inner 

 on the left, the outer on the right which meet above it. On the left of the inner pillar 

 is a hair-cell ; to the left of this a nerve-cell with two nuclei. To the right of the outer 

 pillar is a space ; to the right of this four hair-cells alternating with four supporting cells, 

 which hold up the reticulated membrane through apertures in which the tufts of hairs 

 project. Three nerve-fibres are seen in the spiral lamina ; they cross the tunnel to ramify 

 between the rows of outer hair-cells. The lamina tectoria rests upon the tufts of hairs. 



traversed by a vast number of fibres of the auditory nerve, 

 which, losing their medullary sheaths, pass across the tunnel 

 of Corti as naked axons, to end amongst the hair-cells. Above 

 the organ of Corti, attached by its edge to the spiral lamina, is 

 a thick, gelatinous, fibrillated structure membrana tectoria 

 which rests as a coverlet on the surface of the organ. It has 

 been supposed that it serves to damp the vibrations of the 

 hairs after they have been set in motion by the waves passing 

 across the scala media ; but it not impossibly plays a more 

 active part in hearing than this. 



The organ of Corti is, beyond doubt, the apparatus which 

 analyses sounds ; but the problem of the way in which it 



