THE COCHLEA AND THE ORGAN OF CORTI 619 



figure 435. In the cavities of the sacculus and utriculus are small masses of 

 calcareous particles called otoliths. 



The Semicircular Canals. There are three semicircular canals for each 

 ear, one horizontal and two vertical ones placed almost at right angles to 

 each other. The three canals, therefore, occupy the three planes of space. 

 Each has a considerable enlargement or swelling, called an ampulla. The 

 epithelium of the ampulla is modified at the point of entrance of the nerve 

 into a thickened hillock called the crista acustica. This epithelium is com- 

 posed of rod cells or supporting cells which extend the full thickness of the 

 crista, and of hair cells, which occupy the inner or free half of the crista. 

 The hair cells are the sensory cells. They have hair-like processes which 

 project from the free ends of the cells out 

 into the endolymph of the cavity. Nerve 

 fibrils run up into the crista and apparently 

 form terminal arborizations about the hair 

 cells, or, according to some observers, end 

 in the cells. 



The Cochlea and the Organ of Corti. 

 The membranous cochlea is located in 

 the spiral canal in the petrous bone, called 

 the cochlear canal. It is attached to the pa P; 4 - sc fit vest j^ u : s, porous 



substance of the modiolus near one of 



wall of the cavity between the fenestra modio e ii tioi xs f (Arnold")* 1 ' 8 spiralis 

 ovalis and the fenestra rotunda, and to the 



outer wall of the canal and the free border of the lamina spiralis almost, but 

 not quite, to its summit. A small cavity is thus left around the upper end 

 of the cochlea connecting the scala vestibuli above with the scala tympani 

 below. A cross-section through the cochlear canal shows the relations of 

 the cochlear canal which was named scala media by the earlier anatomists. 

 The free portion of the membranous wall above is called the membrane of 

 Reisner, while that below is called the basilar membrane. The basilar mem- 

 brane supports the special sensory apparatus for the reception of stimuli 

 of sound waves. 



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 expanded 

 foot or base on the basilar membrane, figure 438. They slant inward 

 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 succession 

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

 three thousand of these pairs of rods of Corti between the base of the cochlea 

 and its apex. They are found progressively to increase in length, and be- 



