THE ORGAN OF HEARING. 



columnar, and progressively increase in length until they measure 

 3035 t*. in contrast with their usual height of 3-4 /*. 



The character and arrangement of the cells of the acoustic areas 

 in the saccule and the utricle are the same, including two kinds of 

 elements, the sustentacular or fibre cells and the hair-cells. 



The sustentacular cells are elongated irregularly cylindrical, 

 and extend the entire thickness of the epithelial layer to rest upon 

 the well-developed basement-membrane by their expanded or divided 

 basal processes. The oval nuclei are frequently wider than the 

 average diameter of the cells, and produce corresponding enlarge- 

 ments in the contour of the elements ; the nuclei occupy various 

 levels within the inner half of the cells, but are never situated beyond ; 

 the cell-contents appear faintly granular, and contain yellowish 

 pigment-particles. 



The hair- cells are broader but shorter than the sustentacular ele- 

 ments, and reach from the surface only to about the middle of the 

 epithelium, where they terminate in rounded margins ; these cells 

 possess large spherical nuclei, which usually lie well towards the 

 slightly-expanded inner ends. The protoplasm of the hair-cells is 

 granular, and contains yellow pigment ; the outer part, next the 

 free surface, exhibits a differentiation into a cuticular zone, cov- 

 ering the outer ends of the cells. From the free border of each cell a 

 seemingly single stiff robust hair (20-25 V- l n g) projects into the 

 endolymph ; this conical process, how- 

 ever, is resolvable into a number 

 of agglutinated finer hairs or rods. 



The free surface of the neuro- 

 epithelium within the saccule and 

 the utricle is covered by a remark- 

 able structure, the so-called otolith 

 membrane. This consists of num- 

 berless small crystalline bodies, the 

 otoliths, or ear-stones, embedded 

 within a soft gelatinous ground-sub- 

 stance. The otoliths are minute 

 crystals of calcium carbonate, 1-15 /* 

 length, usually six-sided prisms 



in 



Section of wall of utricle through mac- 

 ular region, from rabbit, showing otoliths 

 (a) embedded within granular substance 

 (g) : h, hair-cells with processes (/) ex- 

 tending between sustentacular elements 

 (s) ; n, nerve-fibres within fibrous tissue 

 (/) passing towards hair-cells and be- 

 coming non-medullated at basement- 

 membrane (m). 



with slightly-rounded angles. The 

 nerve-fibres proceed to the acoustic 

 areas and unite beneath the epithelial 

 layer in a plexus, from which fine 



bundles of fibres pass towards the 'surface ; the nerve-fibres usually 

 lose their medullary substance in their transit through the base- 

 ment-membrane and enter the epithelium as naked axis-cylinders. 



25 



