372 



NORMAL HISTOLOGY. 



side of each of the semicircular canals, which occupy only about one third 

 of the lumina of the bony tubes, is a band-like area of thickened epithelium 

 in which the cells become low cylindrical in type. Over the regions receiv- 

 ing the nerve-fibres, the macula acusticce and the crista acusticce, the epi- 

 thelium undergoes a marked alteration, changing from the indifferent covering 

 cells into the highly specialized neuroepithelium. 



The maculae acusticae are about 3 mm. long by 2 mm. broad, the 

 macula of the saccule being a little narrower (1.5-1.6 mm.) than that of the 

 utricle (2 mm. ). At the margin of these areas the cells are at first cuboidal, 



FIG. 420. Membranous labyrinth of five-months foetus, postero-mesial aspect. I, 2, 3, posterioi, 

 horizontal, and superior semicircular canals ; 4, 5, 6, their ampullae ; 7, common crus of superior and 

 posterior canals ; 8, 9, recess and macula of utricle (10); n, saccule and its macula ( 12); 13, ductus endo- 

 lymphaticus ; 14, utriculo-saccular canal ; 15, canalis reunions, opening at 16 into cochlear duct ; 17, blind 

 sac of cochlear duct (18); 19, basilar membrane; 20, ligamentum spirale; a, facial nerve; f>, yestibular 

 nerve ; c, branch of vestibular nerve to posterior canal ; d, branches of cochlear nerve to Corti's organ. 

 X6. (Retzius.) 



next low columnar, and then abruptly increase in length, until they measure 

 from 30-35 /*, in contrast with their usual height of from 3-4 /*. The 

 special area includes two kinds of elements, the sustentacular or fibre-cells 

 and the hair-cells. The sustentacular cells are long, rather narrow, irregularly 

 cylindrical elements and extend the entire thickness of the epithelial layer, 

 resting upon a well-developed basement-membrane by their expanded or 

 divided basal processes. They present a swelling enclosing an oval nucleus 

 and terminate at the surface in a cuticular zone. The cylindrical hair-cells 

 are broader but shorter than the sustentacular cells, and reach from the free 

 surface only as far as the, middle of the epithelial layer, where each cell 

 terminates usually in a rounded or somewhat swollen end containing a spher- 

 ical nucleus. The central end, next to the free surface, exhibits a differen- 

 tiation into a cuticular zone, similar to that covering the inner ends of the 

 sustentacular elements. From the free border of each hair-cell, a stiff robust 

 hair (20-25 JL long) projects into the endolymph. This conical process, 

 however, is resolvable into a number of agglutinated finer hairs or rods. 



The free surface of the neuroepithelium within the saccule and the utri- 

 cle is covered by a remarkable structure, the so-called otolith membrane. 

 This consists of a gelatinous membrane in which are embedded numberless 

 small crystalline bodies, the otoliths or ear-stones. Between it and the cutic- 

 ular zone is a space filled with endolymph, through which the hair-cells pass 



