VESTIBULAR MECHANISMS 



551 



The saccule is a small, pear-shaped sac situated in 

 the forepart of the vestibule. It lies below and medial 

 to the utricle. The long axis of this sac is very nearly 

 vertical; its dome-shaped portion is directed upward 

 and its bluntly tapered portion downward and slightly 

 posteriorly. From its posterior wall, a slender tube, 

 the endolymphatic duct, arises to extend through the 

 vestibular aqueduct into the cranial cavity; here it 

 terminates outside the dura mater o\cr the petrous 

 portion of the temporal bone in a blind pouch, the 

 endolymphatic sac. The endolymphatic duct and sac 

 serve as a drainage mechanism for the endolymph. 

 The ductus reuniens, finally, is a v'ery slender duct 

 which connects the saccule with the cochlear duct 

 (scala media) near its basal end. 



The macula is covered by a mucous or gelatinous 

 substance which contains aragonite concretions (oto- 

 liths, otoconia) of calcium carbonate. The specific 

 gravity of the otoliths, which ranges from 2.93 to 2.95, 

 is thus greater than that of the surrounding endo- 

 hmph. It has been shown that the otoliths of the 

 utricular macula of mammals are of two, or perhaps 

 three, distinct grades of fineness, each kind being 

 situated in its own particular area of the receptor sur- 

 face, which thus has a mosaic arrangement (61). 



The macula of the utricle is situated on its anterior 

 and medial walls, the two portions being joined at an 

 angle of 140°. When the head is in the normal posi- 

 tion, the macula of the utricle is in an approximately 



Crista 



The sensory epithelium in the ampullae of the 

 semicircular canals is collected into transverse crest- 

 like elevations — the cristae ampuHares — protruding 

 toward the lumen and firmly attached to their bony 

 foundations but free to swing at the other end. These 

 are the receptor organs of the canals. The height of 

 the crista corresponds to about one third the diameter 

 of the ampulla. The epithelium is composed of two 

 main types of cells, the hair (sensory) cells and the 

 nonsensory supporting cells. Recent electronmicro- 

 scopic studies have revealed two types of hair cells 

 that differ distinctly from each other both in structure 

 and innervation (121). One type of cells is bottle- 

 shaped, the other is more cylindrical (fig. 3). The 

 former is mainly localized to the summit and the latter 

 to the periphery of the cristae. The majority of them 

 have a sterocilial structure, though one process from 

 each cell has a kinocilium-like structure. The sensory 

 hairs, or cilia, project into a gelatinous mass, the 

 cupula, and there are found in a large number of 

 canals (55, 56; fig. 4). The cupula may be regarded 

 as a damped structure with a natural period, in the 

 case of the pike of about 20 sec. (106), and acts as a 

 spring-loaded over-critically damped torsion pendu- 

 lum (50). Its chemical structure is not yet fully 

 elucidated, but histochemical investigations conducted 

 in recent years suggest that sulphomucopolysaccha- 

 rides are important chemical constituents. 



Macula 



The receptor organs of the utricle and saccule are 

 called maculae. The sensory epithelium exhibits 

 again two kinds of cells, supporting and hair cells. 







FIG. 3. The ultrastructural architecture of the cells and 

 nerve endings of the crista ampullaris (guinea pig). HC /, 

 bottle-shaped hair cell; HC II, cylindrical hair cell; SC, sup- 

 porting cell; St, sterocilia; AC, kinocilia; .V, nucleus; GA, Golgi 

 apparatus; IM, intracellular membrane system; VB, vesicular 

 body; NC, nerve calyx; RM, reticular membrane; M, mito- 

 chondrion; NE, nerve endings; BM, basement membrane; 

 jV/.V, myelinated nerve; LG, lipid granule; MV, microvilli. 

 [From Wersall (121).] 



