756 



SPECIAL SENSES. 



on one side, and the raucous membrane lining the tympanic cavity, on the 

 other. 



In the bony vestibule, occupying about two-thirds of its cavity, are two 

 distinct sacs ; a large, ovoid sac, the utricle, situated in the upper and pos- 

 terior portion of the cavity, and a smaller, rounded sac, the saccule, situated 

 in its lower and anterior portion. These two sacs communicate with each 



other through a small 

 canal in the form of 

 the letter Y, which is 

 represented in the up- 

 per diagram in Fig. 

 269. The utricle com- 

 municates with the 

 semicircular canals, 

 and the saccule opens 

 into the membranous 

 canal of the cochlea, by 

 the canalis reunions. 

 At a point in the utri- 

 cle corresponding to the 

 entrance of a branch of 

 the auditory nerve, is a 

 round, whitish spot, 

 called the acoustic spot 

 (macula acustica), con- 

 taining otoliths, or oto- 

 conia, which are at- 

 tached to the inner sur- 

 face of the membrane. 

 A similar spot, contain- 

 ing otoliths, exists in 

 the saccule, at the point 

 of entrance of its nerve. 

 Otoliths are also found 

 in the ampullae of the 

 semicircular canals. 

 These calcareous masses are composed of crystals of calcium carbonate, which 

 are hexagonal and pointed at their extremities. Nothing definite is known 

 of the uses of these calcareous bodies, which exist in man, mammals, birds 

 and reptiles. 



The membranous semicircular canals occupy about one-third of the cavity 

 of the bony canals. They present small, ovoid dilatations, called ampullae, 

 corresponding to the ampullary enlargements of the bony canals. They are 

 held in place by a large number of little, fibrous bands extending to the bony 

 labyrinth. 



The membrane of the cochlea, including the lining periosteum, occupies 



FIG. 269. Diagram of the labyrinth (vestibule and semicircular 

 canakt). From a photograph, and somewhat reduced (Riidinger). 



Upper figure: 1, utricle ; 2, saccule : 3, 5, membranous cochlea ; 4, 

 canalis reuniens : 6, semicircular canals. 



Lower figure : 1, utricle : 2. saccule : 3, 4, 6. ampullae : 5, 7. 8. 9, semi- 

 circular canals : 10, auditory nerve (partly diagrammatic); 11, 12, 

 13, 14, 15, distribui.ion of the branches or the nerve, to the vesti- 

 bule and the semicircular canals. 



