756 



SPECIAL SENSES. 



on one side, and the mucous 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 

 three semicircular ca- 

 nals, and the saccule is 

 connected with the 

 true membranous coch- 

 lea by the canalis re- 

 uniens. At a point in 

 the utricle correspond- 

 ing to the entrance of 

 a branch of the audi- 

 tory nerve, is a round, 

 whitish spot, called the 

 acoustic spot (macula 

 acustica), containing 

 otoliths, or otoconia, 

 which are attached to 

 the inner surface of 

 the membrane. A sim- 

 ilar spot, containing 

 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 ampullate enlargements of the bony canals. 

 They are held in place by a large number of little, fibrous bands extending 

 to the bony labyrinth. 



The membranes of the cochlea include the periosteum lining the bony 



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

 canals). 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 : I, utricle ; 2, saccule ; 3, 4, 6. ampullse ; 5, 7, 8, 9, semi- 

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

 13, 14, 15, distribution of the branches of the nerve, to the vesti- 

 bule and the semicircular canals. 



