214 SPECIAL SENSES. 



portions of the vestibule and semicircular canals, the perios- 

 teum is united to the membranous labyrinth, more or less 

 closely, by fibrous bands, which have been called ligaments 

 of the labyrinth. 1 The fenestra rotunda, which lies between 

 the cavity of the tympanum and the cochlea, is closed by a 

 membrane formed by an extension of the periosteum lining 

 the cochlea, on the 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 posterior portion of the cavity, and 

 a smaller, rounded sac, the saccule, situated in its lower and 

 anterior portion. The utricle communicates with the semi- 

 circular canals ; and the saccule opens into the membranous 

 canal of the cochlea by the canalis reunions. At a point in 

 the utricle corresponding to the entrance of a branch of the 

 auditory 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 am- 

 pullae of the semicircular canals. These calcareous masses 

 are composed of crystals of carbonate of lime, hexagonal and 

 pointed at their extremities. According to Sappey, each 

 crystal corresponds to a nerve-fibre. 2 Nothing definite is 

 known of the function 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 little 

 ovoid dilatations, ampullae, corresponding to the ampullary 

 enlargements of the bony canals. 



The membrane of the cochlea, including the lining perios- 

 teum, occupies the spiral canal of the cochlea, which it fills 



1 RUDINGER, in STRICKER, Manual of Human and Comparative Histology, The 

 New Sydenham Society, London, 1873, vol. iii., p. 88. 



2 SAPPEY, Traite tfanatomie, Paris, 1871, tome iii., p. 842. 



