LABYRINTH 



569 



It can now be understood how vibrations of the membrana tympani 

 are comrrftmicated to the perilymph within the osseous labyrinth. The 

 chain of auditory ossicles through the base of the stapes affects the 

 perilymph in the vestibule. The vibrations of the perilymph passing 

 along the scala vestibuli into the scala tympani in turn affect the secondary 

 membrane of the tympanum which is stretched across the fenestra cochleae. 

 With every inward movement of the membrana tympani and of the base 

 of the stapes, there is an outward movement of the membrane of the 

 fenestra cochleae, and vice versa. The vibrations of the perilymph affect 

 the endolymph in the membranous labyrinth, and thus excite the termina- 

 tions of the acustic nerve. 



Ductus endolymphaticus 



Dura 



Vestibule 

 \ 



Osseous j 



cochlea _-:, 



Stapes 



Fenestra cochleae 



Aquseductus cochleae 



Modiolu 



FIG. 250. Diagram of the Osseous and Membranous Labyrinth. 

 (Modified from Testut.) 



U. Utricle. 



S. Saccule. 



Z>.C. Ductus cochlearis. 



Membranous Labyrinth. In the vestibule there are two mem- 

 branous sacs, termed the utricle and the saccule. The utricle occupies 

 the recessus ellipticus on -the wall of the vestibule, and lies above and 

 posterior to the saccule. Into it open the membranous semicircular 

 canals. 



The saccule is smaller, and occupies the recessus sphrericus on the anterior 

 part of the medial wall of the vestibule. It communicates by means of a 

 short narrow tube, termed the canalis retmiens, with the ductus cochlearis 

 or membranous cochlear tube. 



The saccule and the utricle are only indirectly brought into communica- 

 tion with each other ; a slender tube termed the ductus endolymphaticus 

 occupies the aqureductus vestibuli, and divides into two branches which 

 pass respectively into the saccule and the utricle (Fig. 250). 



The diictus cochlearis, or scala media, lies between the two scalre of the 

 cochlear tube. It ends blindly at each extremity, but close to its basal 

 end it is brought into communication with the saccule by the canalis 

 reuniens. 



