THE INTERNAL EAR (J17 



of a roundish tubercle with the stapes. The stapes is a little bone shaped 

 exactly like a stirrup, of which the base or bar fits into the fenestra ovalis. 

 The stapedius muscle is attached to the neck of the stapes. 



The bones of the ear are covered with mucous membrane reflected over 

 them from the wall of the tympanum. They are movable both altogether 

 and one upon the other. The malleus moves and vibrates with every move- 

 ment and vibration of the membrana tympani, and its movements are com- 

 municated through the incus to the stapes, and through the stapes to the 

 membrane closing the fenestra ovalis. The malleus, also, is movable in its 

 articulation with the incus. The membrana tympani which moves the long 

 process of the malleus is altered in its degree of tension by the degree of con- 

 traction of the tensor tympani muscles. The stapes is movable on the process 

 of the incus, the contractions of the stapedius muscle draws it outward. The 

 axis round which the malleus and incus rotate is the line joining the pro- 

 cessus gracilis of the malleus and the posterior process of the incus. 



The Internal Ear. The internal ear, or labyrinth, constitutes the 

 proper organ of hearing. It contains special epithelial structures to which 

 arc distributed the auditory nerve. The organ is located in a cavity in the 

 petrous bone, called the osseus labyrinth. The auditory organ within is 



FIG. 433. FIG. 434- 



FIG. 433 . Right Bony Labyrinth, Viewed from the Outer Side. The specimen here represented 

 is prepared by separating piecemeal the looser substance of the petrous bone from the dense walls 

 which immediately enclose the labyrinth, i, The vestibule; 2, fenestra ovalis; 3, superior semi- 

 circular canal; 4, horizontal or external canal; 5, posterior canal; *, ampullae of the semicircular 

 canals; 6, first turn of the cochlea; 7, second turn; 8, apex; 9, fenestra rotunda. The smaller 

 figure in outline below shows the natural size. X 2.5. (Sommering.) 



FIG. 434. View of the Interior of the Left Labyrinth. The bony wall of the labyrinth is re- 

 moved superiorly and externally, i, Fovea hemielliptica; 2, fovea hemispherica; 3, common 

 opening of the superior and posterior semicircular canals; 4, opening of the aqueduct of the 

 vestibule; 5, the superior, 6, the posterior, and 7, the external semicircula- canals; 8, spiral tube 

 of the cochlea (scala tympani) ; 9, opening of the aqueduct of the cochlea; 10, placed on the lamina 

 spiralis in the scala vestibuli. Xa.s. (Sommering.) 



called the membranous labyrinth. The membranous labyrinth contains a 

 fluid called endolymph; while outside it, between it and the osseous labyrinth, 

 is a fluid called perilymph. This is not a pure lymph, as it contains mucin. 



