THE MIDDLE EAR OR TYMPANUM 651 



being vertical, including the whole length of the handle, and extending from 

 the upper border to the center of the membrane. The head of the malleus 

 is irregularly rounded; its neck, or the line of boundary between it and the 

 handle, supports a short conical process which receives the insertion of the 

 tensor tympani muscle. The incus, shaped like a bicuspid molar tooth, is 

 articulated by its broader part to the malleus. Of its two fang-like processes, 

 one directed backward has a free end lodged in a depression in the mastoid 

 bone; the other, curved downward and more pointed, articulates by means 

 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 f enestra 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- 



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 semicircular 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 removed 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 17, the external semicircular 

 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. X 2 .5. (Sommering.) 



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- 



