THE SENSES. 555 



In its natural position the membrane is -drawn inward, by its attach- 

 ment to the malleus, in such a way as to present a funnel-shaped depres- 

 sion, the deepest point of which corresponds to the end of the handle 

 of the malleus. According to Helmholtz,* the sides of this depression 

 are convex, somewhat like the inner surface of the blossom of a morning- 

 glory. It is only along a line corresponding to the handle of the 

 malleus, that the meridian of the funnel is a nearly straight line ; else- 

 where the radial fibres of the membrane are curved, with their convexi- 

 ties toward the external auditory meatus. 



As the only attachment of the membrana tympani. except at its 

 border, is to the handle of the malleus, any movement of this bone 

 inward will draw the membrane in the same direction, deepen its cen- 

 tral depression, and put its fibres upon the stretch. On the other hand, 

 if the membrane be forced outward, it will draw the handle of the 

 malleus with it ; and, finally, if the elastic and muscular attachments 

 generally be in equilibrium, any movement of the membrane will be 

 followed by a corresponding change of position in the malleus. 



This is the physiological action of the membrana tympani. From its 

 thinness and tension, and from its position at the bottom of the external 

 auditory meatus, it enters into vibration, under the impulse of sounds 

 from the exterior, and communicates its movement to the handle of 

 the malleus at its inner surface. 



The chain of bones consists of three ossicles, articulated with each 

 other by their extremities, and forming a zigzag line of jointed levers 

 across the cavity of the tympanum. They are 

 known respectively, from their configuration, 

 as the " malleus," "incus/- and "stapes," or 

 the hammer, the anvil, and the stirrup. The 

 malleus is about nine millimetres in length, of 

 which a little more than one-third is occupied 

 by the rounded head and the neck, and a lit- 



OSSICLES of the human ear. 



tie less than two-thirds by the comparatively i. Malleus. 2. incus. 3. 

 straight and tapering handle. Its very slender stapes. (Riidinger.) 

 lateral process projects in a nearly horizontal direction from behind 

 forward in the natural position of the bone. The handle is the only 

 part of the malleus adherent to the membrana tympani, the neck cor- 

 responding to the upper border of this membrane, while the head pro- 

 jects above it, lying comparatively free in the tympanic cavity. It is 

 maintained in position by thin ligamentous bands from the bony wall 

 of the cavity inserted into its head and neck, and by the tendon of the 

 internal muscle of the malleus or " tensor tympani." The action of this 

 muscle is to draw the handle of the malleus inward, tightening the 

 membrana tympani, and rotating the head of the bone slightly out- 



* Mechanism of the Ossicles of the Ear. Buck's Translation. New York, 1873, 

 p. 20. 



