650 THE SENSES. 



tj'mpanic cavity on the other ; and all three layers together form a mem- 

 brane which is about 0.10 millimetre thick. 



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

 ment to the handle of the malleus, in such a way that its external sur- 

 face exhibits a funnel-shaped depression, the deepest point or bottom 

 of which corresponds to the situation of the end of the handle of the 

 malleus. According to the observations of Helmholtz, 1 the sides of this 

 funnel-shaped depression are not plane but convex, somewhat like the 

 inner surface of the blossom of a morning-glory. It is only along a 

 single line, corresponding to the attachment of the handle of the malleus, 

 that the meridian of the funnel would be a nearly straight line ; else- 

 where the radial fibres of the membrana tympani are curved, with their 

 convexities looking toward the external auditory meatus. 



As the only attachment of the membrana tympani, except at its cir- 

 cular border where it adheres to the bony walls of the meatus, is to 

 the movable handle of the malleus, any movement of the handle of the 

 malleus inward will draw the membrana tympani in the same direction, 

 deepen the funnel-shaped depression at its centre, and put its fibres 

 more upon the stretch. On the other hand, a movement of the mem- 

 brana tympani outward will draw the handle of the malleus outward ; 

 and, finally, if the malleus be held in a position of equilibrium, by its 

 elastic and muscular attachments internally to the membrana tympani, 

 any movement of this membrane, either outward or inward, will be 

 followed \)y a corresponding change of position in the malleus itself. 



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 sound 

 coming from the exterior, and communicates similar movements to the 

 handle of the malleus attached to its internal surface. 



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

 other by their corresponding extremities, and forming a zigzag line 

 of jointed levers, extending from without in- 

 _*g' 209> ward, across the cavity of the tympanum. 



They are known respectively, from the resem- 

 blances of their configuration, as the " malleus," 

 "incus," and "stapes," or the hammer, the 

 anvil, and the stirrup. The malleus is about 

 OSSICLES of the human nine millimetres in length, of which a little 



stags' M (Rfldin er I"*' 3 ' more tna " one - third is occupied by the rounded 

 head and the neck, and a little less than two- 

 thirds by the comparatively straight and tapering handle. The very 

 slender long process projects laterally in a nearly horizontal direction 

 from behind forward in the natural position of the bone. The handle 

 is the only part of the malleus which is adherent to the membrana 



1 Mechanism of the Ossicles of the Ear. Translated by Albert H. Buck, M.D., 

 and Normand Smith, M.D. New York, 1873, p. 20. 



