404 SPECIAL PHYSIOLOGY. 



audible, or become altogether inaudible, whereas high ones are heard 

 with greater distinctness. 



The vibrations produced in the membrana tympani, are propagated 

 chiefly, most readily, and, indeed, in a concentrated manner, through 

 the tympanic ossicles, to the fluid of the labyrinth. The undulations 

 of this membrane are communicated directly to the handle of the mal- 

 leus, whence they pass to the head of this bone, are then propagated 

 to the incus and stapes, and from the base of the latter bone, which, 

 as already mentioned, is fixed by membrane into the fenestra ovalis, to 

 the perilymph of the labyrinth. In performing this office, the chain of 

 ossicles transmits the sonorous vibrations, communicated to them as a 

 whole, and not vibrations resulting from motions in their particles. 

 The direction of the undulations is unaffected by the angular arrange- 

 ment of the ossicles; for the undulations are propagated, as they would 

 be through a series of levers, from the stapes to the fenestra ovalis, in 

 the same direction as that in which they are communicated to the 

 handle of the malleus, viz.. in a perpendicular direction. It has been 

 maintained by some, that the ossicles are merely employed as levers 

 to regulate the tension of the three membranes of the tympanic cavity, 

 i. e., of the membrana tympani, of the membrane which, with the base 

 of the stapes, closes the fenestra ovalis, and also through the medium 

 of the perilymph in the cochlea, of the membrane closing the fenestra 

 rotunda. The ossicles are, according to this view, not conductors of 

 sonorous vibrations, which, it is supposed, are propagated solely 

 through the air in the tympanic cavity, either to the membrane clos- 

 ing the fenestra rotunda, or else to the inner wall of the tympanum 

 generally, and, in this manner, to the parts within the labyrinth, 

 founds conducted through two such different paths as the ossicles and 

 .air of the tympanum, must interfere with, and confuse, each other, and 

 hence it is probable, that they are conducted solely, either through the 

 one or the other path. Considering the special connection of the os- 

 sicles with the expanded membrana tympani on the one hand, and with 

 the chief aperture of the labyrinth on the other, their almost complete 

 isolation in the atmosphere of the tympanum, and their greater con- 

 ducting power, as compared with that of air, we must conclude that 

 the little chain of ossicles is the actual path for the conveyance of 

 .sounds. In the tympanic cavity, on the contrary, we find contrivances 

 apparently intended to impair the conducting power of the air within 

 it ; for this cavity communicates freely with the Eustachian tube and 

 the mastoid cells ; the inner surfaces of all these parts are moist, 

 .so that sonorous undulations, in whatever manner they may be ex- 

 cited within them, must be damped and deadened; moreover, the tym- 

 panic ossicles being invested by moist mucous tissue, are very bad 

 conductors of vibrations to, or from, the air within the tympanum; so 

 that, in this manner, they are secured against loss in one way, and 

 interference in another. Again, 'the mucous lining of the tympanum is 

 also especially adverse to the propagation of vibrations from the walls 

 -of the cavity, to the tympanic atmosphere, as well as to the reception 

 of any such as, striking on its inner surface, might, if received upon 

 .a dry membrane, interfere with those which impinge upon its external 



