124 Mr. R. Moon on the Structure of the Human Ear, and on 



The argument is not limited, however, to a bare demonstra- 

 tion that the ear is open to the action of one class of waves 

 while it suppresses the action of the other. It may be shown 

 that some of the most remarkable and characteristic portions of 

 the auditory apparatus are expressly contrived with a view to fa- 

 cilitate and regulate the admission and transmission of waves of 

 rarefaction, and have no intelligible function as applying to the 

 transmission of condensed waves. 



If the tympanal membrane were capable of being stretched 

 when a condensed wave becomes incident upon it, it is quite 

 certain that its elasticity, i. e. its tendency to recover its original 

 form, would be sufficient to bring it back to its original position 

 and status. 



But when, through the incidence upon it of a rarefaction, 

 the membrana tympani is pushed outwards, what is to bring it 

 back to its original position ? There is no property of the 

 membrane itself capable of producing this effect. A distinct 

 machinery is requisite for the purpose ; and this machinery we 

 have in the muscles acting upon the bones of the ear. 



To make this clear, it will be necessary to view more in detail 

 the structure of the organ. 



The tympanal membrane is connected with the fenestra ovalis 

 by a chain of small bones, variously estimated as three and four 

 in number, but which for our present purpose may be regarded 

 with sufficient accuracy as consisting of three, stretching across 

 the tympanal cavity, and respectively denominated: — (1) the 

 malleus, next to the membrana tympani ; (2) the incus ; (3) 

 the stapes, or stirrup bone, whose name describes its shape, 

 the base of which is attached to the membrane of the fenestra 

 ovalis. 



The three bones or ossicles are articulated upon one another 

 in the order in which they have been named. The body of the 

 malleus and the body of the incus, which are in juxtaposition, 

 are much more massive than the other portions of the ossicu- 

 lar system. The former puts out. a comparatively slender arm 

 called the handle of the malleus, which extends from the side of 

 the tympanal cavity to about the centre of the membrana tympani. 

 At the centre of the membrane, and nearly along the entire 

 length of the handle of the malleus, the latter is attached to the 

 membrane and moves with its motion. 



The incus sends out a slender process on the other side to 

 the apex of the stirrup, to which it is attached. 



The base of the stapes is described by Sir W. Wilde as fitting 

 into the fenestra ovalis "somewhat like a stopper or the piston 

 of a cylinder, and is attached to its circumference by a ligamento- 

 fibrous membrane." 



