the Mode in which it administers to the Perception of Sound. 127 



requires that such should be the case ; but the mode in which 

 this effect occurs demands very careful consideration. 



If there were no round aperture, it is clear, either that such 

 effect could not occur at all, or could occur only to an extent 

 almost, if not absolutely imperceptible, and certainly very much 

 less than the structure of the stapes with the membrane at- 

 tached to it is calculated to admit of. For in such case the 

 labyrinth would be a closed vessel filled with liquid, and in all 

 parts rigid except at the oval aperture. Consequently the va- 

 cuum which the motion outward of the stapes would tend to 

 produce must be filled up by the liquid contents of the laby- 

 rinth, a result which could only occur (1) through an expan- 

 sion of the liquid in the labyrinth, or (2) through a contraction 

 in the space occupied by that liquid by reason of the expansion of 

 the walls and solid contents of the labyrinth. It may well be 

 doubted whether the expansion of the liquid in the labyrinth, or 

 the contraction in the space occupied by that liquid through the 

 agency just referred to, would be traceable by the aid of the 

 finest instruments, whereas the extent to which the stapes may 

 vibrate is perceptible, I apprehend, to the naked eye. We may 

 conclude, therefore, that the existence of the fenestra rotunda is 

 essential to the production in the stapes of that degree of motion 

 of which it is susceptible. 



The mode in which the fenestra rotunda operates for that 

 purpose may be gathered from the following passage from Sir 

 W. Wilde. 



" That the membrane [of the fenestra rotunda] vibrates is 

 proved by experiment ; and one use of it may be to allow the 

 fluid contained within the vestibule, when pressed upon by the 

 base of the stapes (covering like a lid the fenestra ovalis), to 

 bulge a little into the cavity of the tympanum." (Practical Ob- 

 vations &c, p. 312*.) 



Assuming that such is the case when the organ is in its normal 

 state, the membrana tympani being drawn inwards t, the stapes 



* I take the following still more decisive testimony from one of an in- 

 teresting series of papers in the c Lancet ' by Dr. Allen. " The tensor 

 tympani influences principally and chiefly the drumhead by pulling in- 

 wards the handle of the malleus and the membrane in which it is im- 

 bedded; and in the second, but not less important, place, it stretches the 

 membrane of the round cochlear opening by pressing the base of the stapes 

 into the oval vestibular opening, and driving the liquor Cotunnii (or laby- 

 rinth fluid) through the scala3 against the inner surface of the membrane 

 [of the round aperture] and causing it to bulge outwards." (See ' Lancet ' 

 for May 1, 1869.) 



t According to Politzer (cited by Mr. Hinton), the act of swallowing 

 will produce this effect by diminishing the pressure of the air in the tym- 

 panal cavity. (Diseases of the Ear, p. 443.) 



