336 NATURAL THEOLOGY. 



and puts the apparatus within the tympanum into 

 play. 



Drawing a fair inference from the demonstra- 

 tion, it would appear that the impulses upon the 

 membrane of the tympanum are communicated to 

 the membrane of the fenestra ovalis, and that the 

 opening called the fenestra rotunda, closed by a 

 similar membrane, is for the purpose not of receiv- 

 ing impulse from without, but of yielding to that 

 impulse from within. For example, if we suppose 

 a bottle of water full to the lip, and a bladder 

 drawn over it so that not a bubble of air is con- 

 tained, although that water must be admitted to be 

 compressible, an impulse upon the bladder would 

 produce no such effect as would follow^ were there 

 a hole covered with a bladder upon the side or 

 bottom of the bottle ; for then each impulse upon 

 the top would be attended with a yielding of the 

 bladder below, and a consequent agitation in all 

 the intermediate fluid. Thus, it appears to us that 

 the use of the fenestra rotunda and its membrane 

 is to give play to the membrane of the fenestra 

 ovalis, and that without this provision, although 

 there might be a general impulse communicated to 

 the fluid in the labyrinth, like tliat communicated 

 through the bones generally, there could be no 

 wave or undulation. If the shutting of the Eus- 

 tachian tube, D, page 45, vol. i., so confines the air 

 in the large cavity of the tympanum as to render 

 us deaf, what would be the consequence of the la- 

 byrinth (which contains water) being shut in un 



