35 NATURAL THEOLOaY. 



more within the line. From which account of its action 

 may be understood how the sensation of sound will be excit- 

 ed by any thing which communicates a vibratory motion to 

 the stapes, though not, as in all ordinary cases, through the 

 intervention of the membrana tympani. This is done by 

 solid bodies applied to the bones of the skull, as by a metal 

 bar holden at one end between the teeth, and touching at 

 the other end a tremulous body. It likewise appears to be 

 done, in a considerable degree, by the air itself, even when 

 this membrane, the drum of the ear, is greatly damaged. 

 Either in the natural or preternatural state of the organ, the 

 use of the chain of bones is to propagate the impulse in a 

 direction towards the brain, and to propagate it with the 

 advantage of a lever ; which advantage consists in increas- 

 ing the force and strength of the vibration, and at the same 

 time diminishing the space through which it oscillates ; both 

 of which changes may augment or facilitate the still deeper 

 action of the auditory nerves. 



The benefit of the eustachian tube to the organ may be 

 made out upon pneumatic principles. Behind the drum of 

 the ear is a second cavity, or barrel, called the tympanum. 

 The eustachian tube is a slender pipe, but sufficient for the 

 passage of air, leading from this cavity into the back pa:t of 

 the mouth. Now, it would not have done to have had a 

 vacuum in this cavity ; for in that case the pressure of the 

 atmosphere from without would have burst the membrane 

 which covered it. Nor would it have done to have filled 

 the cavity with lymph, or any other secretion, which would 

 necessarily have obstructed both the vibration of the mem- 

 brane and the play of the small bones. Nor, lastly, would 

 it have done to have occupied the space with confined air, 

 because the expansion of that air by heat, or its contraction 

 by cold, would have distended or relaxed the covering mem- 

 brane in a degree inconsistent with the purpose which it 

 was designed to execute. The only remaining expedient, 

 and that for which the eustachian tube serves, is to open 



