'28 APPLICATION OF THE ARGUMENT. 



which aperture opens into the tortuous canals that lead to 

 the brain. This last bone of the four is called the stapes. 

 The office of the drum of the ear is to spread out an ex- 

 tended surface, capable of receiving the impressions of 

 sound, and of being put by them into a state of vibration. 

 The office of the stapes is to repeat these vibrations. It i« 

 a repeating frigate, stationed more within the line : From 

 which account of its action may be understood, how the 

 sensation of sound will be excited, by any thing which 

 communicates a vibratory motion to the stapes, though not, 

 as in all ordinary cases, through the intervention of the 

 membrana lyuipani. This is done by solid bodies applied 

 to the bones of the skull, as by a metal bar held at one 

 end between the teeth, and touching at the other end a 

 tremulous body. It likewise appears to be done, m a con- 

 siderable degree, by the air itself, even when this mem- 

 brane, 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 di- 

 rection towards the brain, and to propagate it with the ad- 

 vantage of a lever ; w hich advantage consists in increasing 

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

 time diminishing the space through w hich it oscillates ; 

 both of Which changes may augment or facilitate the still 

 deeper action of the auditory nerves. 



The beneftt of the eustachian tube to the organ, may be 

 made out upon known pneumatic principles. Behind the 

 drum of the ear is a second cavity or barrel, called the 

 iympanum. The eustachian tube is a slender pipe, but 

 sufficient for the passage of air, leading from this cavity 

 into the back part 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 membrane, 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 distend- 

 ed or relaxed the covering membrane, in a degree inconsis- 

 tent with the purpose which it was assigned to execute. 

 The only remaining expedient, and that for which the 

 eustachian tube serves, is to open to the cavity a coramu- 



