APPLICATION OP THE ARGUMENT. 29 
rection towards the brain, and to propagate it with the ad¬ 
vantage of a leyer; which advantage consists in increasing 
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 known 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 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 this cavity a commu¬ 
nication with the external air. In one word; it . exactly 
answers the purpose of the hole in a drum. 
The membrana tympani itself, likewise, deserves all the 
examination which can be made of it. It is not found in 
the ears of fish; which furnishes an additional proof of 
what indeed is indicated by everything about it, that it is 
appropriated to the action of air, or of an elastic medium. 
It bears an obvious resemblance to the pelt or head of a 
drum, from which it takes its name. It resembles also a 
drum-head in this principal property, that its use depends 
upon its tension. Tension is the state essential to it. Now 
we know that, in a drum, the pelt is carried over a hoop, 
and braced, as occasion requires, by the means of strings 
attached to its circumference. In the membrane of the 
ear, the same purpose is provided for, more simply, but not 
less mechanically, nor less successfully, by a different ex¬ 
pedient, viz. by the end of a bone (the handle of the mal¬ 
leus) pressing upon its centre. It is only in very large 
animals that the texture of this membrane can be discern¬ 
ed In the Philosophical Transactions for the year 1800, 
tvol. i.) Sir Everard Home has given some curious observa 
