APPLICATION OF THE ARGUMENT. 29 



nication with the external air. In one word ; it exactly 

 answers the purpose of the hole in a drum. 



The meinbrana 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 every thing 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, 

 (vol. i.) Mr. Everard Home has given some curious observa- 

 tions upon the ear, and the drum of the ear, of an elephant. 

 [PI. V. fig. 4.] He discovered in it, what he calls a radiated 

 muscle, that is, straight muscular fibres, passing along the 

 membrane from the circumference to the centre ; from the 

 bony rim which surrounds it, towards the handle of the 

 xnalleus to which the central part is attached. This mus- 

 cle he supposes to be designed to bring the membrane into 

 unison with different sounds ; but then he also discovered, 

 that this muscle itself cannot act, unless the membrane be 

 drawn to a stretch, and kept in a due state of tightness, 

 by what may be called a foreign force, viz. the action of 

 the muscles of the malleus. Our author, supposing his ex- 

 planation of the use of the parts to be just, is well founded 

 in the reflection which he makes upon it : '•' that this mode 

 of adapting the ear to different sounds, is one of the most 

 beautiful applications of muscles in the body ; the mechan- 

 ism is so simple, and the variety of the effects so great.'^ 



In another volume of the transactions above referred to, 

 and of the same year, two most curious cases are related, 

 of persons who retained the sense of hearing, not in a 

 perfect, but in a very considerable degr-^e, notwithstanding 

 the almost total loss of the membrane we have been de- 

 scribing. In one of these cases, the use here assigned to 

 that membrane, of modifying the impressions of sound by 

 D 



