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with confined air, because the expansion of that air by heat, or its contraction by cold, would have diftended or relaxed the covering membrane, in a degree inconfiftent with the purpose which it was affigned to execute. The only remaining expedient, and that for which the euftachian tube ferves, is to open to this cavity a communication with the external air. In one word; it exactly answers the purpose of the hole in a drum.

The membrana tympani itself, likewife, deferves 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 elaftic medium. It bears an obvious refemblance 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 tenfion. Tension is the ftate effential 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 ftrings attached to its circumference. In the membrane of the ear, the fame purpose is provided for, more fimply, but not lefs mechani

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cally, nor lefs fuccessfully, by a different expedient, viz. by the end of a bone (the handle of the malleus) preffing upon its centre. It is only in very large animals that the texture of this membrane can be difcerned. In the Philofophical Transactions for the year 1800, (vol i.) Mr. Everard Home has given fome curious obfervations upon the ear, and the drum of the ear, of an elephant. He difcovered in it, what he calls a radiated muscle, that is, ftraight muscular fibres, paffing along the membrane from the circumference to the centre; from the bony rim which furrounds it, towards the handle of the malleus, to which the central part is attached. This muscle he fuppofes to be defigned to bring the membrane into unifon with different founds: but then he also discovered, that this muscle itself cannot act, unless the membrane be drawn to a stretch, and kept in a due ftate of tightness, by what may be called a foreign force, viz. the action of the muscles of the malleus. Our author, fuppofing his explanation of the use of the parts to be juft, is well founded in the reflection which he makes upon it; "that this mode of adapting the ear to different founds, is one of the most beautiful applications of muscles

muscles in the body; the mechanism is so fimple, and the variety of effects fo great."

In another volume of the Tranfactions above referred to, and of the fame year, two most curious cafes are related, of perfons who retained the sense of hearing, not in a perfect, but in a very confiderable degree, notwithftanding the almoft total lofs of the membrane we have been defcribing. In one of these cafes, the ufe here affigned to that membrane, of modifying the impreffions of sound by change of tension, was attempted to be fupplied by straining the mufcles of the outward "The external ear," we are told, "had acquired a distinct motion upward and backward, which was obfervable whenever the patient liftened to any thing which he did not diftinctly hear; when he was addreffed in a whisper, the ear was feen immediately to move; when the tone of voice was louder, it then remained altogether motionless."

ear.

It appears probable, from both these cases, that a collateral, if not principal, use of the membrane, is to cover and protect the barrel of the ear which lies behind it. Both the patients fuffered from cold; one," a great increase of deafness from catching cold;" the other,

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other, "very confiderable pain from exposure to a stream of cold air." Bad effects therefore followed from this cavity being left open to the external air; yet, had the author of nature shut it up by any other cover, than what was capable, by its texture, of receiving vibrations from found, and by its connection with the interior parts, of tranfmitting those vibrations to the brain, the use of the

organ, fo far as we can judge, must have been entirely obftructed.

CHAP

CHAPTER IV.

OF THE SUCCESSION OF PLANTS AND

ANIMALS.

THE

HE generation of the animal no more accounts for the contrivance of the eye or ear, than, upon the fuppofition ftated in a preceding chapter, the production of a watch by the motion and mechanism of a former watch, would account for the fkill and intention evidenced in the watch fo produced; than it would account for the difpofition of the wheels, the catching of their teeth, the relation of the several parts of the works to one another, and to their common end, for the fuitableness of their forms and places to their offices, for their connection, their operation, and the useful refult of that operation. I do infift most strenuously upon the correctnefs of this comparison; that it holds as to every mode of specific propagation; and that whatever was true of the watch, under the hypothefis above mentioned, is true of plants and animals.

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