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with confined air, because the expansion of that air by heat, or its contraction by cold, would have distended or relaxed the covering membrane, in a degree inconsistent 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 communication 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 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 circumserence. In the membrane of the ear, the fame purpose is provided for, more simply, but not less mechani
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cally, nor less successfully, by a different expedient, viz. by the end of a bone (the handle of the malleus) pressing upon its centre. It is only in very large animals that the texture of this membrane can be discerned. In the Philosophical Tranfactions for the year 1800, (vol i.) Mr. Everard Home has given some curious observations upon the ear, and the drum of the ear, of an elephant. He discovered in it, what he calls a radiated muscle, that is, straight muscular fibres, passing along the membrane from the circumserence to the centre; from the bony rim which surrounds it, towards the handle of the malleus, to which the central part is attached. This muscle 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 explanation 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 mechanism is so simple, and the variety of effects so great?
In another volume of the Transactions above referred to, and of the fame year, two most curious cases are related, of persons who retained the sense of hearing, not in a perfect, but in a very considerable degree, notwithstanding the almost total loss of the membrane we have been describing. In one of these cafes, the use here assigned to that membrane, of modifying the impressions of found by change of tension, was attempted to be supplied by straining the muscles of the outward ear. "The external ear," we are told, " had acquired a distinct motion upward and backward, which was observable whenever the patient listened to any thing which he did not distinctly hear; when he was addressed in a whisper, the ear was seen immediately to move; when the tone of voice was louder, it then remained altogether motionless."
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 suffered from cold; one, " a great increase of deasness from catching coldthe E 2 - other, other, " very considerable 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 transmitting those vibrations to the brain, the use of the organ, so far as we can judge, must have been entirely obstructed.
OF THE SUCCESSION OF PLANTS AND ANIMALS.
The generation of the animal no more accounts for the contrivance of the eye or ear, than, upon the supposition stated in a preceding chapter, the production of a watch by the motion and mechanism of a former watch, would account for the skill and intention evidenced in the watch so produced; than it would account for the disposition 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 suitableness of their forms and places to their offices, for their connection, their operation, and the useful result of that operation. I do insist most strenuously upon the correctness of this comparison; that it holds as to every mode of specific propagation; and that whatever was true of the watch, under the hypothesis above mentioned, is true of plants and animals.
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