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perforated skin, fo as to admit the diffolved food to the papille underneath, which, in the mean time, remain defended from the rough action of the unbruifed fpiculæ.

There are brought together within the cavity of the mouth more distinct uses, and parts executing more diftinct offices, than I think can be found lying fo near to one another, or within the fame compass, in any other portion of the body: viz, teeth of different shape, first for cutting, fecondly for grinding: mufcles, moft artificially difpofed for carrying on the compound motion of the lower jaw, half lateral and half vertical, by which the mill is worked fountains of faliva, fpringing up in different parts of the cavity for the moistening of the food, whilft the maftication is going on: glands, to feed the fountains: a mufcular conftriction of a very peculiar kind in the back part of the cavity, for the guiding of the prepared aliment into its paffage towards the ftomach, and in many cafes for carrying it along that paffage: for, although we may imagine this to be done fimply by the weight of the food itself, it in truth is not fo, even in the upright posture of the human neck; and moft evidently is not the cafe with quadru

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peds, with a horse for inftance, in which, when pafturing, the food is thrust upward by muscular strength, instead of defcending of its own accord.

In the mean time, and within the fame. cavity, is going on another business, åltogether different from what is here defcribed, that of refpiration and fpeech. In addition therefore to all that has been mentioned, we have a paffage opened, from this cavity to the lungs, for the admiffion of air, exclusively of every other fubftance: we have muscles, fome in the larynx, and without number in the tongue, for the purpose of modulating that air in its paffage, with a variety, a compass, and precifion, of which no other musical inftrument is capable. And, laftly, which in my opinion crowns the whole as a piece of machinery, we have a specific contrivance for dividing the pneumatic part from the mechanical, and for preventing one fet of actions interfering with the other. Where various functions are united, the difficulty is to guard against the inconveniences of a too great complexity. In no apparatus put together by art, and for the purposes of art, do I know such multifarious uses so aptly combined as in

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the natural organization of the human mouth; or where the structure, compared with the ufes is fo fimple. The mouth, with all these intentions to ferve, is a fingle cavity; is one machine; with its parts neither crowded nor confused, and each unembarrassed by the rest: each at least at liberty in a degree fufficient for the end to be attained. If we cannot eat and fing at the fame moment, we can eat one moment and fing the next; the respiration proceeding freely all the while.

There is one cafe however of this double office, and that of the earliest neceffity, which the mouth alone could not perform; and that is, carrying on together the two actions of fucking and breathing. Another rout therefore is opened for the air, namely, through the nose, which lets the breath pass backward and forward, whilft the lips, in the act of fucking, are neceffarily fhut close upon the body, from which the nutriment is drawn. This is a circumftance, which always appeared to me worthy of notice. The nose would have been neceffary, although it had not been the organ of smelling. The making it the feat of a fenfe, was fuperadding a new use to

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a part already wanted: was taking a wife advantage of an antecedent and a constitutional neceffity.

But to return to that, which is the proper fubject of the prefent fection, the celerity and precision of mufcular motion. These qualities may be particularly observed in the execution of many fpecies of inftrumental mufic, in which the changes produced by the hand of the musician are exceedingly rapid; are exactly measured, even when most minute; and display, on the part of the muscles, an obedience of action, alike wonderful for its quickness and its correctness.

Or let a perfon only obferve his own hand whilft he is writing; the number of muscles, which are brought to bear upon the pen; how the joint and adjusted operation of several tendons is concerned in every stroke, yet that five hundred fuch strokes, are drawn in a minute. Not a letter can be turned without more than one or two or three tendinous contractions, definite, both as to the choice of the tendon, and as to the space through which the contraction moves; yet how currently does

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does the work proceed! and, and when we look at it, how faithful have the mufcles been to their duty, how true to the order which endeavour or habit hath inculcated! For let it be remembered, that, whilft a man's hand writing is the fame, an exactitude of order is preferved, whether he write well or ill. These two inftances of mufic and writing, fhew not only the quickness and precision of muscular action, but the docility.

II. Regarding the particular configuration of mufcles, Sphincter or circular mufcles appear to me admirable pieces of mechanism. It is the muscular power moft happily applied; the fame quality of the mufcular fubftance, but under a new modification. The circular difpofition of the fibres is ftrictly mechanical; but, though the moft mechanical, is not the only thing in sphincters which deferves our notice. The regulated degree of contractile force with which they are endowed, fufficient for retention, yet vincible when requifite; together with their ordinary state of actual contraction, by means of which their dependance upon the will is not conftant but occafional, gives to them a conftitution of which the conveniency is ineftimable. This their semivoluntary

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