voluntary character, is exactly fuch as fuits with the wants and functions of the animal. III. We may alfo, upon the fubject of mufcles, obferve, that many of our moft important actions are achieved by the combined help of different mufcles. Frequently, a diagonal motion is produced, by the contraction of tendons, pulling in the direction of the fides of the parallelogram. This is the cafe, as hath been already noticed, with fome of the oblique nutations of the head. Sometimes the number of cooperating muscles is very great. Dr. Nieuentyt, in the Leipfic Tranfactions, reckons up a hundred mufcles that are employed "every time we breathe yet we take in, or let out, our breath, without reflecting what a work is thereby performed: what an apparatus is laid in of inftruments for the fervice, and how many fuch contribute their affifiance to the effect. Breathing with cafe is a bleffing of every moment: yet, of all others, it is that which we poffefs with the least consciousness. ` A man in an asthma is the only man who knows how to estimate it. ان IV. Mr. Home has obferved*, that the Phil. Tranf. parti. 1800, p. 8. moft most important and the moft delicate actions are performed in the body by the smallest mufcles and he mentions, as his examples, the muscles which have been discovered in the iris of the eye and the drum of the ear. The tenuity of these muscles is astonishing. They are microscopic hairs; must be magnified to be visible; yet are they real effective muscles; and not only fuch, but the grandeft and most precious of our faculties, sight and hearing, depend upon their health and action, V. The muscles act in the limbs with what is called a mechanical disadvantage. The muscle at the shoulder, by which the arm is raised, is fixed nearly in the fame manner, as the load is fixed upon a fteelyard, within a few decimals, we will fay, of an inch, from the centre upon which the fteelyard turns. In this fituation, we find that a very heavy draught is no more than fufficient to countervail the force of a small lead plummet, placed upon the long arm of the fteelyard, at the diftance of perhaps fifteen or twenty inches from the centre, and on the other fide of it. And this is the disadvantage which is meant. And an abfolute difadvantage, no doubt, it would would be, if the object were to fpare the force of mufcular contraction. But obferve how conducive is this conftitution to animal conveniency. Mechanism has always in view one or other of these two purposes; either to move a great weight flowly, and through a small space; or to move a light weight rapidly, through a confiderable sweep. For the former of these purposes, a different fpecies of lever, and a different collocation of the muscles, might be better than the present: but for the second, the prefent ftructure is the true one. Now fo it happens, that the second, and not the firft, is that which the occafions of animal life principally call for. In what concerns the human body, it is of much more confequence to any man to be able to carry his hand to his head with due expedition, than it would be to have the power of raising from the ground a heavier load (of two or three more hundred weight, we will fuppose,) than he can lift at present. This last is a faculty, which, upon fome extraordinary occafions, he may defire to poffefs; but the other is what he wants and uses every hour or minute. In like manner, a husbandman or a gardener will do more execution, by being able to carry his scythe, L 4 fcythe, his rake, or his flail, with a fufficient dispatch through a fufficient fpace, than if, with greater ftrength, his motions were proportionably more confined and flow. It is the same with a mechanic in the ufe of his tools. It is the fame alfo with other animals in the use of their limbs. In general, the vivacity of their motions would be ill exchanged for greater force under a clumfier ftructure. We have offered our obfervations upon the structure of muscles in general; we have also noticed certain fpecies of muscles; but there are alfo fingle mufcles, which bear marks of mechanical contrivance, appropriate as well as particular. Out of many inftances of this kind we select the following. I. Of muscular actions, even of those which are well understood, fome of the most curious are incapable of popular explanation; at least without the aid of plates and figures. This is in a great measure the cafe, with a very familiar, but, at the fame time, a very complicated motion, that of the lower jaw and with the muscular ftructure by which it is produced. One of the muscles concerned, may, however, be defcribed in fuch a man ner, as to be, I think, fufficiently comprehended for our prefent purpose. The problem is to pull the lower jaw down. The obvious method fhould feem to be, to place a ftraight muscle, viz. to fix a ftring from the chin to the breaft, the contraction of which would open the mouth, and produce the motion required at once. But it is evident that the form and liberty of the neck forbid a muscle being laid in fuch a position; and that, confiftently with the preservation of this form, the motion, which we want, must be effectuated, by fome muscular mechanifm difpofed further back in the jaw. The mechanism adopted is as follows. A certain muscle called the digastric rifes on the fide of the face, confiderably above the infertion of the lower jaw; and comes down, being converted in its progress into a round tendon. Now it is evident that the tendon, whilft it pursues a direction defcending towards the jaw, must, by its contraction, pull the jaw up, inftead of down. What then was to be done? This, we find, is done. The defcending tendon, when it is got low enough, is paffed through a loop, or ring, or pulley, in the os hyoides, and then made to afcend; and, having thus changed |