6 N 2 the zoonic acid, is chiefly discovered in the muscles; that their fluids are the most completely animalized of the whole system; that in animals who lead the most active lives, and in the organs most constantly exerted, it is most frequently found, as in wild animals, the red deer, and hares, the pectoral muscles of the moor fowl, &c; but this seems the effect rather than the cause of the contraction, as we shall soon find; and, on the contrary, the action of a lympathic, for instance, is steady, and constant; that of the heart of the embryo rapid and unremitted, while each is transparent.
On what then does this action depend ? certainly on one of those intricate states connected with life, which we cannot even approach by conjecture. We have nothing analogous to compare it with, nothing which possesses the same properties in different circumstances to enable us to divest it of the adventitious circumstances. We must content ourselves, therefore, with endeavouring to ascertain its phenomena and its laws.
We know, from various experiments, that a free circulation through the arteries of muscles is necessary to their contraction; for tying the artery will render a muscle paralytic; and even tying the inferior vena cava above the iliacs, we are told by Kauw Boerhaave, will render the muscles of the posterior extremities incapable of contraction. The free access of the nervous influence is equally necessary; for though we are vaguely told of animals without a nervous system possessing muscular power, none, which have been examined with care, are found to want this source of motion.
A muscle in action increases in absolute weight, in density, and in its power of resistance. That it increases in bulk may be doubted, for experiments are contradictory; and while the middle or the belly of the muscle is evidently enlarged, the diminution of its length compensates for the increase. Though the tendon is a firm, substantial cord, it is sometimes broken by muscular exertion; while the organ is uninjured, and the patella, as well as other bones, are fractured by a violent jerk, or a sudd en, peculiarly violent, action of the muscles. It is, however, recorded, that the muscles themselves are sometimes ruptured, particularly the recti abdominis in tetanus, and the gastrocnemii in cramps; and, as it is difficult to suppose that increased cohesion should occasion a solution of cohesion, it has been suspected that this is the effect of the action of antagonists, or of unequal excitement. Neither, however, could be the cause in the present instances, nor is it difficult to conceive that the material may be ruptured by excess of power. Cord entwined with great force round any body, and then wetted, will be broken by the violent contraction. The choutchouc, a very elastic substance, is diminished in specific gravity by extension.
Muscular action, then, consists in an attraction of the parts of each fibre in a manner peculiar to animated nature. It is not a contraction in every dimension, since it would be then attended with diminished volume; but in length only attended with a proportional increase of bulk, so as to preserve the absolute volume unchanged, or perhaps somewhat increased. The force exerted by this contraction is enormous. It was calculated, however, by the mechanical physicians in the most extravagant manner, on foundations almost wholly chimerical; but on better grounds, from the very disadvantageous insertion of the muscles, which we shall soon enlarge on, to surmount a resistance of fifty pounds, the deltoid, it is said, must exert a force equal to 2568 pounds. In general, it appears that the force with which a muscle contracts is in proportion to the number of its fleshy fibres, and the extent of the surface to which these fibres are attached; but its degree of contraction, or the extent of its motion, is in proportion to their length. The limits of contraction differ in the long and in the circular muscles; for the former do not contract more than one third of their length; but the circular fibres of the stomach, which, in their utmost dilatation, may be expanded to a foot in circumference, will, after long fasting, be reduced to a circle of an inch. It must, however, be added, that in circular muscles no fibres pass completely round: bundles of fibres are collected, and end at different points, while some begin where others end. Each may, therefore, admit only of a limited contraction, while the dilatation just mentioned may be the sum of the whole.
Muscles are usually attached to bones near their joints, and, running almost parallel with the bone, are inserted at very acute angles. They are consequently levers of the third kind, situated between the fulcrum and resistance. By this contrivance, much force is lost to attain velocity, and to avoid deformity; for the muscle would start out in its action, if conducted in any other manner, and this starting would be in proportion to the celerity of the action of the resisting part. At present, the hand, for instance, sweeps through a large circle with great velocity, while the muscles, which raise the fore arm, viz. the biceps and brachialis internus, swell to a degree scarcely perceptible. In the force supposed to be exerted by the deltoid, a great proportion acts only in pressing the humerus to the shoulder, according to the common doctrine of the composition, and resolution of forces. Another loss of force arises from the oblique insertion of the fleshy fibres into the tendinous expansion, which pervades them: but, with the Author of Nature, power as well as life is obedient to the divine fiat, and each is profusely diffused: yet all this has been resolved into jarring atoms, molecules or-ganiques, and animated filaments! Various contrivances, however, appear to have been adopted to lessen this considerable loss of power. In general, the extremities of the bones are enlarged, so that the muscle is removed from the parallelism, the tendons of the smaller line of direction of the bones are separated, or the angles at which they are inserted are augmented. Sometimes, for the same purpose, the tendons, or the muscles themselves, are reflected over pulleys. When the limb begins to move, the angle of insertion is, of course, increased.