of the muscle, the moveable bone is drawn upon its fulcrum towards a fixed point. This is indisputable; and it is likewise certain, that the muscles are to be distinguished into those of voluntary, and those of natural or necessary motion: that the voluntary muscles have antagonists, which act alternately in a contrary direction, that is, are contracted by the command of the will, while the others are stretched, and again are extended, while the others are contracted: but the necessary muscles have contracting and extending powers within themselves, and need no antagonists. This being the true state of the muscles, the question is, what causes that elasticity, spring, or power of contraction and restoration, which their nervous coats and fibres have, to recover themselves against a given weight or force that stretches them? The reply is, that many unanswerable reasons can be given to prove, that this contractive restitutive force does not depend on the mixture, effervescence, or rarefaction of any fluids, humours, or liquors within the body; and there is one convincing experiment that shows it. Lay open the thorax of a dog, (as I have often done) and take a distinct view of that famous muscle, the heart, in its curious and wonderful motion, while the animal is still alive. In diastole, the muscle is very red and florid, soft and yielding to the touch, and thro' it the vital fluid glows and shines; it appears in this state fully replenished and distended with blood: but in systole, as soon as it begins to contract, and the blood rushes out by the compression of the contracting fibres, the heart loses its florid colour, and becomes pale and livid, compact and solid, and evinces that, during this state of it, the muscle contracts inwardly into its own dense substance, and takes up less space than before, till it returns to its diastole: then the blood which flowed from it with velocity, during systole thro' the coronary veins into the auricles, rushes back into it thro' the coronary arteries, restores the glowing florid colour, and inflates the muscle, in order to strain the nerves for the next contraction. It is plain from hence, that the heart has less blood and fluid in time of contraction, and that the contraction is not caused by the addition of another fluid from the nerves, as the learned have asserted. And as to what they say of the longitudinal fibres being divided into innumerable little cells or bladders, which have communications with the blood vessels and nerves, and that in these vesicles, the blood