1st. They evidently harden, as we may be convinced by placing the hand on the masseter, the temporal or any other superficial muscle in contraction.
2d. They increase in thickness; hence the greater prominence of all the sub-cutaneous muscles when the body is in violent action. Sculptors know this difference very well. A man at rest and a man in motion, have in their statues an exterior wholly different.
3d. The muscles when they are not confined by the aponeuroses, sometimes experience a slight displacement.
4th. They diminish in length, and thus the two points to which they are fixed approximate.
5th. Their volume remains about the same. What they lose in length, they nearly gain in thickness. Is the proportion very exact? Of what consequence to us is this insulated question, to which, since the days of Glisson, so much importance has have attached! it deserves none.
6th. The blood contained in the vessels of the muscles, especially in the veins, is in part pressed out; we increase the flow of the blood by the motions of the arm, the operation of bleeding proves both these facts.
7th. Yet the muscle does not change colour; it is because it is not the colouring portion of the blood circulating with it in the muscular vessels that colours the muscles, but, as I have said, that which is inherent in their texture and combined with their fibres; now this combined colouring substance remains the same in relaxation and contraction. The heart of the frog is pale when it contracts; but it is because the blood it contains is evacuated and the transparency of its parietes renders this phenomenon evident.
8th. In contracting, the muscles become the seat of many small transverse wrinkles, sensible especially in the contractions of oscillation, less apparent in those of the whole of the muscle, and almost nothing, when a muscle being laid bare in a living animal, contracts with a small degree of force.
9th. All authors consider contraction in too uniform a manner; they have described the phenomena of it, as if in every case the muscle contracted alike; but it is evident that there are numerous differences in the state in which it then is. 1st. There is the slow and insensible contraction produced by the contractility of texture, when we cut a muscle or when its antagonist is paralyzed. 2d. The quick and sudden contraction produced by the will, or by the excitement of a nerve, a mode of motion that takes place most commonly either in the ordinary state, or in convulsions. 3d. The species of oscillation of which I have already spoken, and which affecting each fibre in a muscle, does not yet produce any very sensible effect upon the whole, contracts it a little, but scarcely approximates at all its moveable points; this is the kind of motion which takes place in the tremors produced by cold, by fear, by the beginning of a fit of intermittent fever, &c. By laying bare a muscle in an animal that is made to shiver, we see that this kind of contraction resembles precisely that which is produced by pouring salt in powder upon a part of the muscular system. Then, although there may be in all the muscles, an internal motion infinitely more sensible than in the great contractions, yet the limbs are displaced but little, there are hardly any motions of the whole muscles, they are but slight jars. 4th. There are other modes of contraction less sensible than these, but which however exhibit differences. In general, to each species of motion of the muscle is adapted a particular manner of contracting; if we make but few experiments on living animals, we may easily be convinced how much the most judicious authors have been mistaken upon this point.
Two modes of contraction are often combined; for example, when we cut a muscle transversely in a living animal, there is at first a slow contraction of the whole, produced by the contractility of texture, then partial oscillations in all the divided fibres; now these oscillations are foreign to the retraction which takes place without them, often in the living animal and always in the dead body. So the oscillations can be combined with the sudden contraction arising from the nervous influence by the act of the will, or they may be disconnected with it, as happens almost always when the animal is in full life. We may be convinced of this last fact without recourse to experiments, by placing the hand upon the masseter muscle or the biceps of a thin person when they are contracting; we do not feel in them through the skin any motion analogous to these oscillations.