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.