Muscles that contract exhibit different phenomena as follows:
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.