The contractions should be considered under the relation of their
quickness as under that of their force.
1st. If it is by stimulants that they are produced, by laying bare a
muscle and acting directly upon it, they vary according to the state
of vitality of the muscle, and according to the body which stimulates.
In the first moments of the experiment, they succeed with rapidity and
are sometimes connected together with such quickness that the eye can
hardly follow them. As the muscle becomes weak, its contractions become
less prompt; and they cease at the end of some time. We can reanimate
them by employing a very active stimulant; the fibres finally become
insensible to this also.
2d. If it is by irritating the nerve that we make a voluntary muscle
contract, we produce a still greater quickness of contraction than by
stimulating the muscle itself. Running would be almost immeasurably
rapid, if each contraction that it requires was equal to those that we
thus obtain, especially when we act on the one hand on very sensitive
animals, and on the other with very active stimulants, galvanism
for example. Upon this subject I have made a remark, it is that the
quickness and the force of the contractions are not commonly greater if
we irritate at the same time all the nerves that go to a muscle, than
if we irritate but one.
3d. When it is the will that regulates the quickness of the muscular
contractions, this quickness has infinitely various degrees; but
there is always one beyond which we cannot go. This degree is not
the same for all men; there is even among them in this respect very
great differences, which are foreign to the force of organization of
the muscles; it is rare even that individuals with a very powerful
muscular system are the best runners. I do not know that we have yet
observed the exterior habit of the body which indicates the quickness
of the contractions, as there is one which denotes their force; it must
however exist. Animals are like men; the degree of quickness which
each can attain, is infinitely variable. I shall not cite examples of
rapid races, of analogous motions given by the superior extremities, as
those of the fingers in performing on certain instruments, the violin,
the flute, &c.; astonishing ones may be read of in many authors. I
would only remark, that there are but few motions which give us a
greater idea of this quickness, than the sudden and rapid contractions
which, in the inferior extremities, produce a leap, or that powerful
action of these extremities when we give a kick with the foot; which
in the superior serve for the projection of heavy bodies; which in
the same limbs assist to push the trunk back, when we support them
against a resisting point, and afterwards suddenly stretch them to
push this point forward, which not yielding, the motion rebounds upon
the trunk; which preside over the action of giving a blow of the hand;
which in the fingers produce the sudden motion, from which results
what is called a fillip, &c. &c. I confound all these motions almost
entirely analogous to leaping, and which differ from it only in the
more or less evident effects that they produce. Authors, it may be
observed, have not sufficiently established the resemblances between
these various sudden and rapid contractions; they have considered
leaping in too insulated a manner. But let us return. The degree of
rapidity of muscular contractions is greatly subordinate to exercise.
The habit of making certain muscles act renders them more quick in
their contraction; for example, walking which accustoms us to contract
alternately the extensors and the flexors of the lower extremities,
fits us wonderfully for swiftness in running. When any man practises
for a little time this last exercise, he soon attains the greatest
rapidity of which his muscular system is capable. On the contrary, the
motions of adduction and abduction being more rare in the ordinary
state, it requires a longer apprenticeship for dancers to learn to
carry their legs rapidly in and out, for the purpose of executing steps
in which they cross them alternately. In general, habit modifies much
more the quickness than the force of the contractions. Yet there is
always a limit which can never be passed, whatever may be the exercise
that we give to the muscles; this limit depends on the constitution;
each man is by it, a more or less active leaper and runner.