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.
III. Duration of the Contractions.
There is as it respects the duration of the contractions a remarkable difference in the muscles, according as we excite these contractions artificially or naturally.
When upon a living animal or one recently killed, we excite the muscle itself, or we stimulate its nerves, the relaxation succeeds almost suddenly the contraction; neither state is ever lasting, though we continue for a long time the action of the stimulant; the effect which it has produced is immediately exhausted. When galvanism, mechanical or chemical agents are used in our experiments, the phenomenon is the same.
On the contrary, when the will directs the contraction, it can sustain it for a very long time. The support of burthens, standing, &c. clearly prove this fact. When even during life, a morbid irritation is directed upon the nerves, the contraction can be very permanent, of which we have terrible proofs in tetanus.
The permanence of the muscular contraction fatigues the muscle much more than alternate relaxation and contraction. Hence why when we are standing long, we contrive by turns to carry the weight of the body more upon one limb than the other.
IV. State of the Muscle in Contraction.
Muscles that contract exhibit different phenomena as follows: