Part 47

The force of the contraction of the muscles of animal life varies much, according as it is brought into action by stimulants, or by the cerebral action.

Every irritation made upon a muscle laid bare produces only a brisk, rapid motion, but generally not very powerful. I have frequently satisfied myself in my experiments that it is impossible to approximate even at a great distance by this means, the great energy which the brain communicates to the muscles of animal life. The organic muscular system which stimuli directly applied put principally in motion, never has exacerbations of force corresponding to those which the animal contractility exhibits in so great a degree under certain circumstances. It is then especially when the muscles move in virtue of this last property, that the force of their contraction must be considered. Now this contraction can, as we have seen, be produced, 1st, by stimulating the brain in experiments; 2d, when its excitement takes place in the natural state by the will, or by sympathy. In the first case, the force of the contraction is never very powerful, whatever may be the stimulant employed, either upon the brain, or the nerves laid bare. I have uniformly observed a very rapid convulsive motion, analogous to that produced by exciting the muscles themselves, but never as strong as that which is the result of vital action. Notwithstanding what some physiologists have written, we can never by irritating the nerves of the flexors impart to them an energy comparable to that which the will can give them. Irritate for example the sciatic nerve in an inferior extremity which has just been amputated, the toes will never bend with the force which they do in certain cases in the natural state. I have twice made this experiment in amputations performed by Desault. Unacquainted then with physiology, I was much struck with this phenomenon.

In the cerebral excitement and in that of the spinal marrow, we cannot so well appreciate the force of the contractions which result from it, as when we stimulate an insulated nerve; in fact, all the system entering then into convulsive action, the extensors destroy in part the effort of the flexors and vice versa. The muscles simultaneously in action, counterbalance, interfere with and injure each other. The stimulant which gives the greatest force to the contractions, has always appeared to me to be galvanism.

In the living state, the force of muscular contraction depends on two causes; 1st, on the muscle; 2d, on the brain. These two causes are in a variable proportion; it is necessary to consider them separately.

Under an equal cerebral influence, a muscle well nourished, which appears distinctly through the integuments, and has very large fibres, will contract much more strongly than that which is delicate, slender, with loose, pale, small fibres, and which makes but a slight prominence under the integuments. In our ordinary manner of considering muscular force, it is to this state of the muscles that we especially attend. The statues which exhibit strength and vigour, have always as an attribute a powerful development of the muscular forms. When the brain acts upon these muscles with energy they are capable of extraordinary motions. I shall not relate examples of the astonishing efforts of which they are susceptible. Haller and others have cited many of them, either in the muscles of the back in carrying burdens, or in the muscles of the superior extremities in raising great weights, or in the inferior extremities in leaping or in order to preserve attitudes which suppose enormous resistances to be overcome.

It is especially the cerebral influence that increases much the force of muscular contraction. The will can raise this force very high; but the different excitements that are foreign to it, raise it infinitely more. We know the force that a man acquires in anger, that of maniacs, of persons in the cerebral excitement of a fever, &c. In all these cases the impulse communicated by the brain, is sometimes such, that the most delicate muscles of the feeblest woman surpass in energy those of the strongest man in the ordinary state.

The force of muscular contraction is then in a ratio compounded of the force of the organization of the texture of the muscles, and of the force of the cerebral excitement. If both are slight, the motions are almost nothing; if both are at the highest degree, it is difficult to conceive how far the effects may go which result from them; a maniac with thick and strong muscles is capable of efforts that we should in vain attempt to calculate. If the nervous force is very powerful, and the muscular texture feeble, or if an inverse state exists, the phenomena of contraction are less. In general nature has almost always united these two things in this last manner. Women and children who have a weak fleshy texture, have a very great nervous mobility; men on the contrary, those especially with athletic forms, whose nervous systems are less easily excited, receive more rarely the causes of a strong influence upon their muscles.

Whatever may be the point of view in which we consider the force of the contractions of the muscular system of animal life, it is always very great in proportion to the effect which results from these contractions. Nature in the economy follows a law the reverse of that of the motion of our common machines, the great advantage of which is to increase the moving powers, to produce a great effect with a small force. Here there is always a great expenditure of force for a small effect, which is owing to the numerous causes that tend to destroy the effect of this force. 1st. The muscles act almost always upon a very unfavourable lever, upon that in which the power they represent is nearer the point of support than the resistance. 2d. All in contracting have to overcome the resistance of the antagonists. 3d. As in each motion there is always a fixed point, the effort which, after contraction, is carried upon this fixed point, is entirely lost. 4th. Various frictions injure also the motion. 5th. The obliquity of the insertion of the muscles upon the bones, an obliquity that approaches nearer a horizontal than a perpendicular direction, an obliquity not less remarkable for the fleshy attachments upon the tendon or aponeuroses, offers a double cause of weakness. All these and many other reasons which we might with Borelli, who was the first to make these important remarks upon muscular motion, add to them, prove that the absolute or real force of the muscles is infinitely superior to their effective force. Yet all are not so unfavourably arranged; in some, as the solæus, the insertion is perpendicular to the bone; in others, as the muscles which act upon the head, we observe that they are powers of a lever of the first kind. In general, in order to estimate the force of a separate muscle, the deltoid, for example, it is necessary especially to have regard to the distance of its insertion at the point of support, to the degree of opening of the angles formed by the fleshy fibres upon the tendon, and afterwards by the tendon upon the bone, and to the division of the forces between the fixed and moveable points.

Some advantages seem to compensate in a slight degree in certain muscles for their bad arrangement as to the power of motion; such are, 1st, the sesamoids, the patella, the different eminences of insertion, the enlargement of the large bones at their extremities, &c. which remove the fibres to a distance from the moveable points; 2d, the intermuscular fat, that which is in the neighbourhood of the muscles, the fluid of synovial sheaths, which facilitate motions by lubricating the surfaces that execute them; 3d, the aponeurotic expansions that confine down the motions on the extremities; 4th, these motions themselves, those of flexion for example, which, as they take place, diminish the obliquity of the insertion of the flexors, and render it even perpendicular, as has been well observed by a modern author.

Many calculations have been made upon the waste of muscular motion, upon the effort of a muscle which contracts, compared with the effect that results from it. They can never be precise because the vital forces vary to an infinite degree, because they are not the same in two individuals and because the cerebral influence and the force of muscular organization are never in constant proportion in the same subject. It is a peculiarity of the vital phenomena to escape all calculations, and to exhibit, like the forces from which they emanate, a character of irregularity which distinguishes them essentially from the physical phenomena. Let us conclude only from the preceding observations, that the muscular effort carried to the highest point by cerebral excitement, can produce astonishing effects, which suppose a force of contraction hardly conceivable; such is the rupture of the strong tendons, of the patella, the olecranon, &c.; such is also the resistance often opposed to the enormous distensions that are used in luxations, fractures, &c.

II. Quickness of the Contractions.

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.

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:

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.