Part 32

It is only those muscles that are attached on one side to a fixed point and on the other to a moveable one, like those of the eye, and most of those of the face, that can move in an insulated manner, and without requiring a motion in the other muscles. It should be remarked however that in general the contractions destined to fix the point which should be immoveable in the ordinary motions, are less than they at first seem to be. In fact, in these ordinary motions, the point which moves is always the most moveable, that which remains without motion is the least so; for example, it requires a much greater effort in flexors to bend the arm upon the fore-arm, than to bend the phalanges upon the fore-arm, or the fore-arm upon the arm. By supposing their two attachments moveable, the gemelli would act much more powerfully on the foot than on the femur, &c. In the extremities, the superior point is always more moveable than the inferior, now it is this which almost always moves, the other being fixed; then as it offers more resistance by its position, it requires less effort of the muscular powers to retain it. It is only in violent motions, that the previous contraction of the muscles destined to fix one of the points of insertion Is very painful. This takes place on the chest when the trapezius, the great serratus and the great pectoral contract powerfully; then all the other muscles of this cavity contract strongly to dilate it, and thus offer a broader and more fixed attachment to those muscles, which move the shoulder in the support of burdens, or in any other analogous effort. The diaphragm contracts also; hence hernias, the descents which take place from a concussion in those motions which, at first view, have no analogy with the abdominal cavity. When in a horizontal position of the body we raise the head, the rectimuscles of the abdomen contract to fix the chest, and present a solid point to the sterno-mastoideus, &c.

We call especially a compound motion that which two or more muscles, acting upon the same point, contribute simultaneously to produce. In this case, the moveable point follows the direction of neither muscle, if there are two of them, but takes the diagonal of their direction. It is thus that the eye is moved outwards and upwards, outwards and downwards, &c.; that the head is depressed, that it is carried to one side, and that the arm is applied to the trunk, &c. In general nature has distributed muscles only in some principal directions around a moveable point, for example around the eye, in those of elevation, depression, adduction and abduction; the combination of these simple motions produces the compound ones. If the adductor and depressor contract equally, the eye will be carried exactly in a middle direction; if one acts with more force than the other, it will be carried a little nearer the other; so that the four muscles, by moving separately, or two by two in an equal manner, carry the eye in eight different directions. In all the intermediate directions, there is also a simultaneous action of two muscles, but always a superiority in the action of one of them. Thus almost all the motions of circumduction operate.

When two opposite muscles contract, the moveable part is not moved; they are perfect antagonists. When two muscles which contract at the same time are placed in the same direction, there is no loss of power; this is what takes place when the genio-hyoideus and the mylo-hyoideus depress the jaw or elevate the os hyoides; these muscles act completely together. But when two muscles are in part opposed and in part in the same direction, as the sterno-mastoidei, one portion of the forces is destroyed and the other remains. The action by which the sterno-mastoidei tend to carry the head to the right or the left, is nothing; that alone by which they direct it downwards produces its effect which is double, considering the action of the two muscles, which are thus at the same time acting together and antagonists. Hence we see that this applies not only to the motion produced by the contractility of texture, but also very often to those which the animal contractility occasions.

VI. _Phenomena of the Relaxation of the Muscles._

When a muscle ceases to contract, it becomes the seat of phenomena precisely opposite to the preceding, which it is sufficient to know in order to understand these. The muscle becomes longer and softer; its wrinkles disappear; it returns exactly to the state in which it was found. It is needless to give in detail the series of these phenomena.

I would remark that in the state of relaxation of the muscles, the parts often execute motions which are only owing to their weight; such are the flexion forwards of the head in sleep, the fall of the fore-arm and the arm in the same case. Then the weight is often opposed to the limbs, remaining in their middle position, which are not supported. We see particularly these phenomena in paralysis.

ARTICLE FIFTH.

DEVELOPMENT OF THE MUSCULAR SYSTEM OF ANIMAL LIFE.

The muscular system exhibits great differences, according as we examine it before the completion of growth, or in the ages that follow that in which this growth is terminated.

I. _State of the Muscular System in the Fœtus._

In the first month of the fœtus, this system is, like the others, a mere mucous homogeneous mass, in which can be distinguished scarcely any line of demarcation. Aponeuroses, muscles, tendons, &c. all have the same appearance. Gradually the limits are established, the muscular texture at first takes a deeper tinge, from the blood that enters it. Yet this tinge is at first much less evident than in the adult; it remains nearly the same till birth. If we make use of the bones as a means of comparison, this becomes striking. In the adult the interior of the bones is less red than the muscular texture; the difference is remarkable. It is the contrary in the fœtus; much more blood penetrates the already ossified portion of the bones, than the interior of the muscles. Nature distributes the blood in an inverse manner at these two periods of life in these two systems.

I presume that this phenomenon is principally owing to the kind of inertia in which the muscles remain before birth. Observe in fact that though some motions announce in the last months the presence of the fœtus in the womb of the mother, yet these motions are infinitely less than they are to be afterwards. The proof of this is the constant semi-flexed position which the limbs and trunk have, and the small space that there is to execute these motions in, especially in the last periods in which the waters are wonderfully diminished. In the early periods of pregnancy, though the space may be greater, by opening the females of animals, we constantly find the fœtus drawn up upon itself, and in an attitude almost immoveable.

Many respectable philosophers have found the muscles of the chick in its shell much less irritable than after birth, either by ordinary agents, or by galvanic influence. I have made the same experiment upon small guinea-pigs that were never born, by irritating directly their muscles, or by stimulating their nerves, their spinal marrow and the brain. The nearer we approach the term of conception, the less are the motions obtained. That which is especially remarkable is the rapidity with which, when the fœtus is dead, the muscles lose their irritability; the instant that extinguishes life seems to destroy this property. In the latter periods that precede accouchement, it is a little more permanent, and more susceptible of being brought into action, but always less than after birth. We can hardly doubt then that the motions are less at this age, though however they exist. We shall see that the nutrition, size and redness of the muscles are in general in the adult in proportion to the number of the motions they perform; it is not then astonishing that less blood penetrates them in the fœtus. Besides the nearer we approach the period of conception, the less abundant is this fluid in them. I have had occasion to make this remark on guinea-pigs killed at different periods of gestation. In the early periods, the muscles of the small ones really resemble those of frogs; white like them, they are marked with reddish lines, which indicate the course of the vessels.

I presume also that the kind of blood which circulates at this age in the arteries and which penetrates the muscles, is less proper to support and develop their mobility. In fact it is the black blood that then enters the muscles by the vessels. We know that in the adult, whenever this blood circulates preternaturally in the arterial system, life is altered, the muscular motion is weakened, and soon asphyxia comes on. It is to the nature and the colour of the blood of the fœtus, that must be attributed the livid and often deep tinge that its muscles exhibit; for this is also a character that distinguishes them from those of the adult. Not only their colour is less evident and they are paler, but their tinge is wholly different; and this tinge has uniformly the character of that of the fœtus before it has respired.

The muscles are slender, but little developed in the fœtus. Their development is infinitely less than that of the muscles of organic life. The size of the limbs arises especially from their sub-cutaneous fat. When this fat is in small quantity, and we compare the limbs with the trunk, they are much less in proportion than they will be afterwards. In the fœtuses that have much cutaneous fat, from whom we remove all the skin, we also see this disproportion of size. We know that at this age all the cavities of muscular insertion, all the apophyses destined to the same use, are almost nothing. The parietes of the temporal fossa, for example, more curved outward, enlarge the cerebral space, and contract that which the temporal muscle fills. This is a small anatomical fact which is the consequence of a great law of nutrition, viz. of the predominance of the nervous system to which the brain belongs, over the animal muscular, in respect to development. Let us remark that this predominance, whence arises at this age an evident disproportion between the muscular and nervous systems, when compared to what they will be afterwards, would alone prove that the muscles are not, as has been said, a termination and expansion of the nerves; in fact two species of organs whose development is inverse, cannot belong to one and the same system.

Many authors have pretended that the fleshy portion was in proportion much more developed in the fœtus than the tendinous, that this even did not exist. I cannot imagine whence this opinion arose. It may be conceived that they have thought that the aponeuroses of the limbs were wanting in the first months; I have uniformly observed that they have not then that white colour which characterizes them afterwards, a colour that they only take when their fibres are developed; they are transparent, like a serous membrane, and cannot at first sight be perceived. But the tendons have a very evident white colour; we distinguish them very well; they are quite as large and as long in proportion as they will be afterwards.

II. _State of the Muscular System during Growth._

At birth, the muscular system of animal life experiences, like all the others, a remarkable revolution. Until then black blood only penetrated its arteries; then the red blood immediately enters them; for this blood is formed when respiration takes place; now this takes place in almost all its perfection at the very instant the fœtus leaves the womb of the mother. We evidently see besides that the livid tinge of the skin gives place almost immediately to a red colour, which arises from this difference of the blood. This new fluid entering the muscles, is a new cause of excitement, and consequently of motion. Add to this cause the sudden increase of cerebral action. Till then, the brain penetrated with black blood, was as in a kind of inertia, which was principally owing also to the absence of sensations, as I have elsewhere proved. Suddenly the red blood enters it; it stimulates it either by the principles that it contains, or because it was different from that which had penetrated it; for such is the nature of sensibility that it is capable of being affected in an organ, merely because the stimulus that is applied to it is new. Suddenly excited by the red blood, the brain re-acts upon the muscles, and determines them to contract. This cause, joined to the preceding, appears to me to be one of those which have the most influence on the sudden disappearance of the kind of inertia in which the fœtus was, or at least of the small degree of motion that it performed, by the general agitation of its limbs, its abdomen, chest, face, &c.; for immediately after birth all the muscles are moved more or less strongly.

Let us not, however, exaggerate the influence of a cause which is certainly not the only one; for example, the motions of the diaphragm and the pectoral muscles, are certainly prior to the entrance of the red blood in the brain, since their action is necessary to the production of this red blood. These muscles enter into action, because the excitement of the air on the whole exterior of the body, and on the mucous membranes in contact with this fluid, stimulates the brain which is the centre of all sensation. Moved by this excitement, this organ re-acts upon the muscles, and begins to make them contract. The contractions increase, when to this external and indirect excitement is added the internal and direct excitement of which we have just spoken. This second excitement is not absolutely necessary for the fœtus, for we often see infants that remain livid some instants after birth, move very well; but in general the motions are not so decided as when the red colour of the skin indicates the entrance of the arterial blood, which has undergone the influence of respiration.

The entrance of the red blood into the muscles does not give them immediately the colour they will afterwards have. For some time after birth, they have a deep tinge, as dissections clearly prove, because, as I have said, their colour does not come from the colouring portion circulating in their texture, but from that combined with this texture. Now nutrition alone produces the combination; but this function takes place gradually; it is truly a chronic function, in comparison with exhalation, absorption, and the circulation, which are evidently rapid in their progress.

As we advance in age, the muscles assume a redder tinge; more blood penetrates them; they are nourished in proportion more than various other organs. This is particularly remarkable in those of the lower extremities. I would remark, however, that as long as growth continues, it is especially upon the length and not upon the thickness of the muscles, that the energy of nutrition is carried. Hence why they are but slightly visible through the integuments and are scarcely at all prominent; why their forms are rounder and more graceful, but less masculine at this age. The exterior of a young man is in this respect wholly different from that of the adult, by considering each, separate from every cause that can have an influence upon their conformation. The external appearance of the infant and the young man is in general very analogous to that of woman.

Though we do not know so well the difference of the substances which penetrate the muscles in the first years and in the adult age, as we know it in the bones in which the addition of the phosphate of lime to gelatine exhibits a very striking phenomenon, yet we cannot doubt that these differences really exist. Treated by ebullition, combustion, maceration, &c. the flesh of the fœtus does not give the same results as that of the adult.

The broth made with the muscles of a young animal contains much more gelatine, a substance which greatly predominates at this period of life. It has less flavour than that of adult animals. The extractive substance consequently appears to be less. A mawkish, nauseous taste characterizes broths made of veal. The difference of the principles they contain has an influence even upon the gastric organs, of which they excite the contraction; they loosen the belly, as it is called, a phenomenon unknown to common broths. It does not appear that the fibrin is in as great a proportion in the muscles at this period of life; the following considerations make me think so.

1st. Instead of this substance, Fourcroy has found in the blood of the fœtus a soft texture, without consistence, and like gelatine; now the blood appears to be the reservoir of fibrin. 2d. The force and energy of the contractions are in general in proportion to the quantity of this principle contained in the muscles; now this energy is small in the first age. 3d. The muscles burn then, and crisp and contract less than in the adult. I have even two or three times seen their texture, when placed upon live coals, become puffed up like gelatine treated in the same way.

In general it appears that this last substance occupies in the muscles the place the fibrous system is afterwards to hold in them. Those who frequent dissecting rooms, have observed, no doubt, that other things being equal, the muscles of young subjects putrify less quickly than most other substances, and that when they do, they give out a less fetid odour. We know that broth made of veal turns sour more easily than that made of beef. It is always whitish, and never has the deep colour of the broth made with this last. It becomes like jelly much more easily. Young and old roast meats exhibit also great differences. Every kind of stewing either by the fire alone, or in any fluid, is much quicker and easier in the first age. The gravy that is then extracted from the muscles has a character wholly different, it is less strong. The effects of maceration are also more rapid; we obtain sooner that mucous pulp, to which the action of water finally reduces almost all animal substances.

III. _State of the Muscular System after Growth._

After general growth is finished in length, our organs then increase in thickness; and it is especially in the muscles that this phenomenon is remarkable. To the slender and delicate body and round forms of the youth and young man, succeeds a large, strong and thick body with well developed forms. The muscles can be traced through the integuments, eminences and depressions are observed in them; different depressed lines serve as limits to various prominent ones. The animal muscular system is then more prominent in a state of repose, than it is in youth in its greatest motions. Painters and sculptors have studied more than anatomists the different degrees of the development of the muscles.

The period when the hairs grow, when the genital organs begin to become active, is principally that in which the muscles begin to become prominent in man. In woman, this last period does not present a similar phenomenon; the muscles preserve the original roundness, they scarcely ever lose it. In this sex, the roundness of the limbs, their agreeable forms, make a contrast with the kind of rudeness of those of man.

The increase in thickness in the muscles appears to be much more in the fleshy than the tendinous portion, and especially than the aponeurotic. The intermuscular aponeuroses principally do not appear to grow in proportion to the fibres that are inserted into them; so that these make a prominence, and at the place of the aponeurosis there is a depression. This is what we see very well in muscles cut for their insertions by many of these fibrous expansions, in the deltoid in particular. Not only the prominence through the skin of the whole of the muscle, makes the depressions evident that separate it from the others, but each fleshy bundle has a prominence which a groove separates; this, it is true, is only distinguishable upon thin subjects.

As the muscle grows in thickness, it increases in density. It becomes firmer and more resisting. If we place for comparison the hand upon two similar muscles of an adult and an infant, whilst they are in contraction, we feel a sensible difference in their hardness. Weights suspended for comparison to the muscles of the two ages, taken in the dead bodies, prove the different degree of their resistance. The muscular texture of adults yields more slowly to all re-agents.

The colour of the muscles continues to be red in the adult; but in general, and all things being equal in respect to the causes that make this colour vary, it begins to become of a less bright red after the thirtieth year. It is usually in the last years of growth, and even from the tenth to the twentieth, that the colour is the most brilliant.

In the adult this colour exhibits a very remarkable phenomenon. All men have their muscles red, but hardly two have the same shade. Those who have opened many dead bodies are easily convinced of this; a residence at the dissecting rooms proves this assertion. A thousand causes have an influence upon this colour; the temperament is the principal. The external appearance of the muscles without the skin indicates by their shades of colour the temperament, as well as the integuments do. Diseases make this colour vary wonderfully. All those that have a chronic progress alter it remarkably; it then becomes pale, dull, &c. Dropsies whiten it, when they are of long standing. In general, every thing that has upon the powers of life a slow and debilitating influence, diminishes the brightness of it. Acute diseases, whatever may be their nature, change it but little. Fevers with the greatest prostration, if they suddenly produce death, leave it untouched, because this colour can only change by nutrition; now as this function is slow in its phenomena, it is but little affected by acute diseases; it is only at the end of some time that it feels the affections reigning in the economy.

I would observe that the varieties of colour that are seen in the muscles of adults, even in the healthy state, distinguish them especially from those of the fœtus, which have in general an uniform paleness. This difference is owing to the fact, that in the first age, we are not subject to the action of the numerous agents which modify, in an infinitely variable manner in the after ages, the great functions, and of course nutrition which is the end of them. It is in these varieties of colour of the muscular system of the adult, that we clearly distinguish that the blood circulating in the arteries is wholly foreign to it; in fact it is uniform, and never partakes of those varieties of colour whatever they may be.

Many circumstances in the adult make the muscular nutrition vary; motion is the principal. The man who passes his life at rest is remarkable for the small prominence of his muscles, especially if we compare this prominence with that of the muscles of a man who takes great exercise. Not only general motion exhibits this phenomenon, but also local motion, as we see in the arms of bakers, the legs of dancers, the backs of porters, &c.

IV. _State of the Muscular System in Old Age._

In old age, the texture of the muscles changes remarkably; it becomes resisting and stiff; the teeth tear it with difficulty. This too great density is injurious to its contractions, which can now only take place slowly; the action of the brain becomes less upon the muscles; the continuance of their motions is not as long; they are sooner fatigued.