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.