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.