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.