In the fœtus and in childhood, absorption relative to nutrition is not
in proportion to exhalation. Many substances remain in the organs, but
few go out, hence there is growth.
But little is known of the differences which the internal absorptions
of synovia, serum, fat, marrow, &c. then present.
The external absorptions appear to be more active, for we know that
contagions are taken with much more facility in the first age. We
know not however whether the skin and mucous surfaces then constantly
introduce more foreign substances into the body, or if they are only
more disposed to introduce them.
We are deficient in positive data as to the state in which absorption
is found in childhood. To judge of it however by that of the lymphatic
glands, it would appear that it was very energetic. In fact these
glands are in proportion much developed; they appear to be the seat of
very active functions; they have a peculiar life more developed than
afterwards, hence a greater disposition to diseases. We know that until
puberty, or rather until the end of growth, they are the seat of many
affections which entirely disappear after that age, and lessen the
numerous series of those to which we are exposed.
This double circumstance, 1st, the precocious and proportionably great
development of the lymphatic glands in childhood; 2d, their very great
disposition to diseases, indicates certainly remarkable activity in
their functions; for it supposes a great development of the vital
forces; now these vital forces being more developed ought necessarily
to preside over more energetic functions. See in fact the organs
whose functions we know, which are on the one hand much developed in
childhood, and on the other much disposed to diseases; the functions
of these organs are more active. Thus the brain and nerves being more
developed, have more sensibility; thus the vessels with red blood have
an activity of nutrition, in proportion to their size, &c. In youth,
it is when the genital organs are more developed and they become more
exposed to diseases, that their functions are greater. Examine all
the organs and their functions, you will see that a general law of
the economy is, that these three things, 1st, great development; 2d,
greater disposition to diseases; 3d, greater activity of functions, are
constantly united. Now when these two first exist in the glands of the
absorbents, we ought to conclude that the third is there also, though
we are not certain of it, since, from what I have said, we are ignorant
of the uses of these small organs. Grimaud has considered them, it is
true, as essential to nutrition; he even calls the nutritive system
the combination of these glands and the cellular texture, a gratuitous
supposition, which is not proved. All that we know upon this point, is
that on the one hand nutrition, and the development of these glands on
the other, are very great in the fœtus. But does it follow from this,
that the first phenomenon proceeds from the second? Undoubtedly not;
no more than because the brain, the liver, &c. are early developed in
the fœtus, and nutrition is very active, they should be considered as
the agents of this function. Besides, nutrition is a function that has
no particular organ for its centre and agent. Each organ is itself
the machine which separates from the blood or the fluids that enter
it, the nutritive materials that are suitable for it, and afterwards
appropriate them to itself. The muscle separates its fibrin, the bone
its phosphate of lime, &c. But one common and central organ does not
elaborate these nutritive materials, as one common viscus moves the
blood, as one central organ presides over sensibility, &c.
As to the anatomical state of the absorbents in the fœtus and
childhood, we can know but little; I do not know that any author has
injected them comparatively in this age and in the adult. I have but
one fact upon this point, it is that the lacteals, examined in an
experiment upon two young dogs, who had only left off sucking eight
days, appeared to me larger in proportion than at a more advanced age.
I will make one remark that has often struck me; it is that the size
of the animal has much less influence than would be thought upon the
diameter of these vessels. For example, an adult dog, twice as large
as another, has not by a great deal, vessels of double size. Accident
led me to examine them the same day, three years since, upon two large
grey-hounds, that were among the dogs brought to me, and upon one of
those vulgarly called cur dogs; they were nearly equal in all three;
this struck me.
We know but little of the different revolutions that absorption
undergoes in the ages that succeed infancy. Only there is no doubt that
puberty is the limit of this kind of predominance which the lymphatic
glands enjoyed in the economy. The age of their diseases is then
passed; often even these diseases, heretofore beyond the reach of art,
are spontaneously cured. The predominance of the genital organs which
succeeds to this and some others, those of the sensitive-organs, &c.
seems to destroy the germ which this first supported.
Soemmering has described in a particular work the part that the
absorbents perform in the different diseases of the adult and the other
ages. This part has appeared to me to be often very difficult to be
known, notwithstanding what he has said of it. I refer however to his
work on this point.
In old age, nutritive absorption continues very active; for it is that
which decomposes the body, which takes from it the substances that
nourish it, and which consequently withers and dries the organs.
The external absorptions, on the contrary, are feeble; the skin takes
with great difficulty the different contagions, as I shall say in
treating of this organ; the mucous surfaces absorb slowly; but little
chyle passes into the blood in proportion to what enters it in the
adult. The two absorptions, the nutritive and the external, are exactly
reversed at the two extreme ages of life; the second is superior to
the first in infancy, the first predominates in old age.
As to the internal absorptions, as those of synovia, of the serous
surfaces, of the cellular texture, &c. I should think that they
predominated in old age, and that to this was to be attributed the
numerous serous effusions which happen at that age, and which we
observe in dead bodies. We have not however upon this point so good
data as upon the other two.