The origin of the absorbents can hardly be demonstrated by inspection;
it is like the termination of the exhalants. Such is in fact the
extreme delicacy of these vessels at their origin, in most parts,
that they cannot be seen with the best optical instruments. In some
places we see pores; but it is difficult to distinguish their nature,
whether they are exhalant or absorbent. Their origin then must be
determined by the phenomena they produce in different places. Wherever
absorption takes place, it is evident that there they begin. Now in
examining attentively the phenomena of absorptions, we see that they
are discoverable whenever there are exhalations; so that the same table
may serve both for the absorbents and the exhalants; the following is
the table for the first.
┌1st. exterior, arising ┌1st. mucous.
│ from the systems └2d. dermoid.
│
│ ┌1st. serous.
│ │2d. cellular, ┌1st. fat.
│ │ and taking │
│ │ up there, └2d. serum.
ABSORBENTS.│2d. interior, │
│ arising from │ ┌1st. the short and flat
│ the systems │ │ bones, and the extremities
│ │3d. medullary │ of the long ones.
│ │ │2d. the middle of the
│ │ └ long bones.
│ │
│ └4th. synovial.┌1st. the articulations.
└3d. of nutrition. └2d. the tendinous grooves.
Let us examine these different absorptions, of which I shall not give
the proofs here in detail, because these proofs will be shown in each
system from which the absorbents arise. 1st. The external absorptions
do not correspond precisely with the exhalations of the same nature.
In fact, neither the sweat nor insensible transpiration exhaled by
the skin, are taken up by the cutaneous absorbents; these fluids
are excrementitious. So the mucous absorbents allow the pulmonary
transpiration to evaporate, and the other fluids exhaled upon their
surface, to mix with the aliments and afterwards to pass off. It is the
substances contained in the atmosphere, in the surrounding bodies, &c.
that this kind of vessels takes up by a very irregular absorption, as
we shall see, except however that of the chyle, which is not made in a
continuous manner, which is subject to great intermissions, and which
at other times takes place with remarkable activity.
2d. The internal absorptions, on the contrary, every where correspond
with the analogous exhalations. Thus the absorbents take up, upon the
serous system, the serum, upon the cellular system the serum and fat,
upon the medullary system the marrow, upon the synovial system, the
synovia; fluids, all of which as we have seen, had been thrown out by
exhalation upon their respective surfaces, and had remained for an
instant upon them. These absorptions are made in a constant and regular
manner; it is in this that they differ from the preceding. The internal
absorbents, incessantly in action, take up in a given time the same
quantity of fluids; their action corresponds precisely with that of the
exhalants. Observe that there are two essential differences between the
external and internal absorptions; viz. that the one are exerted on the
one hand on fluids different from those exhaled upon their surfaces,
and that on the other they are subject to continual variations and
irregularities; whilst the others always take up the fluids exhaled
upon their surfaces, and are constant and regular, at least in a state
of health. I shall point out in the mucous and cutaneous, systems, the
cause of this important difference.
3d. As to the nutritive absorptions, we know much less of them than
the preceding; but nutrition evidently supposes them. There is in fact
in this function a double motion, one of composition, the other of
decomposition. No organ, no part of an organ is formed at one period
of the same elements that composed it at a former one. The ancients
thought, without positive proof, that the body was renewed every seven
years. Whatever may be the period of its renewal, we cannot deny that
it is continually composed and decomposed; now the exhalants perform
the first nutritive motion, the absorbents the second. Observe in
fact, that the internal substances never re-enter the circulation to be
afterwards thrown out, except by the way of the absorbents.
The nutritive absorptions differ then from the preceding, in this, that
the substance deposited by exhalation and taken up by them, remains in
the organs, makes a part of them, and contributes to their composition;
whilst the fluids with which the internal exhalations and absorptions
are concerned, after having been furnished by the one, and before being
taken up by the other, remain out of the organs, upon their surface or
in their cells, but do not make a part of their structure.
It is difficult perhaps to conceive how solid nutritive substances
can be absorbed by vessels so delicate. Hunter, to whom anatomy owes
much both as it respects the absorbents and their uses, has already
removed this objection. It may be added to what he has said, that the
distinction between solids and fluids is not substantial except when
they are in mass; but when they are considered as separate particles,
they do not differ; this is so true, that the same particle makes
alternately part of a solid and a fluid, as in water that is not frozen
and that which is, as in solid or melted lead, &c. Now the nutritive
substances are absorbed particle by particle; then the distinction of
fluid and solid is of no consequence in the function of absorption.
Since the origin of the absorbents is beyond the reach of our senses,
it is difficult, it is impossible even to determine the manner in which
they arise, the peculiar structure which distinguishes them at their
origin, their communications, &c. They undoubtedly differ essentially
according to the mucous, cutaneous, serous, synovial, cellular,
medullary surfaces to which they belong; undoubtedly the nutritive
absorbents differ remarkably from the others; but nothing can be proved
by inspection. What has not been said upon the villous coat of the
intestines considered as the origin of the lacteals, upon their small
bladders, upon the form of the pores of the peritoneum, the pleura,
&c. upon the cellular sponge? I shall not notice here these anatomical
hypotheses, which have been made by an abuse of the microscope, and
which besides, if they had any real foundation, would not lead to any
inference useful to science.
Do the absorbents arise from the capillary system? Judging by
injections, it would seem that they did, for many distinguished
anatomists, by forcing a fine injection through the arteries, have
filled the absorbents in the neighbourhood. I never saw any thing
similar to it myself, yet I am far from denying a fact attested by
Meckel. If many other experiments should confirm it, it would be
established incontestably, it is evident, that the origin of the
absorbents is in the capillary system, as the origin of the excretories
and exhalants are proved to be in the same system. Besides, the
phenomena of absorptions cannot give us any light upon the mode of the
origin of the absorbents.
Where they go off from the surfaces or the organs from which they
arise, the absorbents are extremely delicate; they elude all kinds of
injection. They appear to anastomose with each other, interlace, form a
complicated net-work, which contributes much to the structure of some
parts, especially of the serous membranes. We know however but little
of this interlacing. It is not until they have run a certain course,
that these vessels are cognizable by our senses, and that we can
consequently study them in a general manner.