All the known absorbents unite into two principal trunks. One of
these, which is the thoracic duct, receives all those from the lower
extremities, abdomen, the greater part of the thorax, and those of the
left side of the superior parts. The other is formed by the union of
the absorbents of the right side of the superior parts, as well of the
head, as the extremities, and of some of those of the thorax. These
two principal trunks go into the vena cava superior; around them, many
smaller branches also terminate there.
If we examine but little the number of the absorbents distributed
in all the parts, it will be easily understood, how enormous the
disproportion of their capacity is with that of these two trunks. How
is it that all the serum contained upon the serous surfaces and in the
cellular texture, that all the residuum of nutrition, that all the fat,
the medullary fluid, and synovia, that all the drinks, all the product
of the solid aliments that constantly enter the circulation, can pass,
in order to get there, through vessels so small? This observation has
struck all authors; and it is, I confess, very difficult to explain.
In fact, 1st. When there is a disproportion in the capacity of the
blood vessels, it is compensated for by an increase of velocity where
the caliber is less; thus, though the capacity of the veins exceeds
that of the pulmonary artery, still all the blood of the first passes
through the second. Now if we examine in a dog the thoracic duct during
digestion, which can easily be done by opening quickly the thorax on
the right side, raising the lungs of that side, and by cutting the
pleura along the aorta, which allows you to see immediately this canal
then very white on account of the chyle that is passing through it, if,
I say, we examine the thoracic duct in action, we shall see that the
circulation goes on there nearly as in the veins. By opening it then,
a more powerful throw of fluid does not indicate a greater velocity
than that of the venous blood. 2d. It might perhaps be said that during
life the thoracic duct is sufficiently dilated to correspond to all the
absorbents; but observation proves precisely the contrary. The thoracic
duct, full of chyle, is undoubtedly a little more dilated than in the
dead body; but I have satisfied myself repeatedly that the difference
is not very great. 3d. By supposing that a great quantity of fluids
passes through the thoracic duct, notwithstanding its size, the vena
cava superior ought to be proportionably dilated between it and the
heart; yet it continues nearly of the same size after having received
this canal. 4th. Hewson, by taking the fluid of the lymphatics, has
proved that it was analogous to that of the serous surfaces; its
transparency, when examined in the vessels of a living animal, makes me
also believe it, though this would not be a conclusive reason. How can
a fluid of the same kind, result from the combination of such different
elements, viz. of those which compose the mucous, cutaneous, nutritive,
fatty absorptions, &c.
I confess that the different substances that enter the black blood by
the thoracic duct and the one corresponding to it, may enter it at
different times; that the lymph, the fat, the chyle can each have their
time of passing. But first this explanation is not supported by any
fact, and the disproportion would also then be very great.
Many distinguished anatomists have thought that the veins absorb,
and in relation, to their use, they have joined these vessels to the
lymphatics. Haller, Meckel, and before them, Kaw Boerhaave, were of
this opinion. Such names deserve undoubtedly an examination of the
reasons advanced; let us now consider these reasons. 1st. The thoracic
duct has been seen obliterated, and absorption continuing to go on,
while life was preserved in the animal. But as they have not observed
whether the great right lymphatic and its accessories were obliterated,
nothing can be concluded from this fact. Besides the observations upon
this point do not appear to me to be well ascertained. They could
decide this question very easily, I think, by tying during digestion,
the thoracic duct at its entrance into the jugular; it could be
readily come at on the inferior part of the neck, where it could be
distinguished by its whiteness; no important part need be wounded.
This experiment would throw great light upon the general question of
absorptions. 2d. Fine injections, made through the mesenteric vein,
cover the peritoneum; hence it has been concluded that the absorbents
terminate in this vein. But as the venous extremities communicate
with the capillary system, and as this gives rise to the exhalants,
injections, by going through its numerous anastomoses, can easily be
spread in this way, which vitality shuts during life, but which the
flaccidity of the parts and the absence of sensibility open after
death. 3d. Compression of the superficial veins produces swelling of
the limbs; but as this compression is made at the same time upon the
absorbents, no inference respecting venous absorption can be drawn from
it. 4th. Kaw Boerhaave having introduced water into the intestinal
canal, it was afterwards found in the mesenteric veins; but this
experiment has been many times repeated since without giving the same
result. 5th. Add to these considerations the numerous experiments of
Dr. Hunter, to prove that venous absorption does not take place on the
surface of the intestines, and you will see that this absorption will
appear very uncertain, in these first respects.
But if you look at the question in other respects, you will be unable
to deny that certain facts present probabilities in favour of this
absorption. 1st. It is almost certain, that the venous extremities
take up by the way of absorption, the blood effused in the corpus
cavernosum. 2d. We do not see absorbents in the placenta, and yet the
umbilical vein takes up all the fluids of this body. 3d. Meckel having
injected a lymphatic vessel that went to a gland, the injected mercury
passed into a neighbouring vein.
All these observations throw great obscurity upon the termination of
the absorbents. I think that if on the one hand, we cannot doubt that
the greatest number of these vessels, those especially that come from
the serous surfaces, from the cellular texture, from the intestines,
have known terminations, we ought on the other to suspend our judgment
as to the manner in which the others terminate, and that the question
must remain wholly undecided upon this point, till it has been settled
by new experiments. Here, as in so many other points, physiology
has need of great light. 1st. The enormous disproportion between
the absorbents and their common trunks; 2d, the impossibility of
understanding from the analogy of the veins, the lymphatic circulation,
with the apparatus that injections exhibit as its vessels; 3d, many
probabilities against and many in favour of venous absorption; 4th, no
other known way for the fluids which enter the blood by the absorbents,
than the trunks noticed above. There is nothing but obscurity and
contradiction in the different data which would assist us to resolve
this problem.