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.
IV. Structure of the Absorbents.
This structure, capable only of being seen in the great trunks, the thoracic duct, for example, presents us at first in its common organization, a layer of dense cellular texture, of the same nature as that of which we have already so often spoken, of which we shall speak again, and which is found around the arteries, the veins, the excretories, under the mucous surfaces, &c. &c. This filamentous texture, connected only to a certain degree with the vessel, strengthens it however much, by surrounding it with an external membrane superadded to that which is peculiar to it. If, as Cruikshank has done, we turn this duct inside out, and introduce into it a tube of glass of a diameter a little larger than its own, this last membrane will break. It is as in the arteries, in which a ligature cuts the internal membrane and not the cellular. The same phenomenon takes place from inflation; a much greater effort is necessary then to break the cellular texture, than to rupture the peculiar membrane of the thoracic duct.
No fleshy fibre is observed, in an evident manner at least, in the absorbents. Some authors have admitted that they were there, but injection contradicts them, even as it regards the thoracic duct. Blood vessels probably run over the parietes of the absorbents; in ordinary injections they are often very conspicuous on the thoracic duct. We know not if there are nerves there; there is but little appearance of them, if we may judge by the analogy of the veins, which have a great relation in structure with these vessels.
The internal membrane which forms the peculiar texture of the absorbents is continuous with that of the veins, and forms with it an uninterrupted series of small tubes. Delicate, transparent, it is moistened in the dead body by an unctuous fluid, which is, I believe, unknown to it in the living, as that of the arteries is to those vessels. It adheres to the external membrane by a compact cellular texture, which, as in the veins, is rarely subject to ossification. Mascagni has however mentioned an instance of it in the absorbents of the pelvis. But there is another affection analogous to this, which I have already seen many times in this kind of vessels. Their cavity often contains a white matter, like plaster, especially on the external surface of the lungs. Then without any preparation, the absorbents exhibit almost the appearance which they have when mercury fills them.