Note 27 — Chapter VII · GENERAL DIFFERENCES OF THE TWO LIVES WITH RESPECT TO VITAL POWER.

All that is here said of the sensibility of the lymphatic vessels, which makes them sometimes admit and sometimes reject the effused fluids, is the more hypothetical, as it is not as yet proved that these vessels are the agents of absorption. It should be remarked, that the fluids that are supposed to be absorbed by them, differ essentially in their chemical composition, from the fluid that is usually found in their cavity. This fluid besides varies but very little in its composition, though its appearance is not uniformly the same; now, if it were the result of the absorption of fluids differing from each other, its composition ought also to vary as that of the chyle does, according to the nature of the aliments.

Before the lymphatic vessels were known, the principal phenomena of absorption were observed, and it was natural to attribute them to the action of the veins. This opinion was maintained for a long time after the discovery of the lymphatics. Finally, towards the middle of the last century, Hunter being engaged in examining these vessels, which he has done more to make known than any other man, thought that they should be considered as the agents of absorption, and this opinion was soon generally admitted. If we look for the means by which he overthrew the ancient theory, we are astonished to find that it was by five experiments only. Harvey did not with equal facility obtain the acknowledgment of the circulation, and perhaps there does not exist a second example of an opinion, which was for a long time established, being abandoned so readily. It should be remarked, that physiologists had not yet recovered from the surprise produced by the discovery of a system of vessels so extensive, and yet for so long a time unknown; they were impatient to know the use of them; the veins had already the function of returning to the heart the blood brought by the arteries; they thought it would not impoverish them too much to deprive them of the faculty of absorbing, in order to enrich the lymphatics with it. Of the five experiments of Hunter, two are designed to prove that the veins do not absorb, the object of the other three is to show that the lymphatics do.

In the first experiment he injected tepid water into a portion of intestine, and the blood which returned by the vein appeared to be neither more diluted nor lighter than before. We cannot conceive how by mere inspection, it is possible to judge if the blood contains a certain quantity of absorbed water, a quantity which must be proportionably very small, if we consider the whole amount of blood that passes through the mesentric veins during the period necessary for the absorption of the fluid. Hunter in the same experiment tied the artery which went to the portion of intestine, and examined the state of the vein. It did not swell, and its blood did not become aqueous. But after this ligature, did the absorption continue to go on in this portion of intestine, which still had no doubt lymphatic vessels? This the author does not say. How moreover should he think that the vein could continue its action when the artery was tied?

In the second experiment Hunter injected milk into a portion of intestine, and was unable to discover this fluid in the blood of the mesentric veins; but at the period in which this experiment was made, mankind were very far from being able to detect in the blood a very small quantity of milk, and at the present day, with all the aid derived from chemistry, we can hardly discover in it a small quantity which is mixed directly with it. These two experiments prove then nothing against the absorption of the veins; as to those which he brings forward in favour of absorption by the lymphatics, they are not more conclusive. I shall content myself with relating one of them. He injected, into a portion of intestine that was empty, a certain quantity of warm milk, and confined it there by two ligatures. The veins that came from this portion were emptied of their blood by several punctures made in their trunk. The corresponding arteries were tied. He then returned the parts into the abdomen, and drew them out again in half an hour. Having examined them with attention, he observed that the veins were almost empty, and that they contained no white fluid, whilst the lacteals were almost full of it. But was not this white fluid that filled them chyle rather than milk? Was it not there before the injection of this liquid? In order to ascertain what takes place in the lymphatic vessels during absorption, we must begin by examining the state of these vessels before the experiment. But this is what Hunter did not do, and it is this that renders his experiment of no value. It is not very astonishing that he mistook the chyle for the milk, since milk has for a long time been mistaken for chyle. Flandrin, Professor of the Veterinary School at Alfort, has several times repeated this experiment of Hunter; but he took care before the injection of the milk to ascertain that the lymphatics contained no white fluid; and he never found any in their cavity after the experiment. I have myself many times performed this experiment, with the same precaution, and I have uniformly obtained the same results as those of Flandrin.

It would occupy too much time to examine all the reasons that have been advanced for and against the absorption of the lymphatics; I shall only relate some experiments I have made myself; but I ought first to observe, that absorption undoubtedly takes place in parts such as the eye, the brain, and the placenta in which the most minute dissection has been unable to discover any lymphatic vessel.

First experiment.—Four ounces of the decoction of rhubarb was given to a dog, in half an hour after he was killed, and it was found that more than half of the liquid had disappeared; the urine evidently contained rhubarb, but the lymph in the thoracic duct exhibited no trace of it.

Second experiment.—A dog swallowed several ounces of alcohol diluted with water; at the end of a quarter of an hour, the blood of the animal had a very distinct odour of alcohol, but there was nothing of the kind in the lymph.

Flandrin made a similar experiment on a horse, to whom he gave half a pound of assafetida mixed with an equal quantity of honey. Six hours after, the horse was killed. The odour of the assafetida was very perceptible in the blood of the veins of the stomach, of the small intestines and the cœcum; but it could not be perceived in the lymph.

Third experiment.—A dog was made to swallow six ounces of a solution of Prussiate of Potash in water. In a quarter of an hour, the urine very evidently contained some of the Prussiate; but the lymph taken from the thoracic duct showed no appearance of it.

Fourth experiment.—I gave to a dog, in whom I had tied the thoracic duct, two ounces of a decoction of nux vomica. The effects of absorption were as rapid as if the duct had been open. After the death of the animal I satisfied myself, that the duct had been well tied, and that there was no other branch, as there sometimes is, by which the lymph could get to the subclavian vein.

I have varied this experiment by putting the poisonous fluid, into the rectum, the sacs of the pleura and peritoneum. The results have been uniformly the same.

Fifth experiment.—M. Delille and myself made an incision into the abdominal parietes of a dog, who had been fed very heartily some hours before, so that the lacteals might be easily seen, and we then drew out a portion of the small intestine upon which we applied two ligatures three inches from each other. The lymphatics that went from this portion of intestine were full of chyle and very distinct. They were all tied and cut. The blood vessels were also tied and cut, with the exception of an artery and a vein; the portion of intestine also was cut off beyond the ligatures, and thus it had no communication with the rest of the animal except by the vein and artery which were left. These two vessels were dissected with the greatest care, and even stripped of their cellular coat, lest there might be some lymphatics concealed in it; we then injected into the cavity of this portion of intestine a decoction of nux vomica, and we retained it there by means of a new ligature. This portion of intestine, covered with fine linen, was restored to the abdomen; six minutes after, the effects of the poison were manifested with their usual intensity.

Sixth experiment.—M. Delille and myself separated the thigh of a dog from his body, leaving only the crural artery and vein, which kept up the communication between the two parts. These two vessels were dissected with care, insulated to an extent of from two to three inches, and even stripped of their cellular coat, for fear it might conceal some small lymphatic vessel. Two grains of a very active poison (the upas) were then inserted into the paw, and the effects were as sudden and as intense as if the thigh had not been separated from the body.

As it might be objected, that notwithstanding all the precautions taken, the parietes of the artery or vein might still contain some lymphatic, we varied our experiment so as to leave no doubt on this point. The artery was cut entirely off, the communication was reestablished between the two ends, by means of a leaden tube introduced into their cavity, and fixed by proper ligatures. The same was done for the vein. Thus there was no longer any communication between the thigh and the rest of the body, except by the arterial blood which came to the thigh, and by the venous blood which returned to the trunk: the poison afterwards introduced into the paw produced its effects in the ordinary time, that is in about four minutes.

From these different experiments, it is right to conclude that the minute branches of the veins possess the power of absorbing; that they exert it on the surface of the mucous and serous membranes, and in the interior of the organs; that the experiments that have been quoted in favour of the absorption of the lymphatics are inaccurate or incorrectly understood, and finally that there is no proof that these vessels absorb any thing but chyle.

Is it now necessary to refer to the venous branches this sensibility that has been attributed to the ultimate ramifications of the lymphatics? But this sensibility, as we have already said, would be constantly in error; the absorbent vessel does not select one fluid in preference to another; all are indiscriminately absorbed, even the most irritating, those in fact whose action is sufficiently powerful to destroy the vascular parietes. Besides, the phenomenon then continues, when it is no longer possible to suppose the existence of this sensibility. After death even, the venous branches absorb still as they do during life, if they are placed in analogous circumstances; and to do this it is evident, that an internal current must be established, which resembles the course of the blood. I shall now relate an experiment, which I made on this subject, and which I selected from many others, because it appeared to me to be very conclusive.

I took the heart of a dog that had died the day before; I injected into one of the coronary arteries some water of the temperature of 30 degrees of the centigrade thermometer. This water returned easily by the coronary vein to the right auricle, whence it flowed into a vessel or dish. I poured half an ounce of slightly acid water into the pericardium. At first the injected water exhibited no sign of acidity; but in five or six minutes it presented unequivocal marks of it.

Absorption then can take place without the assistance of this sensibility, as well as of this insensible organic mobility, which is supposed to be in the ultimate vascular extremities, in the absorbing mouths, as they are called. But do these mouths really exist? Do the last capillary branches terminate abruptly with a large opening on the surface of the membranes or in the texture of the organs? Can the absorbed fluids pass through their parietes as oxygen does in the lungs to arrive at the blood which it modifies? We are unable to make experiments on these small vessels, that are not cognizable by our senses; let us make them on the large ones, and if they permit fluids, in which they are immersed, to pass through them, for a stronger reason we may suppose that it takes place in the capillaries, whose parietes are so much more delicate and consequently more permeable. Now we have confirmed by experiments what we had suspected; the first attempts were made on dead vessels.

I took a portion of the external jugular vein of a dog; I stripped it of the surrounding cellular texture; I attached to each of its extremities a glass tube by means of which I established a current of warm water through its interior; I then immersed the vein into a liquor slightly acid.

It is seen by the arrangement of the apparatus that there could not be any communication between the internal current of warm water and the external acid liquor.

During the first minutes the liquid that I collected did not change its nature; but after five or six minutes the water became perceptibly acid; absorption had taken place.

The same experiment was repeated on veins taken from human subjects; the effect was the same; it was the same also with the arteries, but a little slower from the greater thickness of their coats.

It remained to be seen if in a living animal absorption thus took place through the parietes of a large vessel. I know that the textures that were permeable after death, are almost all so during life, though the contrary is generally believed. If we inject into the pleura of a living animal a certain quantity of ink, at the end of an hour, and often sooner, we shall find the pleura, the pericardium, the intercostal muscles, and the surface of the heart itself, evidently of a black colour. It is true that the signs of this exudation are not always apparent. Thus after death, the transudation of the gall bladder is rendered evident by the colouring of the neighbouring parts. During life, on the contrary, as fast as the colouring particles are deposited, they are absorbed by the serous membrane which covers the surrounding parts, and carried off by the sanguineous current which runs through this membrane and the subjacent organs.

From these considerations we must believe that absorption may take place through the parietes of the vessel during life as after death. To be satisfied of this I made the following experiment:

I took a young dog of about six weeks old. At this age the vascular parietes are delicate, and consequently more likely to render the experiment successful. I laid bare one of the jugular veins; I insulated it perfectly in its whole length; I stripped off carefully every thing which covered it, and especially the cellular texture and some small vessels that ramified on it; I placed it on a card, that it might not be in contact with the surrounding parts; I then let fall, on its surface and opposite the middle of the card, a thick aqueous solution of an alcoholic extract of nux vomica, a substance the action of which is very powerful on dogs; I took care that none of the poison could touch any thing but the vein and the card, and that the course of blood was free in the interior of the vessel. Before the fourth minute, the effects that I expected appeared, at first feeble, but afterwards with so much power as to render inflation of the lungs necessary to prevent the death of the animal. I repeated this experiment on an adult animal of a much larger size than the preceding one; the same effects appeared but slower, on account of the greater thickness of the parietes; they began to appear in fact after the tenth minute.

After satisfying myself with this result respecting the veins, I thought I would ascertain if the arteries exhibited analogous properties. These vessels are in a less favourable condition; their texture is less spongy than that of the veins and with an equal caliber, their parietes are much thicker. It was easy then to foresee, that if the phenomenon of absorption showed itself, it would appear much slower than in the veins; this was confirmed in an experiment on two large rabbits, in whom I dissected perfectly clean one of the carotid arteries. It was more than a quarter of an hour before the solution of nux vomica passed through the parietes of the artery. As soon as I saw the symptoms of poisoning distinctly, I stopped moistening the vessel; yet one of the rabbits died. In order then to convince myself that the poison had really passed through the arterial parietes, and that it had not been absorbed by small veins which might have escaped my dissection, I carefully detached the vessel that had been used in the experiment; I cut it open in its whole extent, and I made those who assisted me taste a little of the blood, that was still adhering to the internal surface; they all perceived in it, and I did myself, the extreme bitterness of the extract of the nux vomica.

To these experiments may be objected a fact that is observed, which is, that absorption does not take place the same under all circumstances; its activity is redoubled or diminished, according to the state of some other functions. Thus during a paroxysm of fever, a medicine, which would usually act with great effect, often produces, when given in a double or treble dose, no perceptible effect. Now if absorption, was a purely mechanical phenomenon, would it undergo modifications in relation with those of the vital functions? Without doubt it would; for these modifications of the functions may introduce new physical circumstances favourable or injurious to the production of a mechanical phenomenon. Thus in the present case, the state of fever, by accelerating the circulation distends with blood the arteries and the veins. The fluid that is to be absorbed must pass from the exterior to the interior of these vessels. Now it may be easily conceived, that the quantity of blood which they contain must have a great influence upon the production of the phenomenon by the greater or less degree of tension of their parietes. This is moreover completely confirmed by experiment.

We can, without producing a very great disturbance in the functions, increase at pleasure the quantity of fluid which passes through the blood-vessels, by carefully injecting into the veins water the temperature of which is near that of the blood. An artificial plethora is thus produced, followed by very curious phenomena, of which I shall have occasion hereafter to speak. One day while making this experiment, the idea occurred to me of seeing what influence the plethora thus produced would exert upon the phenomenon of absorption.

In consequence, after having injected into the veins of a dog of middle size about a quart of water, I placed in the pleura a small dose of a substance, the effects of which were well known to me. These effects did not show themselves till many minutes after the period in which they usually appear. I soon made the same experiment on another animal with the same result.

In many other trials the effects showed themselves at the period in which they ought to have appeared; but they were evidently weaker and prolonged much beyond the ordinary time.

Finally, in another experiment in which I had introduced as much water as the animal could bear and live, the effects did not appear at all. I waited nearly half an hour for effects which commonly show themselves in two or three minutes. Presuming then that the distension of the vessels prevented the absorption, I endeavoured to satisfy myself of it, by seeing if after the distension had ceased, absorption would be any longer prevented. In consequence, I bled the animal copiously from the jugular, and I saw, with the greatest satisfaction, the effects appearing as the blood flowed out.

It was proper to make the opposite experiment, that is to say to diminish the quantity of blood, in order to see if absorption would take place sooner. This took place in fact, as I thought it would; about half a pound of blood was taken from an animal; the effects, which did not usually appear till after the second minute, showed themselves in thirty seconds.

Yet it might still be suspected, that it was less the distension of the blood-vessels than the change of the nature of the blood that opposed absorption. To remove this difficulty I made the following experiment; a dog was bled copiously; the place of the blood which he had lost was supplied by water at the temperature of 40 degrees of the centigrade thermometer, and a certain quantity of a solution of nux vomica was introduced into the pleura. The consequences of it were as prompt and as powerful, as if the nature of the blood had not been changed; it was then to the distension of the vessels that must be attributed the want or diminution of absorption.

The consequences that may be deduced from the experiments I have just related will acquire new force, if we connect with these facts a multitude of pathological ones, which are every day seen; such as the cure of dropsies, engorgements and inflammations by bleeding; the evident want of action of medicines at the moment of a violent fever, when the vascular system is powerfully distended; the practice of certain physicians who purge and bleed their patients before administering active medicines to them; the employment of cinchona at the period of remission for the cure of intermittent fevers; general or partial oedema from organic disease of the heart or lungs, and the application of a ligature upon the extremities after a puncture or a bite of a venomous animal, to prevent the deleterious effects which are the consequence of it.

On the whole, I think, it may be concluded from the preceding experiments that the capillary attraction of the small vessels is one of the principal causes of the absorption called venous. If the lymphatics do not appear to enjoy in the same manner the faculty of absorption, it probably arises not from the nature of the parietes, the physical properties of which are nearly the same as those of the veins, but from the want of a continuous current in their interior.

In this note I have brought together the absorption of the gases and that of fluids. This resemblance holds only as it relates to the permeability of the textures by these two orders of bodies. As to the cause of the absorption of the two, it cannot be the same, since gases are not subjected to capillary attraction.[28]