Part 8

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.

II. _Course of the Absorbents._

The absorbents, arising from the different parts that we have pointed out, go in different ways.

1st. In the extremities, they are divided immediately into two very distinct courses, the one superficial, the other deep-seated. The former accompanies at first the sub-cutaneous veins, then runs along in their interstices; so that when injections have succeeded well, the whole exterior of the limbs appears to be covered with a kind of lymphatic net-work. The second goes along the muscular interstices, principally in the course of the arteries and the veins. Both tend towards the superior parts of the limbs. When the vessels arrive there, they approximate each other, and are collected into a bundle, in which they are fewer but larger than below, and which passes through certain openings that lead them into the trunk; for example, those of the superior extremities almost all terminate in the axilla, those of the inferior in the groin and some in the ischiatic notch. Now as it is a general rule, that every absorbent should pass through one or more glands, nature has placed at these openings of communication of the extremities with the trunk, a certain number of these glands. Yet before arriving at them, some have already passed through similar glands placed, in a less number, it is true, in the ham and the bend of the arm. It is in the extremities that the absorbents run the longest course without passing through glands.

2d. In the trunk, the absorbents take at first two courses, the one sub-cutaneous, the other deep-seated, which is found upon the internal surface of the parietes of the cavities, for example, between these parietes and the peritoneum in the abdomen, between these parietes and the pleura in the thorax. The first belongs especially to the fleshy parietes and the abundant cellular texture that is found on them. The second belongs to these parietes and the serous surface that lines them. Besides these absorbents, each viscus contained in the preceding cavities, has deep-seated and superficial ones; the first go into the interior of the organ, we see the second on the surface. This distinction is easily made upon the liver, the spleen, &c. The external absorbents of the parietes of the trunk, run a long course without meeting with glands. Those that are spread on the internal surface of these parietes exhibit a similar arrangement. But those of the viscera hardly come out of them, before they meet these glands, and pass through a great number at a time, because they are very near each other.

3d. There are many absorbents upon the exterior of the cranium; but anatomists have not yet found them in its cavity, which agrees perhaps with the almost total absence of cellular texture in this cavity. There are many on the face, superficially, in the muscular interstices, and around the organs that occupy this region. They descend to the neck, where they find in their course a very great number of glands which they successively pass through.

_Form of the Absorbents in their course._

The absorbents differ essentially from the veins in this, that for a great distance they keep of the same size. Whilst in the venous system the vessels are constantly becoming larger, so that a branch can hardly go a few inches without doubling its size, those in the absorbent system remain for a long time the same. When injected these vessels appear like long white threads running upon the organs.

It follows hence, 1st. that the lymph never circulates like the blood, in considerable masses, but always in very fine streams; 2d. that the absorbents are very numerous; for their number compensates for their size; thus all the surfaces are covered with them, whilst they have but few veins and those of considerable size; 3d. that the absorbent system has not really the form of a tree, like the arterial and venous systems; the manner of division is wholly different. The absorbents are very commonly straight; when they are tortuous, their curves are entirely unlike those of the veins or the arteries. In fact in these last, when the tubes have become as fine as the absorbents, their curves are brought near each other and are small in proportion to the size of the vessel. On the contrary the windings of the absorbents are great; the curves that result from them have often a very considerable extent; they wind in long folds upon the extremities, when they are not straight there.

Viewed externally, the absorbents are not always cylindrical. When filled with injection, they often appear full of knots; this undoubtedly arises principally from the valves. Many authors have represented them as a series of successive contractions; this is however true only to a certain extent.

I have often seen in living animals, in dogs in particular, evident dilatations, a kind of little bladders in the course of a lymphatic, containing serum. It is upon the concave surface of the liver and the gall-bladder, that I have most often observed them. When these bladders are pricked with a lancet, the fluid flows out and they disappear immediately. In making experiments with other views, I once saw two or three of these small dilatations in the neighbourhood of the gall-bladder. Having suffered the liver to fall back, to examine the intestines, I was astonished at not being able to find them an instant after; they disappeared without doubt by the contraction of the vessel. I would remark upon this subject, that the liver is the organ upon which these vessels are best seen in living animals; but it is necessary to examine its concave face the instant the abdomen is opened; for the contact of the air by contracting them, soon prevents their being distinguished.

Besides I believe that in no case are the absorbents as much distended during life by serum, as they are by mercury from injections. When these have succeeded well, we see upon many parts a net-work of very evident vessels. On the contrary, most commonly nothing similar is to be seen in living animals. With whatever promptness we examine most of the surfaces which the serous membranes cover, surfaces that can be laid bare without making the blood flow, nothing is seen, except sometimes small transparent striæ, which soon disappear. Now it is impossible that if the absorbents were as full during life, as they are by injections, but what their transparency contrasted with the colour of the surrounding parts, would render them evident. I have selected however very large dogs, to try to see their course better. I believe that injections double at least the diameter of these vessels.

_Of the Capacity of the Absorbents in their course._

The capacity of the absorbents is remarkably variable; it depends entirely, in the dead body, on the state in which these vessels were in the last moments. In subjects of the same stature and age, they are sometimes very apparent, at others hardly visible. They are double, treble even, in some dropsical patients, what they are in a natural state. Many authors say that they have seen branches almost equal to the thoracic duct, and larger than the trunk of the right side. To be convinced of the extreme variety of the absorbents, without the assistance of injections, take the lymphatic glands in different places, then dissect carefully the parts in the neighbourhood; you will easily find all the absorbents that go to them. Then you will be able to satisfy yourself of the extreme variety of their size; we can even in this way trace them far enough without injection. Sometimes in order to find the end of the thoracic duct, I take a gland in the neighbourhood of the second lumbar vertebra; thus following the empty lymphatic tubes that go from it towards the canal, I find it without difficulty.

When we are not in the habit of finding immediately the absorbents, this method of searching for them by means of the glands, which are always very evident, infallibly succeeds; it cannot be used it is true in the extremities; but in the thorax and especially in the abdomen, it is very convenient. For example, by taking the inguinal glands we can trace these vessels to the thoracic duct, by injecting them, or even without. Some authors have advised making an opening in the gland and placing a tube in it; this rarely succeeds; it is much better to open the vessels that go from the gland, at the place where they go off.

The absorbents usually flat in the dead body, because they are empty, never exhibit in this state a diameter proportional to that which injections give them; whatever may be the varieties of capacity, the fluids that we force into them always increase this capacity. It is this flattening after death, that often makes us in attempting to open them with a lancet, cut through both their parietes, and thus render it more difficult to inject them.

The best proof of the extreme variety of the capacity of the absorbents, is the necessity of choosing particular bodies in order to inject them, the very great difficulties that often take place in finding them in some subjects, whilst they are seen immediately in others, and can be traced in the inferior and superior extremities, through the cellular texture, without having glands for a guide. It is not necessary then, after what has been said, to consider the caliber of the absorbent vessels in a determinate manner. Constantly varying, according to the state of the lymph they contain, they have no standard size to which we can refer their increase or diminution. This is the peculiarity of all the extensible and contractile canals, like those in the animal economy; it is that which prevents us from making any kind of calculation of their capacity.

These varieties of the absorbents are not general as in the veins, all the great trunks of which, for example, are simultaneously dilated when there is an obstruction in the lungs. Here sometimes one only, sometimes many branches enlarge; the others remain contracted. Sometimes the dilatation is general in a part, very often there are remarkable disproportions of capacity in the same vessel; it is double in one place what it is in another, though it has not received branches.

Authors have been much puzzled to determine the capacity of the thoracic duct. I believe it, for it is never found twice the same. These varieties do not depend on the constitution of the subject, but only on the functions, and the state in which these functions are found at death. Whether it be dilated above, contracted in the middle, exhibiting below a little bladder, called by some the reservoir of the chyle, &c. are circumstances the greatest number of which vary incessantly during life, according to the quantity, the nature of the lymph, and the obstacles to its course in this or that part. We find a hundred varieties of the thoracic duct and the absorbents in a hundred different subjects. The same subject has perhaps undergone these hundred varieties at the different periods of his life. If life returned and was destroyed many times in the same man, the venous and absorbent systems would exhibit a number of varieties equal to the number of times he had died.

We see from these considerations to what are reduced all these minute examinations of proportion in the capacity of the vessels, which fill our books of physiology.

If we compare the amount of the veins with those of the absorbents, it is difficult undoubtedly after what has been said, to form any precise idea of it; but we can make approximations. Now the absorbents do not appear hardly inferior to the veins; as to the branches, for example, the whole of the lymphatics of the lower limbs, placed by the side of the capacity of the venous trunks, does not appear much inferior to it. So in all the other parts, the veins being larger, but the absorbents more numerous, the disproportion is not very great.

From this it seems as if there would be but little difference between the trunks that terminate the veins and those that are the terminations of the exhalant system; however this difference is enormous, as we shall see.

_Anastomoses of the Absorbents, in their course._

In the extremities, on the exterior of the trunk and the head, in the intermuscular spaces, &c. the anastomoses are very evident. We see branches of communication going from one absorbent to another; so that we might often say that these vessels are bifurcated. But this appearance is most usually deceptive; for each branch of the bifurcation is almost always as large as the trunk.

Under the serous surfaces, as on the convex face of the liver, the lungs, the spleen, &c. the anastomoses are infinitely more numerous; it is a kind of net-work in the plates of authors; for I confess that I have never injected this portion of the absorbent system.

The anastomoses of the absorbents are made, 1st. from one vessel to another that is contiguous to it; 2d. from the sub-cutaneous divisions to the intermuscular in the extremities, and in the organs, from the sub-serous divisions to those that occupy the interior of these organs. 3d. They take place between the absorbents of the superior regions and those of the inferior; 4th. between those that go to the thoracic duct and those that go to the great lymphatic vessel of the right side, &c.

By these anastomoses we understand how a tube with mercury, being placed in one absorbent, many others around it are filled. They are so much the more necessary, in the system of which we are treating, as the lymph is subject, like the black blood, to an infinite variety of causes of delay in its course, from the want of an agent of impulse at the origin of the absorbents.

Gravity, external motions, different compressions, &c. have upon the motion of this fluid, the same influence as upon that of the veins; gravity especially has much influence. We know that if the powers are a little diminished after long diseases, too long standing renders the legs œdematous; hence why they are always more swelled in the evening than in the morning. As to compression, if it is only moderately great and acts upon many absorbents, it also produces œdema. It is not the size of the surface compressed that has an influence upon this phenomenon; it is only the number of absorbents that pass through this surface. Thus by the head of the humerus being in the axilla, the arm is frequently made to swell, whilst more extensive compressions across the deltoid muscle, where there are fewer absorbents, do not produce this effect.

From these phenomena, it is necessary then that there should be the same means to favour the lymphatic circulation, that there are to aid the venous. These means are especially the anastomoses; it is by them that the first of these circulations is continued, notwithstanding all the external obstacles that our clothes in certain places create, notwithstanding the different pressure that the organs make upon each other. It is only when the whole of the absorbents of a part is compressed, that the motion of the lymph is interrupted. Thus the womb becoming very large in pregnancy, and pressing upon all those of the lower extremities, these extremities become dropsical. I hardly know any organ in the abdomen but this, which by its position can produce these general infiltrations by compression. The liver and all the other organs are not capable of producing a similar phenomenon. When dropsy takes place from an affection of them, it is rather because the functions of the exhalants are increased.

_Remarks upon the Difference of Dropsies that are produced by the increase of exhalation, and those that are the effect of a diminution of absorption._

This leads to an observation that appears to me to be very important in dropsies, viz. the determining when the defect of the action of the absorbents produces them, and when they arise from the increase of that of the exhalants.