Part 35

The veins of the inferior parts are generally more dilated in old age than those of the superior; this arises from the habitual weight of the column of blood, which constantly acting, produces finally a real effect; for, as we have said, the venous circulation is much influenced by mechanical causes, owing to the want of power in the cause that circulates it; hence why varices are infinitely more frequent in the inferior than the superior parts, in which they are hardly ever found.

In women who have had many children, we see this dilatation of the veins of the inferior parts in a very evident manner; very often there are varices in them. Observe that this disease seems to be the companion of old age more particularly than that of every other age. On the contrary, we rarely see aneurisms in old people. The rupture of the veins has been almost constantly observed at this or the adult age. I hardly know an example of it in infancy.

The pulmonary artery does not dilate in old age in proportion to the veins; because, removed from the action of foreign bodies, and provided at its origin with an agent of impulse formed of a firm and resisting texture, it has not been in the habit of yielding like them.

IV. _Accidental development of the Veins._

The veins are accidentally developed in two ways. 1st. In cancerous tumours, in fungi, &c. in which more red blood enters, they acquire a size in proportion to that of the arteries; now, as they are superficial, we see more easily their increase than that of the arteries; this increase, which has been taken for a characteristic of cancers and other analogous tumours, is only a consequence of the increase of nutrition. The motion of the blood is as rapid there as in the other veins; there is no obstruction to it. 2d. There are cases on the other hand, in which the veins dilate, because the blood cannot easily circulate in them, and because the velocity of its course is diminished. For example, the whole venous system of the abdominal parietes is often increased in ascites; it is not because there is more blood circulated; there is less than in the ordinary state; but it is because the venous parietes having in part lost their elasticity, like the neighbouring parts, the circulation becomes slower; now the slower it is, the more the blood accumulates and the more the venous parietes are dilated. It is then a kind of general varix in a division of the veins. There is not more blood brought by the arteries, as in the preceding case; the same thing in part happens in old age.

ARTICLE FIFTH.

REMARKS UPON THE PULMONARY ARTERY AND VEINS.

Though in the exposition of the two systems of black and red blood, I have considered the pulmonary artery as making a part with the veins, and the pulmonary veins as a continuation of the arteries, yet their nature is wholly different. There are in truth but two general membranes, forming the two great tubes in which are contained the two kinds of blood, which are every where of the same nature, from the capillary system to the pulmonary. The textures added to the exterior of these two common membranes are essentially different. Thus the texture of the pulmonary artery, though added to the membrane with black blood, is, in point of thickness nearly of the same nature as that of the aorta and its divisions. So the texture of the pulmonary veins, though united to the membrane with red blood, is the same as that of the other veins.

This uniformity in texture supposes an uniformity in the functions, and this is really the case. The mechanical laws of the circulation of black blood are the same in the pulmonary artery as those of the red blood in the aorta. So the laws of the general venous circulation are the same with those of the pulmonary veins; inspection proves this; and, moreover, it must be so, since the relation of the heart to the two kinds of vessels, the veins and the arteries, is the same.

Each system of blood, then, has its two modes of circulation. Sudden motion, generally communicated, and not the progressive undulation of the fluid; a pulsation by a real locomotion, a general straightening of all the divisions of the same trunk at each impulse of the heart; these are the general mechanical characters of the artery with red blood, as well as that with black. Absence of pulsation, slowness in the course of the blood, want of straightening, &c.; these are the general attributes of the veins of each kind of blood.

There are no doubt general modifications that arise from local causes. Thus, on account of the short course of the pulmonary veins, the weight has scarcely any influence upon the blood; they never become varicose; the motion of the fluid is more rapid in them, since they have less time to lose that which is communicated to the blood in the pulmonary capillary system, &c.; thus the artery of the same name, whose branches are less tortuous, does not seem to me to have pulsations as evident as those of the aorta, &c. But these general phenomena are always the same; they are but different modifications.

This is why the general arrangement is nearly the same in the veins and in the arteries, whether they circulate red blood or black. Thus, for example, each of the two arteries go off from a ventricle by a single orifice, necessary for the unity of the impulse of the blood, for the uniformity of its course in the divisions of its great vessels, and for the simultaneous pulsations in all the divisions. On the contrary, the veins pour into the heart the red and black blood by many separate orifices; this is of no consequence, since, as we have seen, the motion of this fluid in the veins is not uniform, but may be accelerated or retarded in a part, by the influences it receives; thus, it may pass with velocity through the opening of the vena cava superior, and slowly through that of the vena cava inferior, &c.

From the preceding considerations, it seems, that if we have no regard but to the mechanism of circulation, that it is almost of no consequence whether we consider with the ancients the small and great circulation by studying first the course of the blood in the artery and the pulmonary veins, then in the aorta and the general venous system; or of studying, as we have done, the course of the blood, first in the pulmonary veins and the aorta, then in the general veins and the pulmonary artery. But if we consider this great function in the important relations of nutrition, secretions, exhalations, for which they furnish the materials, of the general stimulus it carries to all parts, and which is indispensable to the support of life, of the introduction of foreign fluids in the body of the animal, and of the change of these fluids into its own substance; then I think it must be described as I have done it.

ARTICLE SIXTH.

ABDOMINAL VASCULAR SYSTEM WITH BLACK BLOOD.

_Situation, Forms, General Arrangement, Anastomoses, &c._

There is in the abdomen a system of black blood wholly independent of the preceding, arranged precisely like it, with the difference, that its course is shorter, and that it has no agent of impulse. This system, usually known by the name of Vena Porta, is found in most animals.

It arises from that part of the general capillary system which belongs to the intestines, the stomach, the omentum, the spleen, the pancreas, &c. and generally to all the abdominal viscera connected with digestion. This origin is remarkable. The viscera in the abdomen, foreign to the phenomena of digestion, are also foreign to the origin of this system. The kidnies and their dependancies, as the glandulæ renales, the ureters, the bladder, the urethra, &c. the genital organs, the diaphragm, &c. the abdominal parietes themselves, &c. &c. pour their black blood into the preceding system. Why are the digestive viscera, in their whole extent, different from the others, in the destination of their black blood? To answer this question, we must know the uses of the system of which we are treating; now, of these we are ignorant.

Thus rising from the whole gastric apparatus, this system forms into two or three trunks, which soon unite into a single one, which occupies the superior and right part of the abdomen below the liver.

This common trunk soon divides again into many branches, which spread in the liver into an infinity of ramifications, and are spent upon the texture of that organ.

This system, then, presents the same general arrangement as the preceding; it is composed of two trees united by their lopped summits, that intermix with each other. Place an agent of impulse at these summits, the arrangement will be the same as in the two preceding. The blood is moved from one capillary system to another. Divided at first into small streams, it is formed into masses constantly increasing to a certain point, then it is divided again, and is carried in streams not larger than the first.

In the abdominal portion, the ramifications, the small branches, the branches and the trunks are arranged very nearly as in the general venous system. The ramifications are found in the organs, the small branches in their interstices, most of the branches are situated in the layers of the peritoneum, there accompanying the arteries, and the trunks wind along the subjacent cellular texture. As to the hepatic portion, contained wholly in the liver, it is divided there nearly like the preceding.

The anastomoses present the following arrangement in the system of which we are treating. 1st. Its hepatic portion appears to want them; all the branches, smaller branches, and ramifications, go separately. As the circulation is not subject in the liver to increase or diminution, the solid texture of this organ protecting the vessels, the blood has no occasion for the means by which it can deviate from one place to another. Thus the great divisions of the pulmonary artery and veins, which go immediately into the lungs where they are wholly distributed, do not communicate with each other. Thus the branches of all the arteries and of all the veins contained in the interior of a viscus, as in the kidney, the spleen, &c. are most commonly without communication. 2d. As to the abdominal tree, its anastomoses are very frequent in the smaller branches. We see all along the small intestines arches exactly like those of the mesenteric arteries; less frequent in the great intestines, they are, however, very evident in them, as upon the stomach; in the branches and the trunks they do not exist.

The anastomoses in the system of black abdominal blood are necessary there from the frequent delays that this fluid may experience. For observe, that the circulation is performed for the abdominal portion according to the same laws as in the other veins, and that consequently that the force that can circulate the blood there, can yield to the least effort. Now in the different motions of the small intestines, often too great a fold, the pressure of these organs filled with aliments upon the veins, when we are lying on the back or the side, and which pressure the veins support only by their resistance, and a thousand other causes, impede the course of the blood in one branch, and force it to flow by anastomoses into others. Observe also, that an obstacle which is of no consequence to the red blood, on account of the very strong impetus that is given to it, is very important to the two circulations of black blood, which receive but a feeble impulse.

The influence of gravity is evident in the blood of this system, as in that of the preceding. Thus you see the hemorrhoidal veins, more exposed than all the others to this influence by their position, become much more frequently varicose; and it is even rare to find dilatations in the superior mesenteric veins, splenic, gastro-epiploic, &c. &c. whilst there is no part in which they exist more often than in the hemorrhoidal. Thus we have seen the preceding system dilated rarely above, but very frequently below.

The system with black abdominal blood communicates but very little with the general system; if there are anastomoses, it is only in the last divisions; do these anastomoses exist? I believe we may consider these two systems as independent of each other.

_Organization, Properties, &c._

Many authors, Haller in particular, considering that the system of which we are treating is destitute of an agent of impulse, have admitted in them a force of structure superior to that of the other veins; but after examining it attentively, I am convinced that it is precisely the same. The cellular covering, of a peculiar nature, which surrounds it, and which is analogous to that of the other vessels, is only a little more evident; this makes these veins at first appear thicker; but by raising this covering we see that the internal membrane is of the same nature, only perhaps a little less extensible. We do not discover the venous longitudinal fibres so well as in the preceding system; I doubt even if they exist in the trunks, in which we should be able to see them better.

The two portions of this system, the hepatic and abdominal, appear to be completely uniform in their structure. Only the first is every where accompanied by a kind of membrane, which appears to be cellular, but whose nature is not yet well known, and which is called the capsule of Glisson. This capsule, intimately connected with the substance of the liver, adheres more loosely to the veins; so that when they are empty, there is often a space between them and it; it is this that makes them fold up when we cut the liver in slices. I think that we are entirely ignorant of the object of this anatomical arrangement.

The analogy between the systems with black blood, the abdominal and general, supposes them the same in properties, sympathies, affections, &c. I have often irritated the mesenteric veins, upon which it is extremely easy to act, by drawing through a small wound of the abdomen a portion of the intestinal canal; the results have always been the same as in the preceding system. Only when we inject air, the animal does not struggle, does not appear to suffer, and the experiment is not fatal; this proves still more, that it is not by its contact upon the veins or the heart, that the air is injurious, but by its action on the brain.

The common membrane of the system of which we are treating, is distinguished from that of the preceding, in this, that it is wholly destitute of valves. This appears to be owing to two causes, 1st, to this, that the course of the blood being shorter, it has less need of being supported; and 2d, to this, that the middle part of this system, wanting an agent of impulse, there is no reflux as in the preceding system. In fact, at every contraction, the right auricle sends, as I have said, a portion of its blood into the veins, which resist by the valves. Here, on the contrary, the course of the fluid is always uniform from one capillary system to the other; there is no cause of retrograde motion.

_Remarks upon the motion of the Black Abdominal Blood._

This uniformity in the course of the motion of this black blood, is not merely the result of the absence of the agent of impulse, but also of this, that the liver does not present as many obstacles to it, as the lungs do to the preceding black blood. Observe also that the liver occupies in regard to this system, the same place as the lungs in regard to the other; it is the termination of the circulation of which we are treating. Having no dilatation or contraction, deprived of the fluid which acts incessantly upon the lungs, and which, loaded with different foreign substances, can often alter the vital forces of these organs, so as to interrupt the passage of the blood, &c. The liver, having a solid and granulated substance, in which no extraordinary motion can take place, except those of the general locomotion of the organ, is evidently incapable of frequently interrupting the course of the black blood, which the abdominal system sends there. Add to this the want of the agent of impulse, and you will understand, 1st. why, when the abdomen is open, we never see a pulsation, a reflux in the veins of the abdominal system, as we observe in those of the other system; 2d. why we always find there nearly the same quantity of blood; 3d. why, consequently, we do not discover, either in the common trunk that corresponds to the heart, or in its branches, the numberless varieties of dilatation and contraction, which the right side of the heart, and all the great venous trunks so frequently exhibit, so that scarcely two subjects are alike in this respect, whilst here the arrangement is always nearly the same; 4th. why the liver is not subject to the innumerable varieties in size that the lungs are. This deserves a particular consideration. You will rarely find the lungs twice containing the same quantity of blood; the weight varies enormously in this respect. Now this arises from the greater or less obstacles the blood has met with in passing through these organs in the last moments of life. We can make them more or less heavy in an animal, by making him die of asphyxia or hemorrhage, consequently by filling with blood, or emptying the extremities of the pulmonary artery. Whatever on the contrary is the kind of death, the hepatic extremities of the abdominal system, contain always nearly the same quantity of blood; suppose, that more remains than usual in this system at the moment of death, it is generally distributed, because there is no agent of impulse, which, at the last moments, drives the greatest quantity to the liver, as happens to the lungs. We understand from this, why the liver exhibits a firm, resisting texture, not extensible like that of the lungs. Sometimes the blood enters it in greater or less quantity, it is even, more or less heavy according to the kind of death. But these varieties belong only to the hepatic veins, which enter into the vena cave inferior just below the heart; they arise from the greater or less reflux of blood that takes place there, as in all the great venous trunks; they consequently arise almost always from the lungs; so that when we see that they are loaded with blood, the right auricle consequently distended, we may also be sure that the liver contains more of this fluid than usual. But this phenomenon, of which I shall speak when treating of the liver, is wholly disconnected with the system which I am describing.

The mechanism of the circulation of the abdominal part of this system, is precisely the same as that of the veins. As to that of the hepatic part, it is unlike that of any other part of the economy. It has no analogy to that of the arteries, for in them the heart is almost every thing, and here there is nothing to correspond to that system; for certainly there is no kind of contraction in the common trunk of the two trees, as I have been frequently convinced. It is then the same motion, which is continued from the gastric viscera to the liver. There is still much obscurity to be removed concerning this motion, as well as the preceding. Every judicious mind perceives that there is a great void, in reading what has been written upon the motion of the general venous blood, and upon this.

We cannot deny but that external agents do much in this last circulation as in the first. The uniform elevation and depression of the diaphragm, the corresponding motion of the abdominal parietes, the alternate dilatation and contraction of the hollow viscera of the abdomen, the constant locomotion of the small intestines, &c. all these causes certainly have an influence upon the motion of the black abdominal blood; and I even think, that the absence of most of them contributes to retard this motion in the hemorrhoidal veins, and to occasion varices in them.

This influence is not however such as Boerhaave thought, that the circulation could not go on without it. In fact, when the abdomen of an animal is opened, the blood is transmitted the same to the liver, and spouts in the same manner from an open vessel; but we observe a sensible weakness in a short time, and this before the general circulation is enfeebled.

_Remarks upon the Liver._

The use of the liver, in being the termination of the black abdominal blood, as the lungs are that of the black blood of all the rest of the body, gives it an importance unknown to all the other secretory organs. Some authors, in observing the enormous size of this viscus compared to the quantity of fluid secreted by it, have suspected that it had another use, besides the secretion of this fluid. This suspicion appears to me, to amount almost to certainty. Compare the hepatic excretories and reservoirs, with the same parts in the kidnies, the salivary glands, the pancreas even, and you will see that they hardly surpass them, and that they are inferior to those of the first. Then compare the size of the liver with that of the kidnies, of the salivary glands, &c. and you will see the difference. If on the other hand, we examine the bile, voided with the stools which it colours, if we open the intestines at the different periods of digestion, as I have done, to see the quantity of this fluid that is poured out; if we keep an animal without food in order to let it accumulate by itself in the intestines; if we tie the ductus choledochus to retain the bile, &c. it is impossible not to be convinced, that the quantity of this fluid is disproportioned to the size of the liver. This viscus is alone equal in size to all the other glands united; now, place on one side, the bile, and on the other, all the secreted fluids, the urine, the saliva, the pancreatic juice, the mucous juices; &c. you will see how enormous the difference is.

Since then the secretion of bile is not the only object of the liver, it must have some other use in the economy. What this is we are ignorant; it is however undoubtedly connected with the existence of the system with black blood of which it is the termination, and is especially relative to this system. The following considerations appear to prove that this use is among the most important.