General Anatomy, Applied to Physiology and Medicine, Vol. 1 (of 3)

by Xavier Bichat

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.