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

by Xavier Bichat

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.