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

by Xavier Bichat

By examining these phenomena in a general manner, we see, 1st. that the red blood going from the lungs, is formed into larger and less numerous columns, as it approaches the cavities of the heart; that it is in the greatest masses in these cavities, and that from them to the capillary system, it is continually dividing into smaller columns; 2d. that the black blood going from the general capillary system, is formed into columns larger and less frequent, as it approaches the right cavities of the heart; that these cavities contain the greatest quantity of it, and that from them to the lungs, it is successively divided into smaller columns.

The two kinds of blood circulate then on the two sides in branches that diminish as they go from the heart, and increase as they approach it. Represent to yourself for each of the two circulations two trees united at their trunks, sending their branches, one of them to the lungs, the other to all the parts. Each of the two parts of the heart is between these trunks, which it serves to unite, so as to form a general canal of which we have spoken.

Authors have commonly considered the arteries and veins as forming, each by their assemblage, a cone, the base of which is at all the parts, and the apex at the heart. This manner of describing them arises from this, that the sum of the branches is greater in diameter, than that of the trunks from which they arise; now in adopting this idea, it is evident that each half of the heart is at the summit of two cones, which would be united without it. The pulmonary veins represent one, and the aorta the other for the red blood; for the black blood, there is on one part the venæ cavæ and coronary veins, on the other, the pulmonary artery, which form the two cones. In each circulation, one of these cones is remarkable for its small size; it is that of the lungs; the other for its great extent; it is that of all the parts.

Placed between these two cones, each part of the heart should be considered as an agent of impulse which hastens the course of the blood, on one hand towards all the parts, on the other towards the lungs. In fact, if in each circulation these two cones communicate by their apex, it is evident that the parietes of the vessels that compose them would be insufficient to maintain the motion, from the base of one to the base of the other; that is to say from the general capillary system to that of the lungs, and reciprocally from that of the lungs to the general one. The course is manifestly too long, and the vital forces of the vascular parietes not sufficient to admit of this; hence the necessity of the heart.

This consequence leads to another, which is this. As the red blood has a greater extent to go over from the heart to the general capillary system, than the black blood has from the heart to the pulmonary capillary system, it is necessary that the portion of this organ belonging to the first kind of blood, should be endowed with a greater force than that destined to support the motion of the latter. Nature has effected this object by composing the ventricle with red blood of fibres much stronger than those of that of black blood. As to the auricles, as they only receive the blood and transmit it to the ventricles, their thickness is nearly uniform.

From this we see, 1st. that the part the heart performs in the two circulations, is absolutely relative to the mechanical phenomena of the course of the blood, and that, if it has any influence upon the composition, it can only be by the internal motion it communicates to it; 2d. that if the course of the two circulations, of black blood and red, was of less extent, they might do without this intermediate agent of impulse. This is precisely what happens in the system of abdominal black blood, the two trees of which distributing their branches, one to the gastric viscera, the other to the liver, unite by their trunk in what is called the sinus of the vena porta, which occupies exactly the place of the heart in the great system of black blood and in that of red.

It is then possible to conceive, 1st. how the heart may fail; of this we have many examples, in which the two great circulating systems resemble in a certain degree, the abdominal; 2d. how the blood can oscillate from one capillary system to the other, during a considerable time, though the heart, weak, enfeebled, and even in part disorganized, can hardly any longer accelerate the course of this fluid; 3d. how, this organ having entirely suspended its pulsation in syncope, asphyxia, &c. there is still an oscillation, a real progression of the blood from one capillary system to the other, so that if an artery or vein is then opened, it flows a little at the opening. Certainly this oscillation is very weak; it cannot last a long time; but we cannot deny that it exists without the influence of the heart, since the black blood is carried without the agent of impulse, from the intestines to the liver; hence it follows that the cessation of the pulsation of the heart is not a proof of the want of motion in the blood, as some authors have thought. 4th. We know that in many animals of the lower classes, there is no heart, though there are distinct vessels and circulating fluids.

The importance of the part that the heart enjoys in the animal economy is only in relation to the general impulse that it communicates to all the organs and the constant excitement in which it keeps them by this impulse. It does not send to them the materials of secretion, of exhalation and of nutrition; it only in this respect transmits what it receives from the lungs.

II. Reflections upon the general uses of the circulation.

This leads us to some reflections upon the general differences of the uses of the two circulations; differences which prove the necessity of presenting the single function that results from them in the view in which I have placed it, and not in that in use in the treatises on physiology. The following are the differences.