The veins terminate by two principal trunks, the superior and inferior venæ cavæ. There is also another, viz. the coronary vein, which empties separately into the right auricle; but as this trunk only brings back the blood that went to the heart, we shall pay but little attention to it in our general remarks, and but little to those small venous branches that empty separately from it into the same auricle.
Some authors have thought, that the two venæ cavæ were continuous and formed but one vessel; but it is easy to see how different their direction is. It is particularly in the fœtus, that their separation can be well perceived, since one corresponds with the right auricle, and the other with the left. There is behind the right auricle a kind of continuity of the membrane between the one and the other; it is the membrane of the black blood that is common to them, and which passes from the inferior to the superior; but in this respect there is no more continuity between them, than between the right side of the heart and the pulmonary artery, between the left and the aorta, &c.
By considering the whole of the trunks and the branches as a cone, we can say that there are two great venous cones distinct from each other; one for all the parts which are above the diaphragm, the other for all those that are below it.
The superior vena cava does not answer, then, entirely to the union of the arteries that form the aorta of the same name, which is only destined to the neck, head, and superior extremities, whilst the other belongs moreover to the chest by the vena azygos. For a contrary reason, the descending aorta has a destination much more extended than the inferior vena cava.
The limit between the two cones of the ascending and descending venæ cavæ, is placed at the diaphragm. It is especially in this respect that we can say that this muscle divides the body into two parts. Has not this arrangement some influence upon the difference that is observed in certain diseases between the superior and inferior parts? Should not this cause be connected with those pointed out under the article upon the fœtus? As yet there is nothing certain with regard to this, but I think it not improbable.
Though forming each a distinct cone, the two venæ cavæ communicate, however, especially in the neighbourhood of their common limit, that is to say, in the neighbourhood of the diaphragm; the azygos is the great means of communication. We know, in fact, that its trunk opens into the right renal, into the vena cava itself, or into some of the lumbar veins, and that the semi-azygos that arises from it, goes also to the left renal or to the lumbar of the same side. This anastomosis is very important; physicians have not paid sufficient attention to it. It proves that when an obstacle is situated in the trunk of the inferior vena cava, a great part of the blood of this trunk can flow into the superior. Much has been said of the compression of this trunk by the enlargements of the liver, in the production of dropsies. But, 1st, it is ascertained by the numerous examinations of dead bodies in modern times, that the production of these diseases belongs to every kind of organic affection; that the lungs, the heart, the womb, the spleen, &c. can likewise occasion them in the latter periods of the alteration of their texture; and that, in this respect, they are but a symptom in the greatest number of cases, and a symptom wholly disconnected with any sort of compression. 2d. By supposing that the liver could exert upon the vena cava an analogous compression, in the place where this vein crosses its posterior part, it is evident that the anastomoses of which I have just spoken, would prevent the effect of this compression, at least in great measure.
By supposing that an obstacle was encountered in the vena cava superior, the same anastomoses would undoubtedly answer the same end; but as the azygos is inserted very near the auricle, as the course of the trunk of the vena cava superior is consequently very short, it is evident that it is especially to counteract the obstacles the inferior may experience, that these anastomoses have been established.
When the blood of this vein passes thus into the superior, it goes through certain branches in a direction opposite to that which is natural to them. For example, suppose that the anastomosis takes place in the renal, which most often happens; then the blood of the trunk of the vena cava enters by one extremity of this vein; that from the kidney comes by the opposite extremity, and both pass into the azygos. A similar motion evidently supposes the absence of valves in the renal, from the vena cava to the insertion of the azygos. Now the renal veins never in fact have these folds; the capsular, the adipose of the kidney, all the lumbar, are also destitute of them, as Haller has seen, and I have ascertained it to be uniformly so. This absence of valves at the places of the anastomoses of the azygos, is a remarkable phenomenon; it proves very well the use that I attribute to the communication of the two venæ cavæ by means of this.