In the greatest number of organs, venous branches go out at the same place that the arteries enter. There are however some exceptions to this rule. In the brain, for example, the arteries enter below, the veins go out above. In the liver, the first enter below, the others go off behind, &c. This circumstance is, in general, indifferent to the circulation, which goes on the same whatever may be the relation of the arteries with the veins. In those places where the small veins go out at the same time that the small arteries enter, sometimes more or less of cellular texture serves to unite the small vessels that are in apposition, sometimes there is a space between them, as in the muscles, the nerves, &c.
Besides the venous origins corresponding with the arterial terminations, there is an order of veins which is separated from the arteries at the exit from the capillary system. This order is particularly remarkable at the exterior of the body. We see that all the organs that are found there furnish, 1st, veins that go to the interior to accompany the arteries; 2d, others that go to the exterior to become sub-cutaneous, and to form trunks of which we shall soon speak. In many internal organs, the same venous division is observed.
It follows from this general arrangement, that many more veins go from the capillary system than there are arteries that enter it. This is the principle of the disproportion of the capacity existing between the system with red blood, and that with black, a disproportion of which we shall soon speak.
The veins at their origin frequently communicate among themselves. We see many little spaces that arise from their interlacing, in the places where they can be seen, as under the serous surfaces, &c.
II. Course of the Veins.
At their exit from the general capillary system, as we have just said, the veins are differently arranged. 1st. In the extremities and external organs of the trunk, they form two sets, the one interior, which accompanies the arteries; the other exterior, which is sub-cutaneous. 2d. In the internal organs we frequently make a similar observation; thus there are superficial veins of the kidney, and deep ones, that accompany the arteries; but oftentimes all the veins unite themselves to those that follow the artery.
The cutaneous portion of the veins is very remarkable in the extremities, where there are considerable branches, viz. the saphena in the lower, the cephalic, basilic, and their numerous divisions in the superior. In the trunk and the head, we do not see any great sub-cutaneous branches, except the external jugular in the neck; but there is a number of smaller branches proportioned to the minuter ones that are distributed there.
The external parts, then, are remarkable by the predominance of the trunks with black blood over those with red. Oftentimes these trunks can be traced through the integuments, upon which they show themselves much more than those that are whiter and more delicate; they have, besides, no connexion with the tinge that colours them, which arises from the blood contained in the general capillary system.
In the interior of the body, the veins almost every where accompany the arteries; they follow the same distribution; so that they are not commonly described, because the course of the arteries is sufficient to represent theirs. Usually a common cellular space receives both the trunks of the two sorts of vessels and those of the nerves. Sometimes, however, the veins are separate, as the azygos, for example, which has no corresponding arterial trunk, and which on this account requires in descriptive anatomy, like the superficial ones of the trunk and the extremities, a particular examination and an accurate dissection to obtain an idea of it.
The deep-seated veins have a caliber much more considerable than that of the arteries; most usually they are more numerous, as in the extremities, where each artery is almost always accompanied by two veins.