After passing through the capillary vessels, the blood passes into the venous radicles. At its entrance into the aorta and during its course through the arterial system, it was of a brilliant red colour, now it has become dark; the arterial or red blood is changed to venous or black blood; deprived of the greater portion of its constituent elements it returns to their source for a fresh supply.
The blood moves in the veins from the impulsion received originally from the heart; this force is designated by the tens vis a tergo, because it is exerted behind the fluid column. The elasticity of the veins, and their contractility also, contributes to urge the blood along in its return towards the heart, but it is the valves which most effectually second the cardiac impulse by opposing the reflux of the blood toward the arteries. If a moderately tight ligature be placed around the arm, the veins begin to swell because of the afflux of blood, which can neither go on toward the heart because of the ligature, nor return toward the arteries because of the valves which oppose it in that direction. If the finger be now passed lightly over the course of a vein in a direction opposed to the circulation, the little nodules which mark the projections caused by the distended valves are easily distinguished. Thanks to the play of these valves, any pressure upon the veins, from muscular contraction, or whatever cause, can only carry the blood toward the heart, while without them it would be impelled indiscriminately one way or the other. Therefore the valves are more numerous in the veins which are connected with the muscles; in the deep veins of the limbs, for example, than in those which creep along under the skin.
Gravitation affects the movement of the venous blood, which is much less rapid than that of arterial blood. When the hands hang down for a long time as in walking, they swell so much that the flexion of the fingers is difficult, and it is the same with the feet and legs after long-continued standing, and varicose veins appear on the limbs of persons whose profession obliges them to remain on their feet.
In following the course of the blood on its return to the heart, we find a venous system belonging to the liver and intestines; the portal system—to which reference has already been made—which carries the venous blood from the digestive canal and from the spleen to the liver. This system is remarkable from the fact that it is ramified at both extremities; at the intestinal extremity the radicles or roots, and at the hepatic the branches, are found. From this it has been inferred that the bile is secreted from the blood of the portal vein and not from the blood of the hepatic artery; but this question still remains unsettled.
The inferior vena cava, after receiving the blood from the lower regions of the body, turns toward the heart like the superior vena cava, but before reaching it, the blood receives by the subclavian veins the lymph and chyle which have been brought to it by the two grand trunks of the lymphatic system; the elements of nutrition drawn from the intestine come to replace those which have just been given up for assimilation. Thus partially reconstituted, the blood pours back through the venae cavae into the right auricle of the heart, and the auricle by contracting throws it into the right ventricle.
At last the blood has returned to the heart, and although it is enriched by the assimilable products of digestion, it is Still incomplete, and must be transformed in order to become perfect arterial blood, while at the same time the combustion of a portion of its principles will produce the heat which is soon to be distributed to the organism. It is in the lungs that this elaboration of the blood is to take place—hematosis or sanguification.