3d. Considered as a whole, the venous system represents three trunks; one corresponds with the vena cava superior, another with the vena cava inferior, and the third with the coronary vein; these three trunks have their apex at the auricle and their base at the general capillary system. Anatomists thus represent the whole of the veins, because the sum of their divisions, like the arteries, has a greater capacity than the trunks from which these divisions arise.
There is however an observation to be made upon this subject, and that is, that the relation between the trunks and their divisions is not as exact in the veins as in the arteries; thus the sum of certain divisions considerably surpasses their trunks, whilst this relation is infinitely less in other cases. But all this arises still from the extreme variation of the venous parietes, according to the quantity of blood they contain; thus in dead bodies, sometimes the branches are much dilated by this fluid, and the trunks remain the same; sometimes an opposite phenomenon is observed. 1st. This last takes place especially when the lungs are obstructed; then in fact the blood flows back to the right cavities of the heart, then into the great corresponding venous trunks; these are then almost equal in capacity to the divisions they furnish, sometimes they even surpass them. 2d. When in the living subject, a limb has been situated for a long time perpendicularly, when standing has been long continued, for example, then the branches are more dilated than the trunks. Now as these causes of dilatation vary ad infinitum, the dilatations themselves are very variable.
From these varieties in the dilatation of the venous branches and trunks separately, it is evident that the relation existing between them is extremely variable, that it is affected by the manner of the death, by the diseases that have preceded it, by the habits of the subject, &c. Let us disregard then, upon this point, as upon every other, all calculations, even if they have a solid basis, if they do not lead to a useful result.
Injections are also a deceptive means of estimating this relation; in fact, they dilate the trunks much more than the branches, and especially than the smaller branches. The internal jugular injected, for example, becomes of an enormous size when compared with the sinus that empties into it. The two venæ cavæ, the azygos, the subclavians, &c. dilate a little less than the jugular; but their size however is remarkable when they are injected, in comparison with that of their branches injected.
Anastomoses.
The veins communicate in general more frequently than the arteries. 1st. In the ramifications there is a real net work, the anastomoses are so numerous. 2d. In the smaller branches, they are not so frequent. 3d. In the branches, they are still less numerous; but still we find many of them, and it is this that particularly distinguishes these branches from the arterial, which are almost always separate from each other.
The communications between the branches of the veins unite immediately in an evident manner the cutaneous to the deep division; thus there is a communication between the cerebral sinuses and the temporal, occipital veins, &c.; between the external and internal jugular by one and even two considerable branches; between the basilic, cephalic, and their numerous divisions spread upon the fore arm, on the one hand; on the other, the brachial, the radial and cubital, by different branches that penetrate deep into the muscles; between the saphena and crural, tibial, peroneal, by analogous branches.
Though separated, the two great venous divisions can then evidently supply the functions of each other by mixing their blood. Hence why, 1st, by agitating the muscles of the fore arm, we increase the throw of blood in venesection, though the muscles do not furnish many branches to the open vein, which then receives the blood from veins, from which it is forced out by the muscles; 2d, why in external pressure that obstructs or even entirely stops the motion of the superficial venous blood, the circulation continues as usual; why, for example, if a ligature is left for a long time applied to the arm, the superficial veins at first swelled, gradually become empty, by pouring their blood into the deep ones; why, notwithstanding tight bandages in fractures and luxations, the venous blood returns as usual to the heart, though it passes in less quantity by the superficial veins. 3d. If a strong band is applied high up on the leg, and the saphena vein is injected below, it does not fill above the band, but the injection goes into the crural. In the same manner the internal jugular may be filled by the temporal, &c.
The anastomoses between the superficial and deep veins are more necessary in man than in any other animal, on account of his clothing, by which the neck, the ham, the arms, &c. are subjected to compressions that would be dangerous without these anastomoses. We can say that upon them alone is founded the possibility of a variety of modes in clothing. That they show in fact that these modes are less dangerous than some physicians have thought; that the danger of apoplexy from a tight cravat, of varices from tight garters, &c. is much less than they have said.
When a single trunk is compressed, the blood passes easily into the neighbouring ones; but if the compression is made upon all the trunks of a limb, a certain time is requisite for this fluid to dilate the anastomoses. It experiences, before this dilatation takes place completely, a kind of stoppage in the capillary system, a stoppage that explains the momentary redness of the fore-arm in women, when the arm is covered with too tight a sleeve, that of the hand or the foot when the bandages of the fore-arm or the leg are too tight.