The mode of the venous anastomoses is very analogous to that of the arteries. Sometimes the smaller branches anastomose with the trunks, sometimes the trunks communicate among themselves.
In the last mode, 1st, there is merely a branch of communication, and this is the most common case; we see this between the jugulars, between the deep and superficial veins of the thigh, the arm, &c. 2d. Two branches unite by their extremities and form an arch, the mesenterics afford an example of this. 3d. Sometimes instead of a trunk, there is an interlacing of the smaller branches that form a real venous plexus; such as that which surrounds the cord of the spermatic vessels.
In general, where there are the most obstacles to the blood, there the anastomoses are the most numerous. Hence why the veins that surround the spermatic cord communicate so frequently together, why the smaller branches of the hypogastric vein which are spread in the bottom of the pelvis, form there a plexus so extended, that it is a real net work in which the course of no one branch can be traced, so numerous are the communications. Notwithstanding this, these two portions of the venous system are the frequent seat of varices; they are even found more frequently dilated in the dead body on account of the difficulty the blood experiences there in rising against its own weight.
This leads us to a general reflection upon the venous system in relation to the anastomoses, and that is respecting the necessity there is for the communications being more numerous in this than in the arterial system. In fact, if we compare the course of the black blood with that of the red, we shall see that there are many more causes to modify that of the first.
The black blood evidently obeys its weight in certain cases. 1st. If we remain standing a short time the veins swell, especially after diseases in which the forces have been diminished; this swelling soon disappears if the leg is inclined; it increases if it remains perpendicular. 2d. There are many cases, in which the forces being very weak, the circulation cannot go on in perfection, except the legs are in a horizontal or inclined position. The influence of position upon many tumours and ulcers of the legs is undoubted. 3d. We know that the first effect of the attitude with the head reversed, is a giddiness produced by the difficulty the blood experiences in rising against its weight. 4th. The valves are particularly destined to counteract the effect of gravity.
Every violent motion communicated to the black blood, and independent of gravity, can also disturb the course of this fluid; it is thus that when we move violently in a circular direction, the venous cerebral blood receives, if we may so say, a centrifugal motion, which, turning it from its natural direction, and preventing it from going entirely to the heart, produces a stoppage of it, and hence the dizziness that is experienced.
It is not only gravity and every other external cause of motion, which influence at every moment the motion of the blood in the veins, but there are also external and internal pressures, and a variety of other mechanical causes.
On the contrary, that of the arteries is independent of most of the causes, of weight especially, and of the internal motion. Why? because the rapidity of the motion is so great which the heart gives to the red blood, that the influence of gravity and every other analogous cause is necessarily nothing. Let us take a comparison; the greater the force with which a projectile is thrown into the air, the less influence the weight has in making it deviate; in the case of the blood its influence is still less. If the blood was driven in empty vessels, gravity would have some effect in the arteries; but in the sudden shock impressed upon the whole fluid that fills them, a shock, the effect of which is felt at the extremities at the same time as at the origin, it is evident that its effect is nothing. For an opposite reason, we can understand why it is so powerful in the veins, in which there is no agent of impulse, in which the parietes and capillary system alone produce the motions, where the motion is consequently slow, &c.
From these considerations, it is easy to see the reason of the very different arrangement that the arteries and veins exhibit in their branches, as it respects anastomoses, which are as rare on one side as they are frequent on the other.