In going out of the organs, the small venous branches run into the branches, which take, as we have seen, two positions, one sub-cutaneous, the other deep. The sub-cutaneous branches go in the extremities between the aponeurosis and the skin, in the trunk between this and the cellular layer that covers the muscles. The deep branches lie in the interstices that the organs have between each other, accompanying almost every where the arteries. The cerebral branches have a peculiar arrangement; they are placed in the interstices of the dura mater, and form with them what are called sinuses.
The venous branches differ from the arterial in this, that they are infinitely less tortuous; this is remarkable under the skin and in the interstices of the organs. This is a reason that would prevent locomotion, supposing that there was an agent of impulse at the origin of the veins, and that their parietes were not so loose. Hence a series of arterial tubes is really longer than a corresponding series of venous tubes; this facilitates the motion of the black blood, which has a less extent to go over, and which besides would find causes of delay in the curvatures, greater than the red blood, because this is driven by a strong agent of impulse, and the other is not.
The venous branches unite to form a certain number of trunks that are connected with those that are immediately discharged into the right auricle; these trunks are the internal jugulars, the iliacs, the azygos, the subclavians, &c. They are still less tortuous than the branches; they have, like the arterial trunks, deep positions, far from external agents, from which many organs defend them, as a hemorrhage from them would be followed with serious consequences.
The trunks, branches, smaller branches and ramifications do not always arise necessarily from each other, in the manner we have just pointed out. The branches are often united to the trunks, the ramifications to the branches, &c. &c.; as it is with regard to the arteries.
The angles of union vary; sometimes they are right angles, as in the lumbar, renal veins, &c.; sometimes they are obtuse, as in some of the intercostals; most commonly they are acute.
The arrangement of the smaller branches and the branches is as variable at least in the veins as in the arteries; they partake, in this respect, of the general character of irregularity that the organs of internal life exhibit. It is necessary only to attend to the general position and distribution of the branches, smaller branches, &c. Their union with the trunks and among themselves, is different in almost every subject.
Forms of the Veins.
The same observation may be made upon the forms of the veins as upon those of the arteries.
1st. A trunk, branch, &c. are cylindrical, when examined in a space where they receive no branch. In the dead body they appear flat, which arises from the collapse of the parietes, and this is owing to the absence of blood. But by distending them with air, water, &c. they take their primitive form. In the living body they appear round.
2d. Examined in a considerable extent, a venous branch appears conical, so that the base of the cone is towards the heart and the apex towards the general capillary system. This form arises from the smaller branches, that are successively united to this branch, and increase its capacity as it approaches the heart.