After having formed this external covering to the veins, this cellular texture of a peculiar nature analogous to the sub-arterial, sub-mucous, &c. passes between the longitudinal venous fibres, separates them, forms for them a kind of sheath, and terminates in the common membrane, which appears to contain it in its texture, and which owes perhaps in part to this circumstance, the great extensibility that it possesses.
I would observe that the presence of the cellular texture in the venous parietes is a distinctive and striking character that distinguishes them from those of the arteries, with which their texture has in other respects no kind of analogy.
Exhalants and Absorbents.
It appears that there is no exhalation upon the internal surface of the veins. This surface is always moist in the dead body, though the vessels are empty; but I attribute this phenomenon, as in the arteries, to a transudation that has taken place after death. If there was in fact a fluid exhaled, it would prevent the adhesions of the venous parietes, when during life the blood ceases to flow through them. Now every vein that is empty is obliterated into a sort of ligament, like the arteries in similar cases.
There is no more absorption upon the internal surface of the veins, than exhalation. To satisfy myself of this fact, I have tried upon the external and internal jugular veins, the same experiment before noticed, as having been made upon the carotid artery; I obtained the same result and drew from it the same conclusion. I have been induced to make these experiments, from the opinion of many distinguished anatomists, who thought that the absorbents arose immediately from the veins and the arteries. It is possible that this is the case in the smaller branches, in the capillary system especially, as I shall say in the absorbent system; but I do not believe that any thing similar can be demonstrated in the trunks.
It appears then that the exhalants and absorbents of the venous parietes, like those of the arterial, are confined to the nutritive functions, and that they are consequently few. This remark is applicable not only to the veins, but to the whole of the vascular system with black blood.
Nerves.
1st. The veins differ essentially from the arteries by the few nerves of the ganglions that accompany them. Whilst these nerves form for most of the arteries a kind of covering, they are scarcely spread at all upon the veins. By laying bare the venæ cavæ, jugulars and azygos, it is easy to observe this. 2d. The side of the heart with black blood, receives as many nerves as that with red; this proves that they have no influence upon the contraction, as it is evidently weaker on the right side than the left; whereas if it was produced by the nerves it would be equal, as there is an equal distribution of them. 3d. The pulmonary artery has but very few nerves. I know not as yet the relation that exists between it and the pulmonary veins in this respect.
It appears from this general survey, that the system with red blood has many more nerves than that with black. In fact being nearly equal at the heart, and the difference being very sensible in this particular between the aortic arteries and the veins that go to the right auricle, although the pulmonary artery may have a few more than the corresponding veins, which I think very probable, yet the short course of these vessels would not prevent the disproportion from being very apparent.