ARTICLE THIRD.
PROPERTIES OF THE VASCULAR SYSTEM WITH BLACK BLOOD.
The veins are in general but little elastic, soft, and loose; they partake of the character of many of the animal textures, and are essentially distinguished in this respect from the arteries, which as we have seen are very elastic. We shall now treat of the vital properties and the properties of texture in these vessels.
I. Properties of Texture. Extensibility.
The veins have in regard to this property, an arrangement entirely opposite to that of the arteries, which are very extensible longitudinally, but very little so transversely.
The veins stretch but little in the first direction. When drawn out of a stump after amputation upon the dead body, they lengthen but little in proportion to what they dilate in varices, though here they experience an actual increase of size. Perhaps however this depends less upon the deficiency of extensibility of texture, than upon the circumstance that the folds are less evident than in the arteries, and of course the development is less. Whatever may be the cause, the fact is certain and uniform.
Few organs, on the other hand, exhibit a greater degree of extensibility transversely, than the veins. In the dead body, they can be enormously dilated, by injections of air, water, fatty substances, &c. In the living, we know the varicose dilatations, which arise in the great trunks, from the obstacles to the course of the blood in the lungs. While the arteries do not appear very often more than to double their diameter without breaking their common and peculiar membrane, the veins treble, quadruple, and even quintuple their diameter without this rupture's taking place.
We have however numerous examples of this accident. Haller has related many in his great work. We see these ruptures take place during pregnancy in the veins of the lower extremities; there are examples of them also in the external veins of the head in violent headaches. We have seen the venæ cavæ, the jugulars, the subclavians suddenly break and produce death. Every one knows of the hemorrhages that arise from the rupture of the hemorrhoidal veins, &c. I think that the extreme tenuity of the parietes of the cerebral veins exposes them to being frequently torn by blows upon the head, wounds upon that part, &c. When there is an effusion in the tunica arachnoides, it can certainly come from no other source than the venous trunks, which being surrounded by a fold of the arachnoides, pass through this cavity to go to the cerebral sinuses. Now we know that this case is very common, and that it even takes place at the same time with that, in which the dura mater being detached from the cranium, is found separated by an effusion. Is not apoplexy a sudden rupture of the venous extremities? I have already observed that we have no data upon this point. All these cases are very different from arterial aneurism; they often take place when the dilatation is infinitely less than in many instances where the veins remain whole. Very commonly this does not happen. The whole of the vein, with the cellular tunic containing it, bursts. The arterial rupture in true aneurisms, is on the contrary uniform; when the dilatation is carried to a certain point it always happens. The two arterial coats break easily, the cellular remains whole. I do not believe that there is a solitary instance of a great aneurism, without rupture. Why? because the arterial extensibility can only yield to a certain point. The ruptures take place then from a want of this property; they are disconnected with this cause in the veins. We do not know yet how they are produced. In a great number of cases certainly, there is an affection of the venous texture; this is undoubtedly the case in hemorrhoids, &c. Let us be content to point out the differences between arterial and venous ruptures, and wait till further observation shall discover to us all their causes.
If we bear in mind, that the arterial fibres are very numerous and all circular, that the venous, on the contrary, are on the one hand longitudinal where they exist, and on the other that they are very thinly scattered on their vessels, we shall then see why the first resist much longer a distension in the direction of their diameter than of their axis, and why the opposite phenomenon is observed in the second, though much less decided.
Contractility.
This corresponds with the extensibility. Slight in the longitudinal direction, much greater in the transverse. 1st. It produces the contraction upon themselves, of the parietes of the umbilical vein, of any trunk that is tied, &c. 2d. It produces in a trunk that is pricked, the sudden evacuation of the blood contained between the two ligatures by the return of the parietes upon themselves. 3d. It manifests a decided influence on the flow of blood in venesection. 4th. The numberless varieties of caliber that the veins exhibit after death, according to the quantity of blood they contain, are the result of their extensibility and contractility of texture. 5th. During life, the superficial veins appear very various; dilated in summer, contracted in winter, expanded in the warm bath, as we see the saphenas, especially in pediluvium, lessened in the cold bath, prominent by a long continued perpendicular position, flattened by a horizontal one, &c. they present to him who observes them, at different times, numerous varieties. I very much doubt whether those who have calculated so much the capacity of the vessels, the velocity of the blood, &c. would have undertaken their labours, if they had opened many bodies, or made many experiments upon living animals; now all the varieties depend upon the extensibility and contractility of texture.