The venous fibre, though infinitely more extensible than the arterial, is also more resisting; it will support without breaking, very considerable weight. The experiments of Wintringam have proved this. In the superficial and inferior veins especially, this is very remarkable.
There is a great difference in individuals as it respects the venous fibres. In some they are very apparent; in others, they are hardly distinguishable upon the great trunks; but then they are always very evident in the branches, particularly in the superficial ones.
There are places in the venous apparatus where we cannot discover either external fibres, or external cellular texture; this is the case with the cerebral sinuses, which have the following arrangement. The jugular vein at its sinus, loses its peculiar texture, and keeps only the common membrane, which, entering the lateral sinus, lines it, and extends below into the inferior longitudinal sinus, and above into the superior; in a word, into all the sinuses of the dura mater. Hence every sinus supposes, 1st, a separation of the layers of the dura mater; 2d, the common membrane of the black blood lining this separation. It is not then upon the dura mater that the blood circulates; it is upon the same membrane that it flows elsewhere; it is easy to establish this fact in the superior longitudinal sinus. This sinus is triangular when considered in relation to the separation of the layers of the dura mater; but in opening it, we see clearly, that the common membrane, by passing over its angles, makes it round; this is very evident. It is easy, also, in many other sinuses, to separate in certain places this membrane from the dura mater; but in the greatest number the adhesion is close, like the union of the arachnoides with the internal surface of the dura mater. This common membrane of the black blood is spread over the folds of the superior longitudinal sinus; it forms a singular net-work, which I shall describe in the cavernous sinuses.
From this general outline, it is evident that the coats of the dura mater supply in the sinuses the place of the venous fibres and their external dense cellular texture; the common membrane is always the same; but the texture that is added to it externally is different. At the place where each cerebral vein opens into a sinus, the common membrane of that sinus connects itself with it in its passage and lines it to its extremities. I know of no author who has thus considered the cerebral sinuses, as having the common membrane of the black blood extended into all the separations of the dura mater. How little soever we examine the internal surface of a sinus, it is easy to see that this surface differs as much from the texture of the dura mater, as it resembles the internal surface of the veins.
The cerebral veins, of which the sinuses are the terminations, are analogous to the arteries of that part in the extreme tenuity of their parietes, a tenuity that appears to be owing to the absence of the cellular coat, and which is so great, that you might believe that there was only a common membrane.
There are no circular fibres in the veins.
Common Membrane of the Black Blood.
This membrane generally extended from the general capillary system to the pulmonary, is every where nearly of the same nature. It differs essentially from that of the red blood in a great number of respects.
1st. It admits of much greater distension; consequently it is less brittle. Tie a vein, and it will not break, unless the constriction be excessive; it is almost as pliable as the cellular coat. This pliability renders it much easier of dissection than the common membrane of the arteries. 2d. It appears to be much more delicate; we have a proof of this in the valves, which at first view are hardly visible from their extreme tenuity, when they are lying against the external surface of the vein. 3d. This common membrane is never ossified in old age, like that of the arteries; its organization seems to resist the deposition of phosphate of lime. When it does take place, it is an unnatural state; whereas the ossification of the common membrane of the red blood is almost a natural state in old age, as I have before observed. This difference between the two common membranes of the black and red blood, gives a distinctive character to the diseases of the heart. We never see ossification in the tricuspid valves, or in the semilunar valves of the pulmonary artery, whilst they are so frequent on the left side; this is the uniform result of observations made at the Charity; in the bodies of old people dissection has always shown me the same thing. So the pulmonary artery, though analogous to the aorta by its peculiar membrane, is never the seat of these ossifications, because the common membrane differs essentially from its own. This single phenomenon, so striking in both these membranes, would incontestibly prove their organic differences, while it establishes the necessity of considering them in a general manner, whether, in the black blood, they line the veins, the pulmonary artery, and the right side of the heart, or in the red blood, they are spread upon the arteries, the left side of the heart, and the pulmonary veins.