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Besides, I would have them consider, that the great artery and the arterious vein are of a composition much stronger then the veinous artery or the vena cava. And in fine, that if any drop of bloud enter into these concavities, this heat is able to make it presently swell and dilate it self, as generally all liquors do, when drop by drop we let them fall into a very hot vessel.
When you think you have a "pain across the kidneys," it is usually a pain in the muscles of the back much lower down, and has nothing to do with the kidneys at all. A very large artery carries the blood from the aorta to each side of the kidney, and a large vein carries the purified blood back to the vena cava and heart. The kidneys then are big filter-glands.
I, and indeed others, have sometimes found valves in the emulgent veins, and in those of the mesentery, the edges of which were directed towards the vena cava and vena portae.
Now if the vena cava be taken up with forceps or the thumb and finger, and the course of the blood intercepted for some distance below the heart, you will at once see it almost emptied between the fingers and the heart, the blood being exhausted by the heart's pulsation, the heart at the same time becoming much paler even in its dilatation, smaller in size, owing to the deficiency of blood, and at length languid in pulsation, as if about to die.
These appearances were propagated into the arteria innominata. The aortic valves were indurated...." He remarks, "The delay of blood in the aorta, in the heart, in the pulmonary vessels, and in the vena cave, would occasion the symptoms of which the woman complained during life; namely, the violent uneasiness, the difficulty of breathing, and the numbness of the arm."
Nature thought of that long ago, and ingeniously but very simply guarded against it by causing two little folds of the lining of the blood pipes to stick up both where the vena cava enters the heart and where the aorta leaves it, so as to form little flaps which act as valves.
Zacutus Lusitanus says that he once found a mass of flesh in place of the liver. Lieutaud is quoted as describing a postmortem examination of an adult who had died of hydropsy, in whom the liver and spleen were entirely missing. The portal vein discharged immediately into the vena cava; this case is probably unique, as no authentic parallel could be found.
The valves by closing cause the contracting muscle to push the blood in one direction only—toward the heart. The valves in the veins are, therefore, an economical device for enabling variable pressure in different parts of the body to assist in the circulation. Veins like the inferior vena cava and the veins of the brain, which are not compressed by movements of the body, do not have valves.
In the vessels of the liver there is no intervention of the heart; but the vena portarum, which does the office of an artery, is distended by the blood poured into it from the mesenteric veins, and is by this distention stimulated to contract itself, and propel the blood to the mouths of the numerous glands, which compose that viscus.
From these capillaries the blood is carried by small veins which unite into a large trunk, the hepatic vein, which opens into the inferior vena cava. The portal circulation is thus not an independent system, but forms a kind of loop on the systemic circulation.
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