Venous absorption probably occurs wherever veins exist, but especially in the alimentary canal, where, in conjunction with the lacteals, they perform the important office of extracting from the food the different materials which enter the circulation and nourish the various tissues of the body.
It is not my intention to enter upon any of the elaborate arguments or disquisitions on the evidence ofvenous absorption. I shall content myself with a brief summary of the most im portant salient points.
That absorption by the veins takes place independently of the lacteals has been proved bya series of conclusive experiments conducted by Tiedemann and Gmelin t, who administered to a number of animals various substances of distinct and easily recognisable physical and chemical properties, and afterwards analysed the venous blood, the secretions, and the chyle. Substances having deep colours, such ps cochineal, indigo, litmus, gamboge ; sub stances having strong odours — turpentine, assafcetida, garlic, musk — and saline sub. stances, such as sulphate of iron, chloride of barium, ferro-cyanide and sulpho-cyanide of potassium, were all in turn submitted to ex periment. Almost all of these matters were found in the venous blood, several of them in the urine, and a few only of the latter (the sa line substances) in the chyle. Therefore it ap pears that absorption did take place, in many instances, independently of the lacteals, and by the veins. This conclusion is fortified by other experiments conveying both positive and negative evidence, which have been performed by Majendie, Blake, Delille, Sega las, &c.
Majendie and Delille in conjunction per formed the following experiment. They severed all the parts of the posterior extremity of a dog, excepting the artery and vein by which the circulation of the limb was kept up. They then introduced some upas poison into the foot ; and in ten minutes the animal died. It cannot be imagined here that the lymphatics absorbed the poison, as they were divided; but it may be objected that the matter was mechanically introduced into the divided extremities of the vessels in the wound. This difficulty has been over come in another experiment devised by Ma jendie. He found that death was caused by the introduction of nux vomica into the intes tine of an animal in which the lacteals had been tied, where the veins, by which alone absorption could here take place, remained entire. An experiment exactly the reverse of the last was performed by Segalas, which completely bore out the idea of venous ab sorption. Instead of tying the lacteals and leaving the veins untied, Segalas tied the veins, the lacteals remaining unimpaired. The result was, that when poison was applied to the intestine, no absorption took place, and no result followed. It was further found by Mr. Blake, that ligature of the vena porta prevented the action of poisons introduced into the stomach.