For years, physiologists assumed that the ordinary physical laws of osmosis, imbibition, and diffusion were quite adequate to explain the passage of digested food materials into the blood and lymph. If a substance was soluble and diffusible, that was sufficient; it would quite naturally be absorbed in harmony with its diffusion velocity. This, however, is not wholly true, since experiment shows that the rapidity of absorption of diffusible substances through the wall of the intestine is by no means always proportional to the diffusion velocity of the substance. The lining membrane of the small intestine, where absorption mainly takes place, is not to be compared to a dead parchment membrane. On the contrary, it is made up of living protoplasmic cells; absorption is not a physical, but a physiological, process, in which the living epithelium cells stand as guardians of the portals, ready to challenge and, if need be, modify the rate of passage. Osmosis and diffusion undoubtedly play some part in absorption, but they alone are not sufficient to account for what actually takes place in the absorption of digestion products, and other substances from the living intestine. The primary products formed in the digestion of proteid foods--the proteoses and peptones--afford another illustration of physiological peculiarity in absorption. These bodies are readily soluble and quite diffusible, yet they are never found to any extent in the circulating blood and lymph during health. It is of course possible, as has been previously suggested, that as soon as formed they undergo transformation into simpler decomposition products in the small intestine; but this is by no means certain. If proteoses and peptones are injected directly into the blood, they cause a marked disturbance, influencing at once blood-pressure, affecting the coagulability of the blood, and in many other ways exhibiting a pronounced deleterious action which at once indicates they are out of their normal environment. They are not at home in the circulating blood, and the latter medium gets rid of them as speedily as possible; they behave like veritable poisons, and yet they are the primary products formed in the digestion of all proteid foodstuffs. On the basis of all physical laws governing diffusion they should be absorbed, and help to renew the proteids of the blood and later the proteids of the tissues. Yet, as we have said, they are not normally present in the blood or lymph. Apparently, in the very act of absorption, as they pass through the epithelial cells of the intestinal wall, before they gain entrance to the blood stream, they undergo transformation into serum-albumin and globulin, the characteristic blood proteids. The other alternative is that, as previously mentioned, they are completely broken down in the intestine into amino-acids, etc., and these simpler products synthesized, as they pass through the intestinal wall toward the blood, into serum-albumin and globulin.