The mucous membrane and the coat of the capillary vessel beyond it are probably subject to about the same physical conditions of absorption as those which are thus found by experiment to regulate the passage of fluids through dead animal membranes.
As to the first condition, it is probable that the aliment or medicine which is digested, however solid and dense, may be diluted down by the gastric juice until the solution is of less specific gravity than the serum of the blood. With the second law we are not so much concerned, as the medicine after solution has generally no repulsion for the animal membrane. So also with the third, as the serum of the blood mixes readily with all watery fluids. The fourth, viz. the condition of motion, is of great importance; for by it the motion of the contents of the capillary vessels will tend powerfully to determine the passage through of the liquid on the other side. Fifthly, the influence of pressure on endosmosis is one which is certainly exerted in the case of the stomach and intestines, which are muscular, and during the process of digestion contract on their contents with considerable force. This contraction, together with the circulation of the blood in the capillaries, would doubtless be sufficient to determine the passage of the digested matters inwards, even in opposition to the first law.[23]
Under the first proposition I have shown that the majority of medicines are absorbed into the system. It is now seen where and how they are absorbed. But it still remains to be seen what particular substances or classes of medicaments are thus taken into the blood, and whether while so passing they undergo any modifications.
The following, then, are the chief classes which I believe to undergo solution and absorption in the intestinal canal. They comprise the great majority of all substances used either as food or medicine:--
1. Mineral substances soluble in water. 2. Mineral substances soluble in acids. 3. Mineral substances soluble in alkalies. 4. Vegetable products soluble in water. 5. Animal and vegetable substances rendered soluble by the gastric juice. 6. Fatty and resinous substances soluble in alkali.
1. All soluble mineral substances--acids, alkalies, salts, and soluble compounds of the earths and metals--are absorbed in the stomach and intestines. The conditions and the process of absorption have been already stated. When solid they are first dissolved, when in a dense solution diluted down by the gastric fluid before they are absorbed. Thin watery liquids are quickly taken up. Motion on one side, and pressure on the other, hasten the process. I am not inclined to make any exception to this rule. This leads me to make some remarks on _the Endosmotic theory of the action of saline purgatives_.