The process of stomach-digestion has been cleared up of late years by the decisive experiments of Spallanzani and Réaumur, of Tiedemann and Gmelin, and of Dr. Beaumont of Canada.
Immediately that a substance touches the mucous surface of the stomach it causes the copious outpouring of a thin fluid, which is secreted by a glandular apparatus. This gastric juice is highly acid, and contains besides a peculiar nitrogenous substance called Pepsin. Dr. Prout thought that the reaction was due to free Hydrochloric acid. But it seems more likely that it is due to Lactic acid. Such at least is the opinion entertained both by Liebig and Lehmann. Now the result of the action of this fluid is to dissolve down the solid materials of the food, or other substances presented to it, reducing them to a thin watery pulp. This pulp is then mainly absorbed; and that which is not taken up through the coat of the stomach is absorbed in the earlier portion of the intestinal canal. I shall have to treat of Aliments as one of the orders of Restorative Hæmatics.
We are now concerned with medicines in general. The same juice is poured out to receive them. Such as are dissolved by it are immediately absorbed. Some other matters may be rendered soluble by the agency of the Bile and Pancreatic juice which are poured out into the middle of the Duodenum. The former is an alkaline fluid, containing carbonate of Soda. The Pancreatic juice is also said to be alkaline. That the stomach is absorbent may be proved by the experiment of placing a ligature round the intestine of a dog, just below the pylorus. It is then found that soluble substances placed in the stomach pass rapidly from it into the circulation. It is probable that all substances which are easily dissolved pass through the coats of the stomach. That the surface of the intestines is absorbent may be proved by the disappearance of enemata thrown into them. Liebig states that a solution of common salt, in the proportion of one part to eighty of water, disappeared so completely in the rectum that an evacuation one hour afterwards was found to contain no more than the usual proportion of salt. (_Animal Chemistry_, _p._ 77.)
On the mucous surface of the small intestines are a number of small projections, called Villi. Within these are the origins of the Lacteals, a peculiar set of Lymphatic vessels, which are engaged in the absorption of chyle. This chyle is a thick fluid which is formed by the meeting of the Bile and Pancreatic juice with that part of the digested food which passes into the Duodenum. It is generally white, from the presence of fat. Now these lacteal vessels are no doubt absorbent, but are they ever engaged in the absorption of medicinal solutions? It seems that they are not in ordinary cases at all concerned in this; for three chief reasons. In the first place it appears from the researches of Bernard and others that the lacteal system is a special arrangement for the absorption of fatty substances, and that other matters, such as albuminous compounds, pass generally into the veins, and thence to the liver. Besides, it seems that these lacteal absorbents are only in action during the digestion of food, when the epithelium on the surface of each villus becomes loosened, in order to allow to the chyle an easier access to the lacteal within it.[21] So that it is likely that a small portion of a fluid or soluble substance would be insufficient to rouse them to action. And, inthe third place, direct experiments of a decisive kind have been made on this point. Magendie has found that the ligature of the lacteal trunks does not prevent the occurrence of poisoning from agents introduced into the bowels. And Tiedemann and Gmelin have carefully sought in the chyle for a number of different medicines administered to animals in their food, and have been unable to detect any of them there. So that, with the exception perhaps of fats and fixed oils, we may reasonably conclude that no medicinal substances pass into the system through the lacteals, but that all are absorbed by the veins or capillary vessels.[22]