This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
These are substances taken internally with the view, through their chemical agency, of promoting the solution of indigestible food in the stomach. it is now, I believe, universally admitted that a fluid is secreted by the stomach, in the normal condition, through the agency of which most of the articles used as diet are brought into a liquid state. This, however, is not a case of simple solution. it is believed that the solvent gastric juice contains a principle, denominated pepsin, which, acting as a ferment, causes such changes in the insoluble substances, or most of them, as to render them soluble in the liquid of the stomach. Now this principle is sometimes deficient, or insufficiently elaborated, in debilitated or otherwise diseased conditions of the stomach; and the consequences are all those likely to arise, either from the presence of undissolved organic matter acting as an irritant, or from its decomposition, through chemical agencies, and the consequent production of gases, acids, and other acrid and irritant matters. The use of the remedies belonging to this class is to supply temporarily this deficiency of the gastric juice, and maintain artificially that part of the process of digestion carried on in the stomach, until the diseased condition in which the deficiency originated can be corrected by other means. Besides pepsin, it is pretty well established that an acid, probably the lactic, is normally produced with the gastric juice, which, if not essential, is greatly promotive of its solvent power. These agents, therefore, namely, pepsin and lactic acid, are those from which the influence demanded in this class of remedies might be expected. But, as pepsin can scarcely be obtained satisfactorily in a pure state, we may substitute substances containing it; and hence it is customary to use preparations made from the stomachs of the lower animals, in which that principle may exist in a greater or less state of concentration. Besides pepsin, or substances containing it, and lactic acid, there is one other remedy which may be placed in the same category, as the object of using it is to favour digestion by the complete solution and decomposition of amylaceous substances. This is yeast. The stronger mineral acids generally might be introduced into the class; as they are believed to be capable of performing the same part in digestion as lactic acid; but they have other powers, in reference to which they have been considered elsewhere.
These solvents are capable of being used also externally, and sometimes perform a useful part in dissolving dead bone, or other effete portions of tissue, which it may be desirable to remove more hastily than can be done by the unassisted powers of nature.