Pepsin, the substance contained in the gastric juice and in the mucous membrane of the stomach to which, in addition to its acidity, the gastric juice owes its power of converting the albuminoid constituents of the food into soluble peptones. It has probably never been isolated in a state of purity. Pepsin mingled with more or less foreign material may be obtained from the stomach of animals and used for the promotion of digestion in other animals or in the reagent glass. The various preparations containing it in a state of greater or less purity are called pepsines. These vary, as regards their digestive strength, within very wide limits. Several processes have been employed, usually beginning with the maceration of acidulated water or in wine of prepared and gently washed mucous membranes from calves1 or pigs' stomachs. The latter menstruum gives rennet or pepsin wine. The acid solution may be preserved with glycerine or treated with various reagents to obtain solid pepsin. The process of Mr. Scheffer of Louisville, which gives a product both elegant and effective, precipitates the pepsin with a concentrated solution of common salt. It is then dried, and diluted with sugar of milk to a fixed strength. Other forms of pepsin are mixed with starch.

The mucus may be scraped from the stomach and dried on a glass plate. The filtered gastric juice itself has also been used. If coagulated white of eggs or pieces of fibrine be placed in an acidulated solution of pepsin and kept at the temperature of the body, they will be gradually softened and dissolved. This process is a good one for estimating the relative value of different kinds of pepsin, the best of course dissolving the most. It does not, however, represent the extreme limits of its efficacy in the stomach, since it has been found by Mr. Scheffer that by separating a portion of the digested product, this in its turn is capable of continuing the process in an acid solution, the quantity of albumen which could have been digested by the original pepsin without renewal of acid being thus vastly exceeded.

The pepsin is thus seen to act as a ferment and not as a chemical solvent. Pepsin is used in many forms of dyspepsia. It is best prescribed in as simple a form as possible. It has also been employed as a dressing for malignant ulcers, to destroy the nerve in teeth, and to dissolve a piece of meat impacted in the oesophagus. Rennet wine is used in making cheese and in cookery. The activity of pepsin is impaired by the presence of alcohol, though this defect may perhaps sometimes be compensated by the stimulant properties of the wine.