The digestion of proteids may be accomplished either within the body by prescribing pepsin and hydrochloric acid, or without the body by use of the same agents, or, as it is more often done at present, by pancreatinisation.

Pepsin was the first among the digestive ferments to be isolated and employed for artificial fermentation.

This ferment differs from diastase in the fact that it is capable of continuing its digestive functional activity within the body, and it is therefore frequently prescribed in cases of deficient gastric secretion, to be taken with the meals, or immediately after, as a powder or in solution. The action of pepsin is confined to the conversion of 25 albuminous food, and it requires an acid reaction. It is sometimes claimed that the continual administration of pepsin and dilute hydrochloric acid may weaken the stomach by doing the work of that organ while it rests; but it is difficult to see any true ground for the statement, for this action in no wise concerns the stomach wall or its secretion, but is limited to the conversion of food into more easily soluble material, which increases the general bodily nutrition, and therefore indirectly improves the stomach digestion.

Pepsin is destroyed by dilute solutions of sodium bicarbonate as well as by the alkaline pancreatic intestinal fluids and the bile. It is obtained in a number of forms for use in aiding gastric digestion, such as scales, dry powder, pills, and solution. There is considerable variation in the strength of the preparations, depending upon the care taken in their manufacture and differences in the processes of extracting the ferment; but all of them have some digestive action, and they are very serviceable remedies for gastric indigestion.

Peptones or amphopeptones are the ultimate products of stomach digestion. They are derived from primary and secondary proteoses by pepsin proteolysis through hydration and cleavage. In regard to the artificial digestion of proteid foods by pepsin, Chittenden says: "Peptones are truly formed, and many times in large amount, but never under any circumstances have I been able to effect a complete transformation of any proteid into true peptone by pepsin-proteolysis; there is always found a certain amount of proteoses more or less resistant to the further action of the ferment. Even with a large amount of active ferment, an abundance of free hydrochloric acid, a proper temperature, and a long-continued period of digestion, even five and six days, there is never found a complete conversion into peptone. Indeed, the largest yield of peptone I ever obtained in an artificial digestion is sixty per cent, while the average of a large number of results under most favourable circumstances is somewhat less than fifty per cent." Hence, artificial pepsin is not a complete digester, as often supposed, and too great reliance should not be placed upon it.

Pepsin should never be given in combination with an alkali, such as sodium bicarbonate, excepting in cases of hypersecretion of hydrochloric acid with scanty pepsin formation.

The pepsin-secreting cells predominate over the acid-forming cells in the stomach so largely that they are seldom destroyed to the same degree as the latter, and hence pepsin is of much less importance than hydrochloric acid in the treatment of both functional and organic disease of the stomach. It is a very common practice to prescribe it in tablet form without acid, but as a rule, to which there are almost no exceptions other than that of hyper-acidity in those cases in which the exhibition of pepsin is indicated hydrochloric acid is equally if not more important, and they should be prescribed together.

The pepsin is given in doses of three to five or ten grains immediately after meals when proteid food of any kind has been eaten. Ten grains may be added to a pint of milk, but if long continued in excess of twenty grains per diem, it may excite purgation.

Pancreatin is the name given to a complex fermentative body derived from extracting the pancreatic gland. It is obtained in the form of a dry powder, and also as a solution or liquor,. Its introduction is mainly due to the researches of Dr. William Roberts, of England.

The extract made from the pancreatic gland is a powerful digestive agent and, on the whole, is more serviceable for the predigestion of food than any of the other ferments. It contains both trypsin and amylopsin. This extract also emulsifies fats, and it is as easy, if not easier, to obtain the pancreatic extract than pepsin, and since its action is so powerful and varied, the majority of the pre-digested food preparations - the so-called "peptonised foods" - are made with pancreatin instead of pepsin. Pancreatin is sometimes prescribed by physicians for internal use, but unless protected in a keratin-coated capsule the ferments are entirely destroyed in the stomach by the action of the gastric juice. Keratin is a horny substance derived from feathers which is not soluble in acid fluid, but is quite soluble in alkaline media. Hence a capsule containing five grains of pancreatin coated over with keratin may remain undissolved in the stomach until gastric digestion is completed, when it will pass into the intestine, where the coating is dissolved and the ferments act upon the chyle. It is customary to add some alkali, such as sodium bicarbonate, to the pancreatin in the process of artificial digestion, although it will also operate on protein in the presence of a neutral reaction.

All the products of pancreatin digestion, as well as the ferment derived from the juice itself, decompose if exposed for any length of time to the air. Pure meat peptones, prepared either by pepsin and hydrochloric acid or by trypsin in alkaline solution, are ready for immediate absorption and are nutritive, but a very general objection to their use exists in the exceedingly disagreeable odour and taste which they possess, which to many persons are nauseating and disgusting. This difficulty is overcome when peptones are used for rectal injection, for which purpose they are most valuable. Efforts are often made to disguise the disagreeable taste and odour of beef peptones in various ways. When obtained in solution, wines and aromatic substances are added to them or they are sometimes evaporated to dryness, pulverised, and redissolved in sherry, but the taste is exceedingly persistent and it is often difficult or impossible to disguise it in any manner, so that patients will not sooner or later object to it. Among the principal peptones which are used are Koch's, Kemmerich's, Catillon's and Carnrick's, Savory and Moore's, and Benger's. Some of the pan-creatinised foods are strengthened by the addition of predigested starches and sugars.

Some of them constitute very good foods for cases of enfeebled digestion or diminished absorptive power. Among them may be mentioned Benger's Food, which is made with cooked wheat meal and pancreatic extract. The latter partially converts the starches into malt and diastase, and when added to milk at the proper temperature for fermentation, it acts upon the casein and converts it into a soluble peptone. Oat flour and lentil flour are similarly prepared. Carnrick's Infant Food is made on the same principle with Benger's Food, with wheat flour, pancreatin, and milk, to which is added a certain percentage of lactose. Savory and Moore prepare a concentrated predigested milk and milk with cocoa, which have an agreeable taste and considerable nutrient value. Another preparation is Loeflund's Peptonised Kindermilch.

All peptonised or pancreatinised foods are open to the objection that they are much more expensive if used for a long time than the preparations which can be easily made at the bedside by any intelligent person by the use of the simple pancreatin extracts. Among these extracts are Benger's Liquor Pancreaticus and Peptonising Powders and Fairchild's Zymine, which act upon lean meat as well as milk. If complete peptonisation is required, large quantities of the ferment must be used, and the process should be continued longer than for a lesser degree of peptonisation. One or two drachms of Benger's Liquor Pancreaticus with fifteen grains of sodium bicarbonate will completely peptonise half a pint of milk in a few minutes if the bottle be immersed in water which is heated, but which is not too hot for the hand to bear. Roberts's rule for pancreatinised milk gruel is quoted by Yeo as follows: "A good thick gruel made with oatmeal or other meal while still boiling hot is added to an equal quantity of cold milk.

The temperature of the mixture will then be about 1400 F. To a pint of this mixture two or three teaspoonfuls of Liquor Pancreaticus and twenty grains of bicarbonate of soda are added; it is kept at the same temperature for about two or three hours, and then boiled for a few minutes and strained. This mixture contains not only the casein of the milk peptonised, but the starch of the material is converted into sugar and its albuminates are peptonised. The bitterness of the peptonised milk is scarcely perceptible in this gruel".

Papoid is a vegetable ferment derived from the papaw tree (Carica Papaya) which is thus described by Chittenden:

"Extended study of the reactions of the vegetable ferment papoid shows that it is composed essentially of a mixture of vegetable globulin, albumoses and peptone, with which is associated the fer-mentscharacteristic of the preparation.

"Papoid, so far as my observations extend, has the power of digesting to a greater or less extent all forms of proteid or albuminous matter, both coagulated and uncoagulated. Furthermore, papoid is peculiar in that its digestive power is exercised in a neutral, acid, and alkaline medium.

"Papoid will act in dilute solutions, but the best and characteristic action is seen only when a small volume of fluid is present. In this respect it differs very markedly from the animal ferment pepsin, and for this reason any direct comparison of the two ferments is practically impossible, but blank experiments without papoid [with acid and alkali as used in these experiments] showed that these reagents have in themselves only a comparatively light solvent action on raw fibrin." After ingesting the ferment it is found in the stools, showing that it is not wholly destroyed in the alimentary canal. The dose is from one to three grains after each meal.

Caroid is a vegetable digestive ferment made, like papoid, from the papaw (Carica Papaya) in the form of a dry yellowish powder. According to Chittenden's analysis, it retains a strong proteolytic action in either acid, neutral, or alkaline media. It softens and disintegrates proteids, coagulates milk like rennet, and is also anxiolytic. It is a solvent of gastric mucus.