The gastric juice has in the absence of mucus no effect on the carbohydrates, and probably the amylolytic fermentation set up by the saliva is impeded, if not completely checked, by the free acid in the stomach as soon as the bolus is moistened by the gastric fluid.

Fats are not affected by the gastric juice, but are simply melted in the stomach.

Upon the albuminous bodies the gastric digestion produces a marked effect. The proteids being colloid bodies, cannot readily pass through an animal membrane by the process called dialysis; it has therefore been assumed that they cannot be absorbed through the lining membrane of the stomach. They are often eaten in an insoluble form. To convert the insoluble and indiffusible albumins into a soluble and diffusible substance would obviously be a great step toward their absorption. This power is ascribed to the gastric juice. The steps of the process may be accurately followed in a suitable glass vessel, irrespective of the stomach, by using artificial gastric juice, and attending to the various conditions necessary for its action. The power of artificial gastric juice carefully prepared from the mucous membrane of an animal's stomach differs in no essential respect from that of the natural secretion in the stomach, if all the circumstances which aid the action of the gastric ferments be applied in the experiment. This action consists in a conversion of coagulated albumins into the peculiar, soluble and more diffusible form of proteid known as "peptones".

The change is not effected immediately, but certain stages may be recognized in which the two chief constituents of the gastric juice, the acid and the pepsin, seem to have special parts to play.

Shortly after the introduction of a proteid, such as boiled fibrin, into gastric fluid at the temperature of the body, the masses of fibrin swell up, become transparent, and eventually are easily shaken to pieces and dissolved.

The first step in the process seems to be brought about by the free acid, and consists in the formation of acid albumin. This can be shown by neutralizing the fluid during the process and thereby causing a precipitate of acid albumin. The amount of this precipitate will depend upon how far the conversion into peptone - which is not precipitated by neutralization - has progressed. Thus, in the earlier stages, nearly all the proteid used will be thrown down by neutralization, while only a comparatively small amount is precipitated in the later stages.

The formation of acid albumin may be effected with weak acid without the other constituents of the gastric juice, and therefore the preliminary step may be attributed to the unaided action of the acid; but since this stage in the formation of peptone is constant, and the material may possibly be distinguishable from the ordinary acid albumin, it has been called par-apeptone.

While the parapeptone is being formed by the acid, the pepsin is engaged in changing it into the final, soluble, diffusible and uncoagulable product - peptone. The pepsin by itself cannot convert proteid into peptone, as may be seen in the want of efficacy of a neutral solution of pepsin, in which neither peptone nor parapeptone is formed. In other words, pepsin solution can only change parapeptone or acid albumin into peptone. It would appear probable, however, that it possesses this property to an unlimited extent, since it undergoes no change itself, and with fresh supplies of acid a very minute quantity of pepsin can convert an indefinite amount of proteid into peptone.

The rapidity with which proteid is converted varies according to the circumstances under which it is placed as well as the kind of proteid used. If the same proteid be used, the following circumstances will be found to influence the rapidity of the process: -

1. The temperature. As already stated, the optimum degree of heat for the change is about that of the body, 35°-40° C.

The activity of the gastric juice diminishes when the temperature rises above or falls below this standard. The minimum at which it is capable of producing any effect is about 1° C. and the maximum is below 700 C. Boiling permanently destroys the function of pepsin.

2. The percentage of acid as well as the kind of acid has a marked effect. Though the action will go on with other acids, hydrochloric is the most effective, and that of a strength of. 2 per cent,

3. A condensed solution of peptone or large quantities of salts in solution impede the action, a certain degree of dilution being necessary for the process. In strong solutions of proteid, the peptones must be removed by dialysis in order to allow of the continuance of the action. This occurs in the stomach by means of the blood and absorbent vessels.

4. The degree of subdivision to which the proteid has been subjected materially influences the rapidity of its conversion into peptone. The more finely subdivided the substance the greater will be the relative extent of surface exposed to the action of the digestive fluids. When large masses of coagulated albumin, such as boiled white of egg, are introduced into the stomach, the gastrie fluid cannot reach the central portions, and their digestion must await the completion of that of the exterior part. 5. Motion aids the action of the foregoing factors.

All these requisites are present during normal digestion.

The temperature of the stomach is 380 C. (=1ioo° F.). Hydrochloric acid is present in the proportion of about.2 per cent. As quickly as the peptones are formed they can be removed by absorption from the stomach, and thus the needful dilution is accomplished. Finally, if the mouth has done its duty, the pieces of proteid have been reduced to a pulp, composed of minute particles. These are kept in constant motion by the gastric walls, and thus are repeatedly brought in contact with fresh supplies of the digestive fluid.