As soon as the food reaches the stomach digestion proper begins. In this process the food is first rendered soluble and secondly converted into a form suitable for absorption. It is not sufficient for the materials to be broken down and dissolved. They will not be absorbed by the intestinal walls unless certain definite compounds are formed. Thus, all the carbo-hydrates, such as starch, glycogen, dextrin, maltose, cane sugar, and milk sugar, must be hydrolyzed to monosaccharides, such as dextrose, levulose, and galactose. Fats are broken up into glycerine and fatty acids. Proteins are hydrolyzed to a large number of products belonging to the class of aminoacids.

These changes are effected by ferments. Proteins are attacked in the stomach by pepsin which hydrolyzes them to the hydrated proteins, albumoses and peptone. These substances would be broken down still further by pepsin but the passage of the chyme into the small intestine removes them from its influence and brings them under that of the powerful proteolytic ferment trypsin, which is more far reaching in its effects than pepsin, breaking down protein through albumoses and peptone into the aminoacids of which the molecule is built. A third ferment, erepsin, is contained in the succus entericus and aids in the disintegration of albumoses and peptone to aminoacids.

Starches are attacked by the ptyalin of the saliva with the formation of the polysaccharide dextrin and the disaccharide maltose. Very small quantities of dextrose are also formed. In the small intestine the carbo-hydrates are digested by the powerful amylase (amylopsin) of the pancreatic juice which forms maltose and a little dextrose. The succus entericus contains a maltase converting maltose to dextrose, an invertase converting cane sugar to dextrose and levulose, and a lactase converting lactose to dextrose and galactose.

Fats are split into fatty acids and glycerine by a lipase of weak action in the stomach and by a much stronger one, steapsin, in the small intestine.

The ferments are of the highest importance, for not only do they play the chief part in the chemical changes which food suffers in the digestive canal, but it is in all probability by similar agents that the nutritional processes of the body as a whole are carried on. We must, therefore, consider briefly their general properties. Ferments have not been separated in a pure form, and we know little of their structure. This is because the actual amount of the ferments present in any solution is extremely small and because they are very unstable; even if a large quantity of a fluid having a strong ferment action be taken for analysis, by the time that the proteins and other bodies accompanying the ferment are removed a large part of it has been lost or destroyed. As far as has been ascertained ferments generally contain nitrogen, and it is probable that they are similar in structure to the bodies upon which they act. They are extremely sensitive to external influences, being easily destroyed by heat, and very susceptible to dilute acids and alkalies; pepsin, for instance, is only active in an acid medium and trypsin in an alkaline. All these ferments act by a process of hydrolysis, that is to say, the molecules upon which they exert their influence take up water and break down into simpler bodies. This method of disintegration is not in any way peculiar to ferment action. Weak acids or alkalies will cause hydrolysis of proteins and starches. At ordinary temperatures the change is small but it is much greater at boiling point. Boiling water alone will hydrolyze many substances, and even cold water does so, although very slowly. But the characteristic of ferment action is that the hydrolytic change is produced with great rapidity and at ordinary temperatures, each ferment having an optimum temperature which, in the cases we are discussing, is that of the body. This may be illustrated by the following experiments. The hydrolysis of a solution of muscle proteins in 4 per cent hydrochloric acid is accompanied in the early stages of the digestion by a fall in the viscosity of the solution which may be measured by the time taken by a given quantity of the fluid to flow through a capillary tube. In some observations of this kind the writer found that the change produced by pepsin in a few hours at body temperature was greater than that produced by hydrochloric acid alone in days at the same temperature or in months at ordinary temperatures. Substances which possess the property of hastening chemical reaction without being themselves affected are known as catalyzers. To this class the ferments belong. They act by rendering the transition from one molecule to another easy, and have been likened to a ladder by the help of which it is possible to get over a wall. The ladder does not supply any energy and can be used an indefinite number of times without suffering change, and the same is true in a general sense of the whole class of catalyzers. It is to be noticed that the ferments are specific in their action; each one can only attack substances of a definite chemical structure and moreover, in the case of isomeric substances, only those having the same stereo-chemical arrangement of their molecules. The way in which these substances act is the subject of research at the present time. It seems probable that a ferment combines with the material it affects, known as the substrate, and that this combination then takes up water and breaks down into simpler molecules, setting free the ferment which can then combine with another portion of the substrate. This is supported, as Bayliss has shown, by the time relations of ferment action at different concentrations. The likelihood of such a combination is also upheld by the fact that a ferment can resist the action of heat to a much greater degree when in the presence of its substrate. Invertase, for instance, is destroyed at 60° C, but when mixed with cane sugar can survive a temperature 25° higher.