The pancreatic fluid contains an inactive zymogen called trypsinogen, which by means of the entero-kinase of the succus entericus is converted to trypsin. This is a strongly proteolytic ferment which resembles, but acts much more rapidly than, pepsin in an alkaline medium, so that alkali-meta-protein is the first product formed. It also acts much more powerfully on certain proteins, such as elastin, although it cannot digest collagen, whereas raw connective tissue can be digested by the gastric juice. There are other minor differences; but the most important point to be noted is that protein digestion is carried much further by trypsin than pepsin, polypeptides and amino-acids always being formed.

The succus entericus is also probably secreted through excitation by hormones. Cohnheim proved that it contains erepsin, a peptolytic enzyme which attacks the proteoses and peptones produced by trypsin and pepsin, and forms from them their final cleavage products, the amino-acids, thus assisting the action of trypsin. Apparently the only native protein which it digests is caseinogen.

Besides the enzyme activities which we have just described, there is a coincident action of the bacteria. This does not usually arise in the stomach, because of the antiseptic effect of the gastric juice, but originates quite high up in the small intestine, and may take place in two ways: firstly, by the production of enzymes, which act in the same way as the pancreatic juice; and, secondly, by fermentation and putrefaction, due to the organisms themselves. The effects on the proteins depend largely on their relative position in the alimentary canal. In the open air, e.g., indol rapidly makes its appearance, and shortly thereafter phenols betray their presence, especially if the reaction is alkaline - an example of a toxin being followed by its appropriate antitoxin. Precisely the same thing happens in the bowel, and not only amino-acids but members of the aromatic and fatty series quickly arise. The lower end of the small intestine frequently has an acid reaction, and if this be continued into the colon, less putrefaction is likely to arise and the risks of auto-intoxication are diminished. So long as bacterial action is not excessive it is an aid to the pancreatic juice and successful in decomposing poisonous products which might be absorbed. Choline - e.g., a derivative of lecithin, found in large quantities in egg-yolk - is decidedly poisonous, but is usually broken up into carbonic acid, methane, and ammonia by bacterial enzymes.

Operations of a parallel character on fats and carbohydrates are performed by bacteria, acid products being formed from them by lactic, butyric, and even alcoholic fermentation.

Thus the end-products of protein digestion are proteoses, peptones, amino-acids, and ammonia and salts of ammonium, which in the liver are synthesised to urea.