After preliminary maceration under the conditions of the moisture, warmth, and motion in the stomach, and after partial digestion there, the food, mixed with gastric juice, passes in a pultaceous mass, known as chyme, into the duodenum. Here the physical conditions are almost identical with those found in the stomach, but the chemical composition of the new digestive fluids - namely, the bile and pancreatic and intestinal juices - is alkaline, and a number of new ferments complete the solution and digestion of the food.

The starches which were but partially digested by the saliva are converted into dextrin, maltose, and glucose by a diastatic ferment - amylopsin - contained in the pancreatic juice. Cane sugar is converted by the intestinal juice into glucose, any proteids which have been but partially digested in the stomach are completely converted into proteoses and peptones by the proteolytic ferment trypsin of the pancreatic juice, and the fats are emulsified and saponified by the combined action of the bile and a pancreatic ferment.

Pancreatic juice is the most active and comprehensive digesting fluid of the body. It is not only much stronger than the gastric juice in its action upon proteids, being able to form peptones with fewer intermediate products, but it possesses the distinct advantage that it also digests fats and carbohydrates by a ferment, called steap-sin, which acts in either acid or alkaline media.

Its amylolytic power is stronger than that of the saliva, for it digests the raw starch and cellulose which is eaten in such vegetables as celery, lettuce, or radishes, and in fruits like the apple.

The ultimate products of trypsin digestion are antipeptones and hemipeptones. The ferment acts best in a fluid medium rendered alkaline by from 0.5 to 1 per cent sodium carbonate. It also digests proteids energetically in a neutral medium, but free acid soon destroys it. Chittenden has shown that combined acids do not necessarily put a stop to trypsin proteolysis.

The bile plays several roles in intestinal digestion, but its chief action is in aiding the emulsion of fats, described below. It can accomplish the absorption of fats even in the absence of pancreatic juice. When bile is absent the fatty food may decompose in the intestine and develop gases and foul odours.

Voit says that the white colour of icteroid stools is dependent rather upon the presence of undigested fat than the absence of bile, for with a meat instead of a milk diet they may still be of a dark colour. When a biliary fistula is made in dogs and the bile is drained away, they emaciate and have a ravenous appetite if fed upon meats and fats, but not if fed upon carbohydrates (Voit).

The quantity of bile secreted is increased by a nitrogenous diet, and diminished upon an exclusive fat diet (Uffelmann). Vegetable foods colour the bile green; animal foods make it yellow or orange.

Bile is often said to prevent putrefactive changes in the food in the intestine. It is not, however, an antiseptic substance, for, unlike the gastric juice, it easily undergoes decomposition, and the influence attributed to it is due rather to its promotion of peristalsis, thereby keeping the intestinal contents moving and preventing stagnation and putrefaction.