The chemical changes which the food undergoes in the large and small intestines are exceedingly complex and concerning which we have greatly insufficient knowledge.

When the partly digested food from the stomach (chyme) reaches the intestines, it meets several liquids secreted by such special glands as the liver, pancreas (sweet bread), and certain smaller glands that are distributed in the membranes of the intestinal walls. The liver secretes the bile, pancreatic juice comes from the pancreas, and various intestinal juices flow from the small glands along the intestinal walls.

Fig. 4. - Duels introducing the bile and pancreatic juice into small intestine. (Gerrish.)

104. The Bile

This is a secretion of the cells of the liver and from the inner wall of the gall bladder, that from the former source being thinner, less ropy, and poorer in solid matter than that secreted by the gall bladder. After elaboration, bile is stored in part, at least, in the gall bladder. Human bile is a golden yellow liquid, alkaline, and bitter in taste. That from the gall bladder has been found to contain from 82 per cent to 90 per cent of water or from 10 per cent to 18 per cent of solid matter. The liver bile is poorer in solids, analyses showing the proportion of water to be from 96.5 per cent to 97.5 per cent. The solids of bile include a great 'variety of compounds, the chiefest of which are certain bile salts and pigments.

Numerous other compounds are present, such as fats, soaps, cholesterin, urea, and mineral salts.

105. Bile Salts

The bile salts are mainly sodium compounds of glycocholiCy taurocholic, and related acids and the best-known pigments which occur under normal conditions are bilirubin, which is reddish yellow, and biliverdin, which is green. Several other pigments occur in the concretions which form in the gall bladder, known as gallstones. No ferment (enzym) has been found in the bile, at least in more than traces.

106. Secretion Of Bile

The secretion of the bile is irregular in quantity, and, as is the case with gastric juice, appears to be induced by chemical excitants of which acids, especially hydrochloric, seem to be especially effective. Of the nutrients, the proteins exert the most influence in this respect. The acceleration of secretion occurs at a greatly varying time after food ingestion, the maximum flow being determined by the kind of food.

The bile compounds are in evidence in certain pathological conditions. When for any reason the discharge of bile into the intestine is retarded and the organism attempts to eliminate it through the kidneys, the tissues become charged with its compounds and take on a yellowish coloration, and the subject is said to have jaundice, - a condition sometimes attended with serious results. Concretions are formed in the gall bladder that in man are characterized by the presence of cholesterin, a bile compound.

107. The Pancreatic Juice

This secretion has the most comprehensive action on the food nutrients of any one of the intestinal liquids. It originates in the pancreas (sweet bread). Its flow is intermittent, being induced by the reaction especially of the acids in the partially digested foods from the stomach. The amount secreted and its composition appear to change with the kind of food. It contains about 87.5 per cent of water and 12.5 per cent of solid matter. This secretion acts upon all classes of nutrients, as it contains a variety of ferments greatly unlike in function.