The food which enters the duodenum from the stomach through the pylorus is changed from acid to alkaline reaction through the agency of sodium carbonate, which is abundantly secreted in all of the intestinal digestive juices. These intestinal digestive juices, as indicated above, are the pancreatic juice, the bile, and the succus entericus.

The pancreatic juice is about 98.5 per cent water and the rest solid, the solids consisting of enzymes, mucus, and salts, the enzymes being amylase or amylopsin, trypsin, and lipase or steapsin. Principal among the salts is sodium carbonate.

The bile from the liver is a greenish-brown secretion and consists of mucus, bile acids, bile salts, and bile pigment. While there are no enzymes in the bile, and while that secretion apparently exerts no chemical change other than that produced by the sodium carbonate, still, the total influence of the bile in the alimentary canal upon the processes of digestion and absorption seems to be a very important one, from the fact that the disturbance of the flow of bile is followed very early by the disturbance of digestion and absorption.

The following functions have been enumerated as among the more important ones performed by the bile: First, emulsion of fats; second, saponification of fats; third, absorption of fats; fourth, facilitation of the passage of food along the alimentary canal perhaps through lubrication by the mucus, perhaps through chemical stimulation of peristaltic movements, and perhaps through both of these influences. Because of this more ready passage of foods and materials along the alimentary canal, the bile seems to regulate to a considerable degree the regularity of bowel movements, interference with the bile secretions being followed very soon by constipation.

The succus entericus consists of about the same proportion of water as existed in the saliva and gastric juices. The solids comprise four more important enzymes and chemical agents, secretin and sodium carbonate. The enzymes are, invertin, maltase, lactase, and enterokinase. The three digestive juices just described - pancreatic juice, bile, and succus entericus - are all mixed with food which passes into the duodenum from the stomach and produces chemical changes as the food passes through the duodenum and jejunum. To enumerate these changes we may begin with the action of the amylase. This starch-splitting ferment acts upon any starch and dextrin that may have passed into the intestines. As a rule, the digestion of starches has not been completed in the salivary digestion and there still remain portions of starch and dextrin. Any such remaining portions are acted upon by the amylase and changed to maltose, the steps of the change being identical with those of salivary digestion.

The trypsin acts upon any undigested proteins that have passed into the intestines from the stomach. As a rule, there still remain portions of undigested proteins, together with syntonin and proteoses. All these proteins and partly digested mid-products are attacked by the trypsin and changed into peptone. In this connection it is necessary to explain that trypsin, as it comes from the pancreas, is inactive and requires to be activated by some other agent before it can begin its work. This activating agent is the enterokinase of the succus entericus.

Why this particular enzyme, the trypsin, should need to be activated while the other enzymes do not need any such activating agent is not readily seen. It has been suggested that if the trypsin were active when secreted, and if it had at first the power of digesting proteins and alkaline mediums, then the pancreatic gland itself would be digested by its own secretion. As a matter of fact, pathological cases are known where digestive changes have been made by the pent-up trypsin of a pancreatic cyst. But this is a pathological condition that is comparatively rare. Except in these rare cases, the inactive trypsin passes out of the pancreatic gland, to be first activated by the enterokinase. At the time of this activation the trypsin is mixed with partly digested food materials in the alimentary canal, whose living walls are protected by a coating of mucus. It may readily be understood that the active trypsin will exert its influence upon the food materials with which it is intimately mixed, and not upon the intestinal wall from which it is separated by a film of mucus.

If the peptones produced in the gastric and intestinal digestion are not rapidly absorbed, they are likely to be changed in part by further breaking-down processes into various simpler end products which are collectively known as the amino acids. There are two theories regarding the occurrence of these substances in the alimentary canal. One is that they are split off from the complex protein molecules in the regular course of peptone formation. The other is, as stated above, that they represent a continuation of the splitting beyond the normal point of digestion as represented in the peptones. While the former of these two theories is more reasonable, neither theory has been conclusively demonstrated. The fact remains that these amino products are always present in smaller or greater quantities.

Lipase is the enzyme through whose activity the fats are digested. It will be remembered that the fats are not chemically acted upon in the mouth. Mechanically, they are subdivided into minute particles and thoroughly mixed with other foods. While the fats themselves are not acted upon in the stomach, fatty tissue is disintegrating through a digestion of the connective-tissue mesh-work which holds the fat globules together in a tissue formation. This digestion of the connective-tissue meshwork releases the fat globules and they float in the food mass, becoming partially emulsified. The chemical action on the fat itself first takes place in the duodenum and begins with a change of reaction of the acid chyme from the stomach through the agency of the sodium carbonate.

The next step in the process is the breaking up of fat molecules into glycerin and the fatty acids. Free fatty acids are very easily saponified. Sodium carbonate, mild alkali though it is, can saponify fatty acids. The sodium soap thus formed is an active emulsifying agent, and acting in conjunction with the other emulsifying agents present in the small intestines, rapidly changes the whole of the fat to a fine emulsion which gives the intestinal contents a white, milky appearance.

Nature's plan in thus providing for the emulsification of fats is analogous to her plan in the digestion of proteins and starches; that is, the first essential step in digestion is division. Olive oil or other fats that are liquefied at body temperature - and they are practically all so liquefied - can only be brought to a fine state of division through an emulsifying agent. In the emulsion, the fat having been brought to the finest possible state of division into minute particles, the enzyme action can go forward very rapidly. As a result of this combined action of the enzyme and the free sodium carbonate the fats are rapidly saponified. These sodium soaps, being soluble in water, are readily absorbed. As they pass through the epithelium of the alimentary canal they change back again to neutral fats in minute divisions and they circulate in the body as such.

Reference has been made above to a reduction of all starches to maltose by the ptyalin and the amylase. The disaccharids, while evidently absorbable, do not circulate as such in the blood. Only monosaccharids are found in the blood. The disaccharids that are eaten as such - for example, saccharose and lactose, and the disaccharids formed as such by starch digestion (maltose) - all require to be split up into their monosaccharid constituents before they are absorbed. Three enzymes of the succus entericus are devoted to this last chemical act of digestion: Invertase whose action breaks up saccharose into dextrose and levulose; maltase, whose action breaks up maltose into two dextrose molecules; lactase, whose action breaks up lactose into dextrose and galactose.

The final products of carbohydrate digestion are, therefore, monosaccharids consisting very largely of dextrose, either ingested as such, or resulting from the splitting up of starches, or of cane sugar, or of milk sugar, a small amount of levulose either ingested as such from certain fruits, or resulting from the breaking up of saccharose and a minute quantity of galactose from split-up milk sugar. Summing up, then, the final results of the chemical changes of digestion upon the organic foodstuffs, starch, sugar, fat, and protein, we find that the end products are three in number, namely: Monosaccharids, soaps, and peptones. All of these end products are soluble in water, readily diffusible through animal membranes, and simple in chemical structure as compared with the ingested foods.