When the pylorus opens, food, now reduced to liquid chyme, is projected into the upper part of the small intestine, where it usually lies for some time in the curve of the duodenum, until several additions have been made to it from the stomach. While the food rests here the bile and pancreatic juice are poured out upon it, and here also, as well as in other parts of the small intestine, a certain amount of intestinal digestive juice ("succus entericus") is secreted by the glands of the lining membrane and mixed with the intestinal contents. While for purposes of description the pancreatic and intestinal juices and the bile may be discussed separately, it is to be remembered that in normal digestion they always act together. Cannon's observations showed that after a certain amount of food and digestive juices has accumulated as just described in the first loop of the small intestine, the mass all at once becomes segmented by constrictions of the intestinal walls, and the segmentation is repeated rhythmically for several minutes, so that the individual portions are subjected to relatively extensive and energetic to-and-fro movement, which is doubtless very important in facilitating the emulsification of fat. Other effects of the muscular constrictions which cause the segmentation are (1) a further mixing of food and digestive juices, (2) the bringing of the digested food into contact with the absorbing membrane, (3) the emptying of the venous and lymphatic radicles in the membrane, the material which they have absorbed being forced into the veins and lymph vessels by the compression of the intestinal wall. After a varying length of time the segmentation ceases and the small segments are carried forward individually by the peristaltic movement, or join and move on as a single body.

The fluid food mass which the stomach pours into the duodenum contains a small amount of free hydrochloric acid besides a larger amount combined with protein and sometimes organic acids from the food as eaten, or from bacterial fermentation of carbohydrates in the stomach. The pylorus having closed, the alkalinity of the bile, the pancreatic juice, and the intestinal juice combine to neutralize the acids present.

In man the main duct of the pancreas and the bile duct unite and empty into the small intestine about 8 to 10 cm. (3 to 4 inches) below the pylorus. The pancreatic juice is a clear liquid having an alkalinity probably equivalent to a 0.5 per cent solution of sodium carbonate and containing three important enzymes or their zymogens -trypsin, amylopsin (amylase), and steapsin or lipase.

The outflow of the pancreatic juice begins at once when any of the acid stomach contents passes through the pylorus, and has been shown by Bayliss and Starling to be due to a definite chemical substance, secretin, a typical hormone produced as the result of the action of the acid upon some constituent of the intestinal mucous membrane, which is absorbed and carried by the blood to the pancreas and there stimulates the flow of pancreatic juice.

Human bile, which, as already stated, enters the intestine through the same duct with the pancreatic juice, is a slightly alkaline solution containing, in addition to water and salts, bile pigments, bile acids (as salts), cholesterin, lecithin, and a peculiar protein derived from the mucous membrane of the bile ducts and gall bladder. The presence of the bile in the intestinal contents greatly increases the solubility of the fatty acids, while at the same time it diminishes the surface tension between watery and oily fluids. Bile may also accelerate the action of pancreatic lipase in a more direct way. Thus bile aids both the digestion and the absorption of fats. The bile acids are themselves absorbed to a considerable extent and again secreted by the liver. The secretion of bile by the liver, although variable in amount, is continuous. Its ejection from the gall bladder into the intestine occurs, however, only during digestion, and appears to be excited by the passage of chyme through the pylorus, and to run parallel to the outpouring of the pancreatic juice. According to Starling, the rapid flow of bile during intestinal digestion is due not only to the pouring out of what was previously stored in the gall bladder, but also to an increased rate of secretion to which the liver is stimulated by the same chemical mechanism which stimulates the flow of pancreatic juice.

The intestinal juice is a distinctly alkaline liquid secreted by the tubular glands (crypts of Lieberkuhn) with which the small intestine is lined. It contains at least five enzymes: enterokinase, by the action of which trypsinogen is converted into trypsin, erepsin, which produces further cleavage of the proteoses and peptones; and the three enzymes, sucrase (or in-vertase), maltase, and lactase, which hydrolyze respectively the three disaccharides, sucrose, maltose, and lactose. The secretion of intestinal juice is probably stimulated by secretin, and possibly also by another hormone whose production is dependent upon the presence of pancreatic juice.

Careful observations on the reaction of the contents of the small intestine were made by Moore and Bergin in 1897.* Samples taken through a fistula immediately above the ileocaecal valve were always alkaline to methyl-orange, lacmoid, and litmus, but acid to phenolphthalein. Hence neither hydrochloric acid, nor any appreciable amount of the stronger organic acids such as acetic, butyric, or lactic, could have been present in the free state. The acid reaction shown by phenolphthalein was probably due either to traces of organic acids, or possibly to dissolved carbonic acid, or to acid-protein compounds not yet completely digested and absorbed. It seems probable that this fairly represents the condition as to reaction which exists throughout the greater part of the small intestine. Under such conditions all three classes of foodstuffs would be readily attacked by the digestive enzymes present, and brought into condition for absorption - the carbohydrates as monosaccharide; the fats as fatty acid and glycerol; the proteins (chiefly at least) as amino acids.

The rate of passage of different foodstuffs through the small intestine has been studied by Cannon with the aid of the Roentgen rays, according to the general method already described. Fat, carbohydrate, and protein foods, uniform in consistency and in amount (25 cc), were fed to cats which had been fasted for 24 hours. At regular intervals for 7 hours after feeding, the shadows of the stomach and intestinal contents were observed by means of the Roentgen rays.

* Very recently the subject has been reinvestigated by Long and Fenger, using modern methods for the actual measurement of hydrogen ion concentration. See Journal of the American Chemical Society, June, 1917.

The process of rhythmic segmentation above described was seen with all three kinds of foodstuffs, and the frequency of its occurrence corresponded roughly to the amount of food present in the intestine.

Absorption takes place very readily in the small intestine -more readily and completely than can be explained by the purely mechanical laws of diffusion. On this account the process is sometimes called "resorption" to distinguish it from passive absorption such as takes place by diffusion through non-living membrane.

Observations have been made upon a patient having a fistula at the end of the small intestine. In this case it was found that 85 per cent of the protein matter of the food was absorbed before this point was reached, and the absorption of the other foodstuffs is probably equally complete. For this patient the food began to pass the ileocaecal valve in from 2 to 5½ hours after eating, but the time required from the eating of the food until the last portions had passed into the large intestine was 9 to 23 hours.