Bread, biscuits, cakes, and puddings, if they have been carefully masticated, readily form an emulsion in the stomach and are thus prepared for complete digestion in the bowels. The less mastication such foods get, the longer they are in being prepared for transmission into the bowels, or in undergoing the transformation into sugar which is due to the salivary ferment. Potatoes and other vegetables are reduced to a pulp, and otherwise prepared for digestion in the bowels ;but the cellulose covering of vegetable cells is not attacked by the gastric juice.

The whole of the food is thus broken down by the teeth or the disintegrating power of the stomach, some portion of starch is converted into sugar, and a portion of meat, milk, fish, and eggs is converted into proteoses and peptones. The whole mass forms a creamy mixture or chyme, which is slowly propelled through the pyloric sphincter into the small intestines.

It may thus be perceived that, important as is the work of the stomach, the entire process of digestion is not completely performed in that cavity. Indeed, so far as many foods are concerned, it may be said that digestion only begins there. Moreover, the work of the stomach, in certain cases, is entirely replaced by intestinal digestion. This occurs in patients who have had an opening made between the stomach and bowels by the operation of gastro-enterostomy. In these cases the food passes quickly out of the stomach into the bowels, and digestion appears to go on almost as well as in the normal process.

But the normal course of the food is through the pylorus to the bowels, and it goes through very slowly. The opening and closing of the pylorus, like the muscular contractions of the stomach, is governed by a nervous mechanism which causes the sphincter to relax each time a peristaltic wave reaches it, when the chyme passes through like a squirt or jet of water. Just as the sphincter is relaxed by a reflex movement, so it is closed by another reflex action, which is believed to be due to the arrival of the acid chyme in the duodenum. The presence of a small amount of acid chyme acts as a stimulus to the nerves in the duodenum, and a message is sent along them which is reflected to the pylorus and causes it to be closely shut until that portion of chyme is neutralized by the bile and pancreatic fluids; and this action protects the intestinal mucous membrane from injury by the acid fluid.

The bile is an alkaline fluid secreted by the liver; the pancreatic fluid is an alkaline secretion from the pancreas; and they are both important items in the digestive process. When the acid chyme passes over the orifice of the bile duct in the bowels it causes a reflex contraction of the gall bladder and a flow of bile into the bowels. The bile does not flow into the intestines when digestion is not going on ;but, like the gastric juice, the flow of bile only occurs during digestion; nevertheless, it continues for several hours, and the total quantity of bile secreted in a day amounts to two pints or more. This fluid mixes with the partly digested food. It has no influence on proteins (meat, milk, eggs, etc.), except that it precipitates dissolved albumin. Neither has it much influence on starch and sugar; but the pancreatic fluid has more effect on these substances in the presence of bile than when it is absent. But the bile has a definite influence on the absorption of fat, and is therefore an important element in its digestion.

The pancreatic fluid enters the duodenum through a separate duct, which opens into the bowel close to the bile duct. This fluid is of the greatest importance in intestinal digestion. It begins to be secreted as soon as acid chyme arrives in the duodenum, its presence being provoked by the action of a hormone or chemical messenger from the duodenum, called secretin; and it continues to be secreted for several hours after the meal is eaten. It contains three ferments or enzymes. There is trypsin, which digests proteins (meat, fowl, fish, game, eggs, milk, etc.); amylopsin, which transforms starch into sugar; and steapsin, which makes an emulsion of all kinds of fat and splits some of them into fatty acids and glycerine. Pancreatic fluid transforms all proteins into proteoses and peptones, and the latter into amino-acids, which are taken up by the mucous membrane and reconverted into the proteins (albumins and gobulins) of the blood. It digests the nuclei of cells, and acts more powerfully than pepsin on elastin, but has no action on keratin and the cellulose of vegetable tissues and cells. Starch is rapidly converted by it into sugar. The various fats are made into an emulsion, which the cells of the mucous membrane eat up greedily. Some of the fats are split into fatty acids, which combine with carbonate of soda in the pancreatic fluid and form a soap which is easily absorbed.

There is still another digestive fluid. This is provoked by the presence of food in the bowels; it is secreted by the intestinal mucous membrane, and is called the succus enter-icus. Like the former fluids, it contains several ferments. One of these is of importance by assisting the trypsin of pancreatic juice. Another completes the digestion of starchy foods by converting the complex sugars into simpler sugars. Thus also the ordinary domestic sugar, malt sugar, and milk sugar, are converted into dextrose, levulose, and galactose, which are much more simple and readily diffused through the coats of the bowels or are absorbed by the mucous membrane. It has no action on fat or starch. But it has a definite influence on proteins and assists in converting peptones and proteoses into amino-acids, which are quickly absorbed. The amino-acids are of great importance, as it is from these substances new blood and flesh are formed; whence they have been called "building stones"

Thus we have seen that most of the food is transformed by various ferments or enzymes from insoluble substances which cannot get into the blood to soluble materials which readily pass through the mucous membrane or are taken up by the cells of the mucous membrane and passed on to the blood to be circulated to the various tissues and organs of the body. There is naturally a certain amount of residue, consisting of indigestible particles which the enzymes have not affected. The amount of this residue varies according to the nature of the food; but it is mixed with some mucus from the bowels, the degenerated cells from the mucous membrane, the remains of bile and other secretions, and an enormous number of bacteria, which together constitute the faeces.