Steapsin, another ferment of the pancreatic juice, acts on both fats and carbohydrates in either an acid or alkaline medium. The sodium in the bile unites with the fatty acid, forming a soap which coats the tiny particles of fat and emulsifies them. The bile thus aids in the absorption of the fats. It also lubricates the intestinal mass, facilitating its passage through the entire length of the intestines. Thus it is a very potent agent in regulating the bowel movements. A diminution in the flow of bile quickly expresses itself in constipation. Fat and protein stimulate the activity of the liver, while starches, if taken in excess, incline to overload it. The food is forced along the intestinal tract by peristaltic or muscular relaxation and contraction waves, as in the stomach. As it is so forced, the nutrient elements, after being put into condition for absorption, are taken up through the villi of the intestinal walls by the portal veins and the lacteals of the submucous lining. A larger proportion of food is digested and absorbed than was formerly supposed, and the excretions from the intestines are, in many cases, made up almost entirely of refuse, and of the catabolic waste of the system. In an ordinary mixed diet, it is stated that about ninety-two per cent. of the proteins, ninety-five per cent. of the fats, and ninety-seven per cent. of the carbohydrates are retained by the body. _In digestion, it is of the utmost importance that the muscular, mucous, and submucous coats, and the secreting glands of the stomach and intestines be kept thoroughly strong and active, that the digestive juices may be freely poured out, the nutriment be freely absorbed, and the food be moved along the digestive tract. The strength of any organ is gained through the nutriment in the blood; therefore, daily exercise, which calls the blood freely to these organs is imperative. Daily exercise should be directed to the vital organs. A walk for exercise is not sufficient._ * * * * * [Sidenote: Absorption of Food] The greater part of the food is absorbed through the intestines, yet some proteins which have been fully digested by the gastric juice, and certain fats, particularly the fats in milk, which are in a natural state of emulsion, may be absorbed through the walls of the stomach. However, the absorption through the stomach is small compared to that through the small intestine. [Illustration: DIAGRAMMATIC REPRESENTATION OF VILLUS. _a_, 1, cylindrical or “sucking” cells; 2, goblet cell; 3, capillaries; 4, food particles ready for absorption by the cells; _b_, cylindrical and goblet cells seen from above.—Adapted from LANDOIS.] The small intestine is peculiarly fitted for absorption. Its mucous lining is thrown up into folds to furnish a larger surface for this purpose. The folds hold the food as it passes toward the large intestine, until the villi have the opportunity to absorb it.