ALIMENTATION AND DIGESTION 117 



D. DIGESTION AND ABSORPTION IN THE SMALL INTESTINE 



AND IN THE LARGE INTESTINE 



Every few minutes during the process of gastric digestion 

 the pylorus opens and the stomach forces a few cubic centi- 

 meters of chyme into the intestine. Chyme, which consists 

 of water holding in solution certain products of digestion, 

 and carrying in suspension larger quantities of undissolved 

 matter, has the consistency of moderately thick pea soup. 

 The suspended matter consists, among other things, of small 

 bundles of muscle fibers (from meat), fat melted by the 

 heat of the body and set free from adipose tissue by the 

 digestion of its connective tissue, bits of coagulated protein, 

 such as casein from milk or the white of egg, together with 

 starches, fats, and proteins of animal or vegetable foods. 

 Thus far the digestive processes in the mouth and stomach 

 have been essentially preparatory to the main chemical work 

 of digestion, which takes place in the small intestine. The finely 

 subdivided food is now attacked by the digestive juices of 

 the small intestine brought into solution, and otherwise made 

 ready for absorption into the blood. 



25. The general structure of the intestine; the pancreas 

 and the liver. The main functions of the intestine, like those 

 of the stomach, are indicated in the structure of two of its 

 coats, the muscular coat and the glandular mucous mem- 

 brane. The fibers of the former are arranged in two layers 

 an inner layer in which they are circularly disposed around 

 the mucous membrane (see Fig. 58), and a much thinner 

 outer layer in which they run lengthwise. The contraction, 

 or shortening, of the circular fibers constricts the bore, or 

 lumen, of the tube, and this constriction of the intestinal 

 tube is the most important work of the muscular coat. 

 Sometimes the constriction is confined to one place ; at other 

 times it moves along the tube, pushing before it the contents. 

 (See under Peristalsis, p. 125.) 



