ASSIMILATION 85 



of the small intestine. The reactions of this region occur in an 

 alkaline medium, owing to the presence of alkahne salts in the 

 three juices. Bile contains no digestive enzymes and serves 

 chiefly as an emulsifying agent for the fats. That is, fat drops 

 in the presence of bile become divided into very fine droplets, 

 thus offering a much greater surface for the activities of the fat- 

 sphtting enzyme. Pancreatic juice contains three digestive 

 enzymes: steapsin, or intestinal hpase; trypsin; and amylopsin, 

 or intestinal amylase. Steapsin brings about the digestion of the 

 emulsified fats into glycerol and fatty acids. Trypsin is a protein 

 enzyme and acts to spUt the undigested or partially digested pro- 

 tein molecules into simpler compounds known as polypeptids. 

 Amylopsin continues the digestion of starch, begun in the mouth 

 by ptyalin and interrupted in the stomach, and converts starch 

 into maltose, a compound sugar. The intestinal juice contains 

 enterokinase, erepsin, and the sugar-splitting enzjTnes. Without 

 enterokinase, trypsin is inactive and has no effect upK)n proteins. 

 Erepsin is the enzyme that finally completes the digestion of pro- 

 teins into their component amino acids. Of the sugar-sphtting 

 enzymes, maltase acts upon maltose or malt sugar; sucrase, or 

 invertin, upon sucrose or cane sugar; and lactase upon lactose or 

 milk sugar. The effect of these enzymes is to produce simple 

 sugars, principally glucose or grape sugar. The final stages of 

 digestion of all foods occur in the small intestine. 



Some of the materials taken in with the food cannot be changed, 

 or for some reason are not changed. These are carried by peri- 

 staltic contractions toward the large intestine, which is separated 

 from the small intestine by the ileo-caecal valve. In man, about 

 four hours elapse from the time of eating until the first part of the 

 food mass enters the large intestine, and the discharge continues for 

 about two hours. After being retained in the lower part of the 

 large intestine for from ten hours to two days, this undigested 

 and undigestible material, now known as fseces, is discharged from 

 the digestive tract upon relaxation of the anal sphincter. This 

 process is known as defecation, or egestion. The amount of food- 

 stuff egested is about 10 per cent of the amount ingested. 



In the large intestine of some animals there are many bacteria 

 which digest cellulose, a carbohydrate present in the walls of plant 

 cells, and produce simple sugars from it. While some of this sugar 

 is absorbed, it must be kept in mind that the bacteria digest this 



