106 PRINCIPLES OF ANIMAL BIOLOGY 



which, although unable to digest the original proteins, attacks the pep- 

 tones which result from digestion in the stomach, reducing them to 

 amino acids. It thus supplements the action of trypsin. The other 

 enzymes convert maltose and the dextrins (resulting from the operation 

 of ptyalin and amylopsin upon starches) into glucose and other simple 

 sugars. 



Secretin, as indicated above in connection with the stimulation of the 

 pancreas, is not an enzyme but a hormone. It exists in the wall of the 

 duodenum as prosecretin which is stable and does not affect the pancreas. 

 The acid from the gastric juice mixed with the food coming from the stom- 

 ach changes the prosecretin into secretin which is absorbed and carried 

 by the blood to the pancreas and the liver, which are thereby stimulated 

 to secrete pancreatic juice and bile, respectively. 



Digestion in the Large Intestine. — The large intestine produces no 

 enzyme. Water and some of the products of digestion are absorbed 

 here. Bacteria flourish in the large intestine. Many of these attack 

 proteins, while others attack the cellulose of plant cells and perhaps so 

 break it down that some sugars are recovered from it. Bacteria which 

 attack proteins are not numerous, however, when the products of protein 

 digestion are removed with normal rapidity. Bacteria may also supply 

 an important vitamin, as is indicated later. 



Absorption. — In the more complex animals absorption occurs along 

 the portions of the alimentary tract. In such simple animals as Hydra 

 all the endodermal cells are bathed in the products of digestion or carry 

 on digestion in themselves, and through these cells absorption takes place. 

 Some of this material not used by the endoderm is passed on by diffusion 

 to the ectodermal cells. In animals with a circulatory system the sim- 

 pler substances pass through the absorbing cells directly into the blood 

 stream. 



In man, as stated earlier, there is little absorption in the stomach. 

 Most of it occurs in the small intestine, whose inner surface is enormously 

 enlarged by the fingerlike protrusions called viUi (Fig. 91). Amino acids 

 and simple sugars are absorbed directly into the blood, which carries them 

 through the liver before delivering them to the general circulation. 

 Glycerol and the fatty acids are absorbed, but in the process are at least 

 partly reconverted into fats. Since fats are insoluble, they exist in the 

 form of droplets and are delivered thus, not to the blood, but to the lymph 

 vessels. However, since the lymph vessels empty into the blood stream 

 (in the left shoulder, page 131), the entrance of fat into the blood is 

 merely delayed. 



While absorption by the intestinal wall is partly simple diffusion, 

 some selection is practiced by the absorbing cells, so that certain sub- 

 stances are passed readily, others are retarded or rejected. This selecti\'e 



