146 VRINCIPLES OF ANIMAL lilOLCKlY 



of a biliary fistula a large share of the ingested fat is not digested and 

 may be recovered in the feces. 



The Intestinal Secretion. — The secretion of the small intestine, 

 sometimes called the succiis entericus, is produced in small tubular glands 

 which are local invaginations of the mucosa of the intestine. This secre- 

 tion consists of eyiterokinase, erepsin, several inverting enzymes, and secretin. 

 Enterokinase, as stated above, converts inactive trypsinogen into active 

 trypsin. Erepsin is a protein-splitting enzyme which acts particularly 

 on peptones and deutero-albumoses, reducing them to amino-acids and 

 supplementing the action of trypsin. The inverting enzymes are three 

 in number. These are m.altase, invertase, and lactase. Maltase con- 

 verts into dextrose the maltose and dextrins resulting from the operation of 

 ptj'alin and amylopsin upon starches. Invertase changes cane sugar 

 to dextrose and levulose. Lactase converts milk sugar to dextrose and 

 galactose. 



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

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

 enz3'mes. 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 pro- 

 tein digestion are rapidly removed. 



Assimilation. — After foods have been converted into simple substances 

 and rendered soluble they are absorbed. These substances are then 

 resynthesized into more complex substances, that is, proteins, carbohy- 

 drates, or fats; or perhaps portions of them without being recombined 

 are oxidized with the hberation of energy. The method of synthesis is 

 unknown but it is presumed that enzymes play an important part in 

 the process. 



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 diffu- 

 sion to the ectodermal cells unchanged, or possibly it is synthesized first 

 in the endodermal cells, in which case the proteins, fats and carbohy- 

 drates must again be broken down into diffusible substances before they 



