800 PHYSIOLOGY OF DIGESTION AND SECRETION. 



gained increased recognition in recent years. The digestion of the 

 protein begun by the pepsin or by the trypsin is carried to comple- 

 tion by the action of the erepsin. 



3. Inverting enzymes capable of converting the disaccharids into 

 the monosaccharids. These enzymes are three in number : maltase, 

 which acts upon maltose (and dextrin); invertase or invertin, 

 which acts upon cane-sugar; and lactase, which acts upon lactose. 

 The maltase acts upon the products formed in the digestion of 

 starches, the maltose and dextrin, converting them to dextrose 

 according to the general formula: 



CisHtfOu + H 2 = C 6 H 12 6 + C 6 H 12 6 



Maltose. Dextrose. Dextrose. 



In the same way invertase converts cane-sugar to dextrose and 

 levulose, and lactase changes milk-sugar to dextrose and galac- 

 tose. This inverting action is necessary to prepare the carbohy- 

 drate food for nutritive purposes. Double sugars cannot be used 

 by the tissues and would escape in the urine, but in the form of 

 dextrose or dextrose and levulose they are readily used by the tis- 

 sues in their normal metabolic processes. 



4. Nuclease. An enzyme to which this name may be given is 

 said to occur in the small intestine. It acts upon the nucleic-acid 

 component of nucleoproteins splitting it with the formation of the 

 corresponding purin and pyrimidin nucleotides (see p. 851). 



5. Secretin. As explained above, this hormone plays an im- 

 portant role in the control of the secretion of the pancreas. It is 

 not an enzyme, but a more stable and definite chemical substance 

 which is secreted or formed in the intestinal mucosa in a preliminary 

 form, prosecretin, and under the influence of acids is changed to 

 secretin. In this latter form it is absorbed, carried to the pancreas, 

 and causes a flow of pancreatic secretion. 



It seems probable that the entire physiology of the intestinal secretion is 

 not yet known. Recent work* shows that when a closed loop of the duodenum 

 is produced by ligatures and is left in the abdomen, the continuity of the 

 alimentary canal being established by a gastro-enterostomy, the animal (dog) 

 dies within a short period with signs of acute intoxication. When the contents 

 of the loop are injected into another animal they cause a similar fatal result. 

 The evidence at hand indicates that the toxic material is formed by the mucosa 

 of the duodenum. Similar loops made at lower points in the intestine do not 

 produce this toxic material. These results throw some light on the fatal effect 

 of high intestinal obstruction in man, and from a physiological standpoint 

 they suggest the existence of a peculiarity in the duodenal mucosa which may 

 prove to be of importance in its normal activity. 



Absorption in the Small Intestine. Absorption takes place 

 very readily in the small intestine. The general correctness of this 

 statement may be shown by the use of isolated loops of the intestine. 



* See Whipple, Stone, and Bernheim, "Journal of Exp. Medicine," 17, 

 286 and 307, 1913. 



