796 PHYSIOLOGY OF DIGESTION AND SECRETION, 



gained increased recognition in reoent 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: 



C12H22O11 + H2O = CeHisOs + CeHijOe 

 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. 848). 



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. The chemical nature of 

 secretin is not known, but there is reason to believe that it is a 

 basic substance related in structure to histamine.* 



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. 

 Salt solutions of varying strengths or even blood-serum nearly 

 identical in composition with the animals' own blood may be ab- 

 sorbed completely from these loops. Examination of the contents 

 of the intestine in the duodenum and at the ileocecal valve shows 

 that the products formed in digestion have largely disappeared in 

 traversing this distance. All the information that we possess in- 

 dicates, in fact, that the mucous membrane of the small intestine 

 absorbs readily, and it is one of the problems of this part of physiology 

 to explain the means by which this absorption is effected. Anatomi- 



* Koch, Luckhardt, and Keeton, "American Journal of Physiology," 52, 

 508, 1920. 



