Chemical Changes in Animal Organism. 299 



Enzymes are catalysts derived from living objects 

 of unknown but complex chemical character; they are 

 readily destroyed by heat, and act only within compara- 

 tively narrow limits of temperature and of acidity or 

 alkalinity of the medium. They are highly specific in 

 their reactions, attacking only substances of certain de- 

 finite constitutions and stereochemical configurations. A 

 limited number of cases is known in which enzymes 

 ivhich bring about hydrolysis can also cause the reverse 

 process of synthesis. 



Some Examples of Enzyme Action in the Animal 

 Body. It is of interest to trace out the various degrada- 

 tive processes which food undergoes during its passage 

 through the alimentary tract. In the mouth it comes 

 into contact with the saliva which is secreted by the 

 salivary glands and contains a ferment known as ptyalin, 

 which is a diastase and degrades starch into dextrins and 

 simpler carbohydrate. In the stomach it comes into con- 

 tact with the secretion poured out as the result of the 

 stimulus of the food ingestion, which contains hydro- 

 chloric acid and the proteoclastic enzyme pepsin which 

 hydrolyzes the proteins into amino-acids and compara- 

 tively complex proteoses and peptones. The gastric juice 

 also contains a small amount of lipase. As the partially 

 digested food passes through the pyloric valve of the 

 stomach into the small intestine, mixed with the secretion 

 from the stomach, it enters the small intestine. The acid 

 of the gastric juice then acts upon the mucous membrane 

 of the intestine, and produces a substance called secretin 

 which is absorbed and carried in the blood-stream. This 

 acts on the pancreas, and causes it to secrete through the 

 pancreatic duct a juice which comes into contact with the 

 mucous membrane of the small intestine. It contains a 

 lipase, and also trypsinogen, a " pro-ferment " from which, 



