56 PHYSIOLOGY 



classes of enzymes to act upon them. One changes the 

 starches, another the proteids, and the third the fats, 

 into soluble substances which can readily be absorbed. 

 These enzymes are found in the various digestive juices 

 of the body. The saliva, a digestive fluid found in 

 the mouth, contains an enzyme which changes starch 

 into a soluble sugar called maltose. In order that it 

 may have time to do its work starchy foods should be 

 thoroughly chewed. The other food substances are not 

 changed by saliva, but they are moistened and are 

 consequently swallowed more easily. 



The gastric juice which is secreted by small glands in 

 the wall of the stomach also contains only one enzyme, 

 pepsin. Pepsin changes proteids into soluble peptones 

 but has no effect on starches and fats. The pancreatic 

 juice contains three enzymes. The most important of 

 these is called lipase. It changes fat into soluble fatty 

 acid and glycerine. The other two enzymes act upon 

 starch and proteid that escape digestion in the mouth 

 and stomach. 



Reversible Action of Enzymes. — Very curiously the 

 action of enzymes is reversible; that is, if an enzyme 

 changes starch into sugar, it also changes sugar into 

 starch. These actions take place simultaneously, and 

 result in an equilibrium, or a condition of balance. 

 Whenever there is an increase or a decrease of either 

 substance the equilibrium is disturbed, and the action 

 becomes more vigorous in one direction than in the other. 



If, for example, there is a surplus of starch, the 

 starch is transformed into sugar more rapidly than 

 the sugar into starch. 



