THE DIGESTION OF THE NUTRIENTS 



39 



certain conditions, of bringing about a chemical reaction 

 without itself entering into the composition of the final 

 products of the reaction. In other words, an enzyme is a 

 substance which, when added to certain inactive substances, 

 causes a chemical change to take place w^ith the formation 



AfotUA 



Dmphmffm 

 Spleen,. 



DlUKto. 



/i > er, iifiprreurtyfiiiily 



/Aijye ciilon 



Ccr, 



SrnaiJ iftt^U*/ir . 



FlofUing coioii. . 



Anius. 



//•/? kiettury find Us u/r^rr 



Bladiitr 



HrrtJcra . 



Pig. 7. — Digestive tract of the horse. (U.S. Department of Agriculture.) 



of new products. The enzyme itself remains unchanged, 

 and may be recovered entirely at the end of the reaction and 

 used over and over again to produce similar reactions. It 

 acts as a catalytic agent. Thus a dilute solution of starch 

 may be allowed to stand at room temperature for a con- 

 siderable length of time and no changes will take place. 

 However, if a few drops of saliva, which .;ontains the enzyme 

 salivary amylase or '' ptyalin," are added, all the starch 



