DIGESTION 69 



second, to bring about such changes in their composition as 

 will insure their absorption and appropriation by the tissues. 



Enzymes. The chemical changes taking place in digestion 

 are of a peculiar nature, in that they are effected largely by 

 the presence of substances known as enzymes, correspond- 

 ing in an obscure way with ordinary chemical reagents. 

 These have been called unorganized or unformed ferments, 

 to distinguish them from such organized ferments as bac- 

 teria, yeast, fungi, etc. They are not themselves possessed 

 of any vital activity, though formed in living organisms, like 

 plants or animals. They are of indefinite chemical composi- 

 tion, contain nitrogen and are supposed to be of proteid 

 structure. The characteristic point in their action has been 

 supposed to be that they produce a chemical change without 

 themselves being affected by that change. This is doubtless 

 practically true, but it is found in experimental work that "a 

 given solution of enzyme cannot be used over and over again 

 indefinitely." It finally loses its identity. 



According to the foods on which they act and the effects 

 they produce, enzymes are classified as : ( i ) Proteolytic 

 enzymes, which convert proteids into soluble peptones ; ex- 

 amples are pepsin and trypsin. (2) Amylolytic enzymes, 

 which convert starches into sugar ; examples are ptyalin and 

 amylopsin. (3) Fat-splitting enzymes, which convert neu- 

 tral fats into glycerine and fatty acids ; an example is steap- 

 sin. (4) Sugar-splitting enzymes, which convert the non- 

 absorbable (saccharose) into absorbable (dextrose) sugar; 

 an example is invertase. (5) Coagulating enzymes, which 

 precipitate soluble proteids ; an example is rennin. 



Characteristics of Enzymes. Some of the characteristics 

 of enzymes are as follows : ( I ) They are soluble in water 

 and in glycerine. (2) In solution they are destroyed before 

 the boiling point is reached (140 to 180 Fahrenheit). Very 

 low temperatures do not destroy them, but suspend their ac- 

 tion. (3) They never completely convert the substance upon 

 which they act. It is supposed that the substance produced, 



