20 PHYSIOLOGICAL CHEMISTRY 



since they may be stored away in large quantities. The body 

 contains on the average about 15%. Proteins vary less from 

 an absolute standpoint, but relatively the percentage depends 

 upon the amount of fat. ' The amount in the body averages 

 also about 15%. Extractives, a heterogeneous group of com- 

 pounds classed together because they are water soluble, and 

 belong in none of the other classes, make up less than 1% 

 of the body weight. This group will receive no further treat- 

 ment as such, but the substances included in it (urea, creat- 

 inine, etc.) will be discussed individually in connection with 

 the tissues or fluids in which they occur. 



