SOME OF THE LIFE PROCESSES 125 



Excretion. The last of the stages of metabolism is excre- 

 tion. Excretory material is only that waste which has been 

 formed by the breaking down of the living protoplasm and 

 the food material stored in it. The carbon dioxide and 

 water removed from the body in the process of respiration 

 are excretory products. In addition various nitrogen com- 

 pounds, of which urea and uric acid are the most familiar, 

 are produced by the processes of katabolism. These wastes, 

 dissolved in water, are collected by the blood and carried 

 to the kidneys. Here they are separated from the blood 

 and passed from the body as urine. 



Other Wastes. The undigested food and the indigestible 

 substances that pass through the length of the intestine 

 are not excretory products, — a fact that has already been 

 stated in the chapter on the grasshopper. These wastes 

 are called feces, or fecal matter. 



Vitamins. Until recently it was very generally thought 

 that growth and health depended only upon sufficient quan- 

 tities of food. It was discovered, however, that ample food 

 under certain conditions may produce diseases. Sailors 

 living on canned and dried foods became ill of a skin disease 

 called scurvy, which disappeared when fresh fruits and 

 vegetables were given to them. Similarly, Orientals eating 

 practically no food but polished rice became subject to a 

 disease called beriberi. The disease disappeared when they 

 ate unpolished rice, containing part of the outer coating. 

 Some of the nutritional diseases and their cures were dis- 

 covered before anyone knew a scientific explanation of their 

 causes. It is now known that these diseases are due to the 

 lack of certain substances called vitamins. The vitamins are 

 substances which must be present in the food if the animal 

 is to be perfectly nourished (Fig. 69). If entirely deprived 

 of vitamins for a long time, experimental animals die. No 

 one knows exactly what vitamins are. They occur in most 



