THE UTILIZATION OF FOOD IN ANIMALS 343 



cells. People often think of respiration as merely involving 

 the process of taking air into the lungs and then expelling it. 

 That process is breathing, and is merely the preliminary part 

 of respiration. The animal cells are able to obtain energy from 

 carbohydrates, fats, and proteins more or less interchangeably. 



The more work a person 

 does, the more food he 

 needs. Work in this sense 

 means something done by 

 the muscles, and may be 

 either work or play in the 

 ordinary sense. Lumbermen 

 and boys of high-school age 

 frequently eat about the 

 same amount of food. If a 

 person spent all his time in 

 bed he would still be doing 

 a good deal of work just 

 because he was alive. When 

 we are resting, the heart con- 

 tracts about seventy-two 

 times a minute, we breathe 

 about sixteen times a min- 

 ute, and we make many 

 motions of which we are un- 

 conscious. If a lumberman 

 broke his leg and so had to 

 sit quietly, he would need 

 about a third as much food as when working. The weight 

 of the body ordinarily furnishes a good test of whether the 

 right amount of food is used (see fig. 162). An adult of nor- 

 mal weight for his height who maintains his weight over a 

 long period of time evidently has about the proper amount 

 of food. A young person should increase steadily in weight, 

 but this should be good muscle and bone and not all fat. 



Effects of poor and good 

 nutrition 



The upper photograph is of a rat at the age 

 of five hundred and five days and after 

 prolonged stunting from insufficient food. 

 The rat weighed 53 grams when the upper 

 picture was taken. The same animal when 

 being fed a suitable quantity and quality 

 of food gradually increased in weight until 

 at the time the lower picture was taken it 

 had reached 205 grams. Photographs by 

 Professor Lafayette B. Mendel 



