PLANTS AND ANIMALS 41 
12. Function of protein—The protein of a food is used 
primarily to replenish the body wastes of tissues, blood, 
organs, brain and 
nerves, etc. Young ani- 
mals require a larger 
supply than adult and 
aged animals because of 
the requirements of 
growth and_ enlarge- 
ment. Protein may be 
used also for energy and 
fat, but usually not eco- 
nomically. A very large 
consumption of protein 
may lead to ill health. 
BEEF CuT SHOWING PROTEIN 
Protein is_the “muscle Observe distribution of large and small 
maker of the body. The particles of fat. This improves the quality 
protein of the plant is of the meat. 
changed into the protein of 
the animal. In the animal this constituent comprises the muscular 
tissues, blood, hair, nerves, internal organs, skin, etc. In addition 
the protein is used in the repair work of the body. Every beat of 
the heart, every circuit of the blood, and every move of a muscle, 
demands that some protein substance be used up. To keep the 
animal machine in good working order these parts must be kept in 
repair through protein. If this supply satisfies the waste, the weight 
of the animal will remain unchanged. When the supply is liberal, 
‘or exceeds the demands of the system, material may be stored in the 
body as flesh or fat, and the animal will gain in weight. 
13. Heat and energy.—Food is needed to keep animals 
warm. As wood burned in a stove gives off heat, so 
food consumed in the body furnishes heat. This con- 
sumption of fuel food is so well regulated -in a healthy 
animal that the temperature remains at the same point 
at all times. Carbohydrates and fats are mainly the 
sources of heat supply. These same ingredients are used 
for the production of fat and muscular energy, and pro- 
tein may also be used for the same purpose. 
