
88 STORRS AGRICULTURAL EXPERIMENT STATION. 
the work is severe; and more energy, that is, more fuel, for the 
extra work. In other words, when the horse is at rest in 
the stall it needs food only for maintenance, but when it is at 
work it requires an extra amount for production. $o likewise 
the cow at rest in the stall needs food for maintenance, but 
when she gives milk she needs extra food to form the casein, 
fat, milk sugar, and other ingredients in the milk; that 1s, 
she must have increased food for production. 
When an animal is fattened it needs food both for mainte- 
nance and for the production of extra flesh and fat. In fact 
whatever the form of production, whether it be work, milk, 
fat, or otherwise, food is required in addition to that necessary 
for maintenance. The same is true of the young, growing ani- 
mal, which needs food for the material that is constantly 
being added to its body in growth. 
When the animal has not food enough to supply its needs it 
draws upon the store previously accumulated in its body. 
When it has more than it needs it stores a part of the extra 
supply. The body is constantly storing up new material and 
consuming that which has been previously stored. ‘The fuel 
of the body is stored in the form of fat which, as we have seen, 
is the most concentrated form of fuel for the use of the animal. 
But the body can store protein, carbohydrates, and other 
materials also, and what is more, it has the power of trans- 
forming one of these into the other. It can transform the 
sugar or starch of its food into fat, which it stores for future 
use, and it can also make fat from protein. But it cannot make 
protein of the fats or carbohydrates; protein contains nitrogen, 
of which the fats and carbohydrates have none. All of these 
classes of nutrients can be burned for fuel. The protein can 
thus do the work of the fats and carbohydrates, but these can- 
_ not take the place of the protein in building tissue, such as 
muscle, tendon, bone, etc., or form the nitrogenous material 
of the blood or milk. The albuminoids of milk, casein, and 
albumin, which contain nitrogen, are made from protein, but 
they cannot be made from. carbohydrates and fats. It seems 
probable that the protein of which this casein and albumin are 
made is largely, if not entirely, that which has formed a part 
of the body, z. e., of the lacteal glands. It has been believed 
also that body protein is transformed into milk fat, and it has 

