208 THE FEEDING OF ANIMALS 



294. Studies of food sources of animal fats. Another 

 class of experiments somewhat more severe in their 

 requirements are those designed to give information as 

 to the relation between the constituents of the food and 

 the growth of the various tissues in the animal body or 

 the formation of milk solids. The experiments conducted 

 by Lawes and Gilbert on the formation of fat with swine 

 may be cited in illustration of the methods used. These 

 were planned so as to learn the amounts of digested pro- 

 tein, carbohydrates, and fat consumed by the animal and 

 also the quantities of protein and fat stored in the body 

 during a given period. "In experiment No. 1, two pigs 

 of the same litter, of almost exactly equal weight, and, so 

 far as could be judged of similar character, were selected." 

 One was killed at once and its composition determined, 

 and the other was fed for ten weeks on a fattening ration 

 of known composition and then slaughtered and analyzed. 

 The quantity of protein and fat which the pig's body had 

 gained during the ten weeks as ascertained from the com- 

 position and weight of the two pigs was then compared 

 with the food-supply of similar compounds. It was 

 assumed that a pound of food fat could produce a pound 

 of body fat and that 51.4 per cent of all the protein not 

 stored in the body as such could be used for fat formation. 

 Even with the most liberal allowances it was found that 

 the protein and fat of the food could not possibly have 

 been the sole source of the new body fat, thus forcing the 

 conclusion that the carbohydrates are fat-formers. Prac- 

 tically the same plan has been followed in studying the 

 source of milk-fat. Several cows were fed on carefully 

 weighed and analyzed rations extremely poor in fat, and 

 the amount and composition of the feces, urine, and milk 

 were ascertained during sixty to ninety days. The fat 



