492 Report of the Department of Animal Husbandry of the 



(4) During fifty-nine consecutive days 38.8 lbs. of milk fat was 

 secreted and the urine nitrogen was equivalent to 33.3 lbs. of 

 protein. According to any accepted method of interpretation, 

 not over 17 lbs. of fat could have been produced from this 

 amount of metabolized protein. 



(5) The quantity of milk solids secreted bore a definite relation 

 neither to the digestible protein eaten nor to the extent of the 

 protein metabolism. In view of these facts it is suggested that 

 the well-known favorable effect upon milk secretion of a narrow 

 nutritive ratio is due in part to a stimulative, and not wholly 

 to a constructive, function of the protein. 



(6) The composition of the milk bore no definite relation to the 

 amount and kind of food. 



(7) The changes in the proportion of milk solids were due al- 

 most wholly to changes in the percentage of fat. 



INTRODUCTION. 



Among the problems important in agriculture that are mosit 

 difiicult of solution are those which pertain to animal metabolism 

 They are difficult because the field of chemical activity where the 

 processes of digestion and reconstruction of compounds occur is 

 inaccessible to the ordinary direct means of study and observa- 

 tion. For this reason such questions as the sources of the fats 

 and other compounds that are formed in the animal body are still 

 only partially answered. Especially inconclusive is the knowl- 

 edge pertaining to the formation of milk fats. A widespread 

 popular belief is that they as well as other fats must first exist in 

 the food, the function of the animal organism being merely to 

 collect and transfer them into the adipose tissue or milk. Apart 

 from certain experimental evidence which proves the possible 

 formation of animal fats in other ways, there ia one fact that ap- 

 I>ears to render this view untenable, which is that great unlike- 

 ness exists in the mixtures of fats that different species construct 

 from the same foods. There seems to be no possibility, either, of 

 simply transferring from any ordinary ration a mixture of fats 

 similar in kind and proiwrtions to that found in tallow, lard or 



