494 Report of the Department of Animal Husbandry of tSe 



state of activity has led some physiologists to believe, not without 

 a show of reason, that milk is the result of the breaking down or 

 liquefaction of proteid tissue in the udder, the food serving merely 

 to rebuild this tissue. Certainly such a theory cannot be regarded 

 as unreasonable in view of the observations which appear to show 

 the formation of beeswax from protein, the fatty degeneration of 

 proteid tissue under the influence of phosphorus poisoning and 

 the apparent production of fat from protein by fly maggots. We 

 have, besides, the generally oibserved and well-established fact 

 that an increase in the supply of protein in the ration of milch 

 cows often stimulates the secretion of niilk in a marked manner, 

 even though there is no increase in the total digestible food con- 

 sumed. 



It must be confessed, however, that the data so far recorded 

 concerning the source of fat are confusing and inconclusive when 

 we attempt to apply them to the narrower question of the forma- 

 tion of milk fat. 



It has been for some time the opinion of the writer tliat in or- 

 der to reach definite conclusions as to the source of milk fat 

 through experiments conducted without the use of a respiration 

 apparatus, the following conditions, among others, must be se- 

 cured: ' 



(1) The use of foods either fat-free or containing such small 

 percentages of fat that, if the milk was formed in the usual quan- 

 tity, a large balance must come either from the cow's body or 

 from the other two classes of food compounds, viz. : protein and 

 carbohydrates. 



(2) The continuation of the experiment over so long a period 

 of time as to show conclusively whether, in the absence of food 

 fat, the cow did or did not draw upon a previous store of body 

 fat for the production of milk fat. 



(3) The variation of the protein of the food from a quantity be- 

 low to one above the actual needs of the animal in order to dis- 

 cover, if possible, the minimum protein metabolism necessary to 

 the maintenance of a given production of milk fat. 



