68 UNIVERSITY OF WISCONSIN STUDIES 



the blood passes, particularly those parts in which lipases 

 are already normally at work. Other enzymes are also in- 

 volved in this activity, for the synthesis of fatty acids and 

 glycerol from which the lipases build the neutral fat is 

 fundamentally reactions of oxidation and reduction. The 

 general hypothesis, however, is at once seen to offer a plausible 

 explanation for the increase in the fat of the milk during 

 underfeeding. It explains how the lipases and other enzymes 

 normally accelerating the formation of milk fat in the mam- 

 mary gland may bring about a greater formation of milk fat 

 at the same time the lipases in the tissue fat of the body are 

 liberating more fat from the cells for energy required by the 

 body. It also explains why the highest fat tests in under- 

 feeding were secured in the cases of the fattest animals, and 

 why animals in thin or only moderate flesh showed the least 

 effects on the percentage of fat in the milk. 



The relation of the lipase activity of the mammary gland 

 to the lipase activity in other parts of the body also explains 

 the low fat percentage of milk often secured during very hot 

 weather. It is a well-known fact that an abnormally low 

 percentage of fat in the milk will accompany a period of hot, 

 humid weather in summer as has been shown by the author, 

 while a period of very cold, dry weather in winter will cause 

 an increase in the percentage of fat in the milk. 



INFLUENCE OF PLANE OF NUTRITION UPON COMPOSITION 

 OF THE FAT 



A striking result of underfeeding a lactating cow is the 

 effect upon the composition of the milk fat. This change 

 becomes noticeable almost as soon as the plane of nutrition 

 drops below the requirements of the animal. In the ex- 

 periment with the Jersey cow mentioned in connection with 

 the relation of the plane of nutrition to the fat percentage of 

 the milk, the fat constants exhibited the typical effect of un- 

 derfeeding during the period when the ration was below the 

 requirements of the animal. When the ration was increased 

 to a normal amount the fat constants quickly returned to 

 normal for the animal in question. 



