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 inerease in the percentage of fat in the milk. 
INFLUENCE oF PLANE or Nutrition UPon COMPOSITION 
Or THE Far 
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. 
