26 BUTTER-MAKING. 
secretion have been studied considerably, many things in this 
connection are not well understood. Previous to the year 
1840 it was thought that the only function of the milk-gland 
was to filter the milk as it transuded from the blood. It was 
supposed that the quality and quantity of milk depended 
entirely upon the food. The theory has also been advanced 
that the major portion of the milk constituents was a decom- 
position of the product of the lymph bodies of the blood. It 
was believed that the lymph bodies were a source of nourish- 
ment to the foetus, and that the calf received its nourishment 
from the same source after it was born as it did previous to 
birth. It was supposed that after the birth of the calf the 
opening on the uterus through which the food was supplied 
was closed, and that a new opening was formed in the milk- 
gland. These two theories have now been practically over- 
thrown. It has been demonstrated that the major portion of 
the milk is formed within the milk-gland. The fat, casein, 
milk-sugar, and part of the albumen are supposed to be formed 
in the udder. This conclusion is substantiated by the fact 
that these substances do not appear in the blood, at least not 
to such an extent as to warrant the assumption that they are 
not manufactured in the cow’s udder. The total amount of 
fat in the blood of the cow would not equal the fat in the milk 
from one milking. 
By some it is maintained that the substances in milk which 
are found in solution may be transuded directly from the 
blood. Here again milk-sugar is found to be in perfect solu- 
tion in the milk, but this substance can be found nowhere in 
nature besides in milk. It is not present in the blood of the 
animal, consequently it must be manufactured within the 
gland itself. The water of milk, and the ash constituents which 
are in solution, are probably transuded directly from the blood. 
No attempts have been made to determine definitely how 
casein and albumen are formed within the gland. 
The theory advanced for the formation of fat is, that the 
epithelial cells break down and form fat. When the breaking- 
