LESSON THIRTY-SEVEN. 
SECRETION OF MILK. 
1. Source of milk.—Milk is secreted from the blood. 
The change from blood to milk occurs in the udder. The 
blood, therefore, is the source from which all milk is de- 
_vived. No matter 
.INTERIOR STRUCTURE OF 
ONE QUARTER OF THE 
UDDER 
how perfect the udder, if the blood is 
impoverished because of poor food or 
ill health, a generous flow of milk is 
not possible. A generous yield of 
milk is dependent upon good health 
and rich blood. 
2. Structure of the udder.—The 
udder is both a factory and storehouse. 
It consists of many hollow spaces or 
cavities of varying sizes, muscular 
tissue, cells, veins, arteries, nerves, 
lymphatics and connecting canals. 
The blood is the raw material, the 
cells the manufactory agents, the 
nerves the stimulating force, and 
the canals the tracks 
of delivery. In  nor- 
mal activity these dif 
ferent creations unite 
in sympathetic  rela- 
tion, with the result 
that milk is secreted. 
Taken together, they 
INTERIOR STRUCTURE OF UDDER 
401 
