32 MILK IN THE UDDER 
eaten by the dairy cow is changed in the cow’s 
digestion into soft liquid material. This is ab- 
sorbed by the circulation which carries it to the 
udder. The cow’s udder is a large sponge filled 
with cavities, in the walls of which the blood 
tubes form a fine network. The surface of the 
cavities is like fine velvet made up of tiny par- 
ticles of animal matter called cells. This velvet 
lining draws liquid nourishment from the cir- 
culation beneath, and transforms it into milk. 
Many of the tiny particles composing the velvet 
become so filled with milk material that they 
burst, and others are squeezed loose and _ fall 
off with the milk into the channels. The blood 
itself contains floating particles of solid material 
called white blood cells. They are round and 
soft like tiny drops of white jelly. These also 
work their way through the walls of the chan- 
nels and fall into the milk. As a result, the 
milk which flows from the small channels of the 
udder into the large channels and finally reaches 
the teats carries with it considerable material 
which is shed from the lining of the udder, and 
which can easily be seen in milk under a mag- 
nifying glass. 
The gray oval spaces in the picture are the 
