402 FARM ANIMALS 
form a delicate mass, red to gray in color, and spongelike 
in texture. 
3. Mammary glands——The udder is a structure in 
which is housed the mammary glands. Located distinctly 
outside of the body cavity, it articulates 
with all that takes place within. The 
mammary gland is the organ of milk secre- 
tion, by means of which the nutrients of 
food, digested, assimilated and changed 
into blood, are converted into casein, fat 
and sugar, which, together with water and 
ash, form milk. In some species the pri- 
mary glands subdivide in more than two, 
or many. In the sheep, goat and horse 
Good Upper there are but two; in the cow, four to six; 
in the cat and dog, eight to ten; and in the 
hog, ten to fourteen. 
4. How the udder is supported.—The udder is inclosed 
externally in a skin covering. It is held fast to its region 
of attachment by a band of fibrous tissue which issues 
from the flesh substance of the body and extends into and 
through the udder mass. This tender tissue, however, 
does not carry blood to the glands. 
5. Milk veins——Milk secretion is dependent primarily 
on the amount of blood delivered into the udder. Heavy 
milkers have large milk veins, which means that a large 
supply of blood is kept circulating through the glands. 
If blood circulation is weak or the blood impover- 
ished of its serum and food constituents, the fact will be 
registered in the udder output. Cows that eat much 
food and that have a strong digestive power, will carry 
a rich blood in their arteries. It is this blood that nour- 
ishes the milk glands abundantly and that enables them 
to yield their product in generous quantities. 
