80 



FEEDS AND FEEDING, ABRIDGED 



II. Production op Milk 



Secretion of milk. — Milk, the marvelous fluid designed by Nature 

 for the nourishment of the young of all mammals, is secreted by special 

 organs, called the mammary glands. Scientists disagree as to the 

 exact process by which the milk is formed in the small, sac-like bodies, 

 known as alveoli, in the udder. However, we do know that the blood, 

 laden with nutrients, is brought by the capillaries of the udder to 

 these alveoli. The nutrients then pass thru the walls of the capillaries 

 into the alveoli. There, by one of Nature's wonderful processes, they 



are converted into milk, 

 which differs entirely 

 in composition from 

 the blood whence it 

 originates. The chief 

 proteins of milk- 

 casein and milk albu- 

 min — differ from all 

 other proteins of the 

 body, and the milk fat 

 likewise has entirely 

 different properties 

 from the body fat of 

 the same animal. Milk 

 sugar, the carbohy- 

 drate of milk, is found 

 nowhere else in the 

 body. While the blood contains much more potassium than sodium, 

 in milk the sodium predominates. 



Prom the alveoli the milk passes into the network of milk ducts. 

 In some animals the large milk ducts open directly on the surface of 

 the teat, but in others, including the cow, they open into a small cavity, 

 called the milk cistern, which is just above the teat. Most of the milk 

 yielded at one milking is secreted during the milking process, for the 

 udder has room for the storage of but a small part of the total product. 

 Tho the secretion of milk is involuntary and cannot be prevented 

 by the animal, any more than can breathing or the circulation of the 

 blood, the flow may be reduced by nervousness caused by fright, an 

 unfamiliar attendant, or other unusual circumstance. The animal 

 has considerable power to "hold up" the milk already secreted in the 

 udder, by contracting the ring of muscle which partially separates 

 the milk cistern from the teat, and similar muscles guarding the milk 

 duets. 



Pig. 24. — Mammary Gland of Cow 



Small portion of gland, greatly magnified. A, 

 alveoli, in which milk is secreted; b, small milk duct; 

 c, larger milk duct. (After Klein.) 



