PRODUCTION OF WORK, MILK, AND WOOL 107 



blood contains much more potassium than sodium, in milk sodium pre- 

 dominates. 



From 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 in the 

 udder there is room for the storage of but a small part of the total amount 

 produced. 



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 un- 

 familiar attendant, or other unusual excitement. The animal probably 

 has no power to ' ' hold up ' ' the milk already secreted in the udder, but 

 fright or pain may cause the glands of the udder to cease secreting milk 

 for the time being. 



Only in most exceptional cases does the true secreting tissue of the 

 mammary gland develop before the animal becomes pregnant. How- 

 ever, when an extract is made from an unborn fetus and injected into 

 the blood of a virgin animal, the mammary gland develops just as tho 

 the animal were pregnant. This leads scientists to believe that in the 

 developing fetus a mysterious chemical messenger, or " hormone," is 

 formed, which is carried by the blood to the udder, and there stimulates 

 the development of the alveoli an example of the surprising degree to 

 which the activities of the body are dependent on each other. 



146. The source of fat in milk. For many years it was believed that 

 the cow could form the fat of milk only from fat in her food. This was 

 disproved by Jordan and Jenter 10 of the New York (Geneva) Station by 

 an ingenious experiment. A thousand pounds of hay and 1,500 Ibs. each 

 of corn meal and ground oats were sent to a new-process oil-meal factory, 

 where nearly all the fat was extracted with gasoline in the percolators 

 employed for extracting the oil from crushed flax seed. (253) The 

 almost fat-free feeds were returned to the Station and afterwards fed to 

 a cow which had freshened about 4 months before. For 95 days the 

 cow lived on these feeds, yet during this period she produced 62.9 

 Ibs. of fat in her milk. The food she consumed contained but 11.6 

 Ibs. of fat, of which only 5.7 Ibs. was digested. Therefore at least 57.2 Ibs. 

 of the fat found in the milk must have been derived from some other 

 source than the fat in the food. This fat could not have come from the 

 body of the cow, for Jordan writes: "The cow's body could have con- 

 tained scarcely more than 60 Ibs. of fat at the beginning of the experi- 

 ment ; she gained 47 Ibs. in weight during this period with no increase 

 of body nitrogen, and was judged to be a much fatter cow at the end ; 

 the formation of this quantity of milk fat from the body fat would have 

 caused a marked condition of emaciation, which, because of an increase 

 10 (Geneva) Bui. 132. 



