CHAPTER XI 

 THE SECRETION OF MILK 



Milk is secreted by the mammary glands or udder of the female 

 after the birth of young, the secretion continuing until the young are 

 mature enough to live on ordinary food. All animals which have 

 mammary glands are classed as mammals, and this group of animals 

 presents wide variations in the number, position, capacity, shape, and 

 prominence of the glands. The mammary glands are present in the 

 male in rudimentary condition, although, in abnormal cases, milk is 

 secreted by males. Milk is easily digested and very nutritious, and 

 no food for the young animal is equal to its mother's milk. The highest 

 development of the udder is found in the dairy cow, the extreme 

 development having been largely secured by careful breeding; the lacta- 

 tion period has become prolonged, and the daily yield and quality of 

 milk have been much improved. Cows have been known to secrete 

 over 150 pounds of milk in a day, their own weight in less than two 

 weeks, and over 15 tons in a year. 



Exterior of the cow's udder. — The cow's udder consists of four 

 mammary glands, usually referred to as the "quarters," and sometimes 

 there are one to four rudimentaries, all arranged in pairs on opposite 

 sides of the median line of the body. The udder begins a few inches 

 behind the umbilicus and continues backward and upward between the 

 legs. The right and left sides are divided on the exterior by a well- 

 defined line or groove, but the line of separation between the front and 

 rear quarters is not so pronounced. The appearance of size depends 

 partly upon the strength and shape of the abdominal wall ; if the wall 

 is weak, the udder is forced downward and backward and there is a 

 seeming increase in size. This sometimes accounts for the apparently 

 sudden development of a large udder after the second or third calving. 

 The skin covering the udder is very thin and elastic, and the hair is 

 finer, softer, and shorter than on other parts of the animal. The skin 

 covering the teats bears little or no hair, and is often of a darker color 

 than upon other parts. 



Structure of the cow's udder. — When dissected, the right and left 

 halves of the udder are found to be enveloped in strong, fibrous capsules. 

 The fibers of the two capsules intermingle somewhat, and are prolonged 

 upward to the median line of the body to form a ligamentous support 

 and partition, the two halves thus being made distinct. The quarters 

 on each side are not so distinctly separated, there being no such well- 



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