CHAPTER XL 

 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 mam- 

 mals, 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 rudi- 

 mentary 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 lactation period has become prolonged, and the 

 daily yield and quality of milk have been much improved. Cows 

 have been known to secrete over one hundred and fifty 

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

 and over fifteen 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 loose, 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 good 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 

 the rest of the body. 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, 



155 



