324 ANIMAL BIOCHEMISTRY 



normal development of the male reproductive organs and to preserve 

 and maintain secondary male characteristics. Two typical androgens 

 are testosterone and androsterone. From their structures it can be seen 

 that the "male" and "female" steroids are closely related chemically. 



MILK 



Mammary Glands 



The mammary gland differs from the organs described above in 

 that the secretion is used for food for the young mammal rather than 

 for regulation of metabolism. Mammary glands develop in the yoinig 

 female mammal at puberty with proliferation of tubules and develop- 

 ment of alveoli. Liberation of the ovarian hormones initiates this 

 growth. Milk is secreted by the alveoli of the mammary gland after 

 proper stimulation by a series of hormones immediately after parturi- 

 tion. Prolactin, a pituitary hormone (page 319), is the chief hormone 

 involved in the initiation of the process. Oxytocin plays a role in the 

 release of milk from the lactating gland through its action on smooth 

 muscles, and adrenal hormones are thought to function although the 

 mechanism is not defined. 



Composition of Milk 



Milk is the principal food of the young mammal. It is a white, 

 opaque fluid containing 64 to 88 per cent water, 1.5 to 10.3 per cent 

 protein, 3.5 to 19.7 per cent fat, 4.5 to 7.2 per cent lactose, and 0.2 to 

 1.5 per cent minerals, depending on the species of origin (page 416). 

 The white color of milk is due to finely dispersed fat globules, coated 

 with a film of protein and phospholipide. 



Milk contains three chief proteins, casein, lactalbvmiin, and lac- 

 toglobulin, with casein constituting approximately 80 per cent of the 

 total protein. Casein is a nutritionally complete protein in that it is 

 rich in all the essential amino acids and therefore has been used as a 

 standard in most experimental protein-feeding trials. It is a phospho- 

 protein with an isoelectric point of pH 4.6, where it is water insoluble. 

 In milk, casein is probably present as the calcium salt. Casein has 

 found extensive use as a food in cheeses, as an industrial protein with 

 a variety of functions in filaments and films, and as an adhesive or 

 filler. 



Milk fat is a mixture of triglycerides containing, as a rule, about 

 two-thirds saturated fatty acids from 4 to 16 carbons in length and 

 approximately one-third oleic acid by weight. Species, age, and state of 



