THE PHYSIOLOGY OF LACTATION 33 



cording to this view milk would be an adaptation product of the 

 mother to the growing embryo. It should be remembered in this 

 connection that the digestive tract of the newborn has not yet 

 entered upon its natural function. Hirth says that milk is a living 

 substance, contains living protein, has the temperature of the 

 living mother, and therefore is an aid to the young in assimilating 

 food which the undeveloped digestive tract is unable to accom- 

 plish. This idea of "living milk" should not be confused with the 

 "life of milk" of some authors, who regard the ferments in milk 

 as the cause of this "life." According to Hirth's view, milk is not 

 only a food, but furnishes the tools for assimilation in the young 

 digestive tract. Whether these "tools" will remain in the domain 

 of hypothesis or be really demonstrated is a matter for the future 

 investigator to decide. 



Mammary glands are the seat of milk formation and secretion 

 and are characteristic of mammals. They occur in parallel rows 

 and their location is determined by the habits of the young in 

 taking food; in short, they offer the most convenient method of 

 feeding. The number of glands is not the same in different species, 

 and corresponds to the number of young usually born in one litter, 

 varying from one to twenty-two pairs. Most quadrupeds have 

 mammary glands on the abdomen; man and apes have them on the 

 breast, while cattle carry them nearer the region of the genital 

 organs. Usually the size of the glands is approximately the same in 

 an individual. In the human, however, the left gland is frequently 

 larger than the right, possibly because the baby is preferably held 

 on the left arm so as to keep the right arm free. 



Sometimes unusual conditions prevail and cause a unique loca- 

 tion of the mammary glands. In some monkeys, for example, 

 they are near the armpits; marsupialia have them in their pouches, 

 while the hippopotamus and myopotamus have them on their 

 backs, so that the young can obtain food while the mother is 

 swimming. 



Mammary glands are part of the anatomy of both sexes. In 

 the male, however, they do not develop, as a rule, to the stage of 

 actual milk secretion. There are some exceptions reported on 

 good authority, although some reports may be legendary. There 

 is the often cited example, reported by von Humboldt, who during 

 a journey in South America spoke to a man forty-five years of age 

 who told him that thirteen years previously he had nursed his son 

 for five months. The mother was ill and the child accidentally 

 sucked the father's breast. This stimulated a milk secretion 

 which sufficed to feed the child for five months. More recently 

 R. C. Creasy reported a case of lactation from the mammary gland 

 of the male. A man of Russian descent, aged twenty-four, was 



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