26 THE MILK QUESTION 



Physiology of lactation 



The mammary glands are included among the glandular 

 structures of the skin. The other skin glands are the sweat 

 glands and the sebaceous glands. 



In the young mammalian embryos generally the mam- 

 mary glands develop from the ectoderm and are first indi- 

 cated by a thickened line of the deeper layers of the skin. 



This thickened line extends from the armpit to the groin. 

 Later, much of this line disappears, leaving a succession of 

 nodular thickenings corresponding with the nipples. In 

 some mammalians this row of nipples remains; in others 

 only the inguinal thickenings, as in the cow; and in still 

 others only those towards the axilla. Thus in man there is 

 normally only one nipple on each side. At birth the nipple 

 is everted and at that time the glands in either sex may dis- 

 charge a little milky secretion similar to the colostrum which 

 precedes lactation. The glands grow in both sexes until 

 puberty, when those in the male atrophy and only the main 

 ducts persist. In the female, enlarged terminal alveoli be- 

 come evident in pregnancy, when they develop into mature 

 and fully formed mammary glands capable of producing 

 a sufficient quantity of milk. 



The circumstances which regulate the position and num- 

 ber of milk glands in animals depend upon the conditions 

 of life, the manner by which the animal takes its food, the 

 position of the animal in rest and action, the form of the 

 thorax, the number of the young, and the way in which 

 the mother holds the young, as well as the chances of in- 

 jury to the glands, etc. 



In those mammalian animals which bring into the world 

 a large number of young at the same time the milk glands 

 lie in two symmetrical rows on both sides between the arm- 

 pits and the groin. Thus the Madagascar tenrec (a small 

 insectivorous animal resembling the hedgehog) has twenty- 

 two pairs of nipples. The domestic pig has four to eight; 



