THE HORMONES IN PREGNANCY 



The proportion of the infant to the space it occupies; the 

 strength of the uterine wall and the pressure it exerts upon its 

 contents; the rate of blood flow through the uterus; the 

 sensitivity of its muscle and nerves ; the balance of the hor- 

 mones that affect it; the nutrition of the infant and the 

 placenta — all these factors (and others beside them, for all 

 we know) are balanced one against the other and when the 

 crisis comes they are all involved at once. The physiologist 

 who looks for one specific cause of the onset of labor is up 

 against the same kind of problem as the economist who tries 

 to find one single cause for a stock market crash or to pin 

 down a nationwide problem of unemployment to one specific 

 factor. When dealing with such complex affairs as those of 

 a nation or a pregnancy the investigator cannot isolate one 

 factor at a time and study it singly. He has to unravel a 

 whole system of balanced forces. In the problem we are con- 

 sidering, the hormones are certainly to be numbered among 

 the most important factors, but it is scarcely safe at present 

 to say more, 



LACTATION 



When the mother's body has completed its provision of 

 shelter and nourishment for the child by means of the hor- 

 mones and has seen him safely into the world, it has yet 

 another service to render on his behalf — namely that the 

 breast for which he will so promptly cry is ready to supply 

 him with milk. 



The recent discovery of a specific hormone for lactation, 

 in the pituitary gland, was a great surprise. It involved a 

 simple little piece of scientific logic which the reader may 

 enjoy after the preceding complexities of this chapter. We 

 had better clear the way for this story, however, by recalling 

 to mind the earlier history of the mammary gland. When a 

 girl or a young animal reaches sexual maturity, the mammary 

 glands are brought from the immature state to the adult con- 



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