THE HORMONES IN HUMAN REPRODUCTION 



we had better leave the subject to the specialists for further 

 study. 



Finally the time comes when gestation can go on no longer. 

 The uterus, overburdened by its rapidly growing tenant, must 

 deliver itself. Degenerative changes begin in the placenta, the 

 nourishment of the infant is thereby impaired, and the uterus 

 commences its efforts to expel the child. This it accomplishes 

 by means of strong contractions, efficiently timed and coordi- 

 nated so that the infant is pushed toward the outlet of the 

 uterus. The uterine orifice, and afterward the vaginal canal, 

 are stretched open to allow passage of the infant, while the 

 rest of the uterus contracts to provide the necessary force. 

 There is no simple explanation of the onset of labor. Many 

 physicians and biologists have tried to discover some simple 

 reason why at a particular time — 9 months in the human, 2 

 years in the elephant, 21 days in the mouse, or whenever, 

 according to the species, the fated hour arrives — the act of 

 parturition begins. It has been thought, for example, that the 

 uterus is at last simply stretched too far and is thereby irri- 

 tated into contracting again; or that the breakdown of the 

 placenta constitutes a stimulus to the uterine muscle ; or that 

 some chemical substance from elsewhere in the body sets the 

 muscle into action. When it was discovered that estrogenic 

 hormones stimulate the involuntary muscle of the uterus, and 

 that progesterone tends to relax it, an attractive theory of 

 the cause of labor at once suggested itself. We need only 

 suppose that when the end of gestation draws near, the pro- 

 duction of progesterone goes down, and estrogenic hormone 

 is thereby allowed to build up contractions of the uterine 

 muscle. This hypothesis is however much too simple, as 

 Reynolds points out in the book cited above. For one thing, 

 the contractions of the uterus in labor are very different in 

 their timing and coordination from those of the nonpregnant 

 uterus. The fact is that the uterus at the end of pregnancy is 

 operated by a very elaborately organized set of adjustments. 



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