THE HORMONES IN PREGNANCY 



pregnancy than in the empty uterus. Other elements of the 

 uterine wall, namely the connective tissue, blood vessels, and 

 nerves, also increase in amount. In this process the estrogenic 

 hormone contributes its general growth-promoting effect, 

 which it exerts by augmenting the blood flow through the 

 tissues of the uterus. Under its influence there is some increase 

 in number of the uterine cells. Progesterone in turn causes a 

 decided wave of cell division in the muscle, and then within a 

 few days at the beginning of pregnancy there is a large in- 

 crease in the number of muscle cells. Subsequent growth of 

 the individual cells, and consequently of the whole wall of the 

 uterus, comes about as the result of stretching by the growing 

 embryo. As the uterus is distended it grows in thickness and 

 strength; if this were not so, the infant would soon rupture 

 the walls that confine it. Everyone is, of course, familiar with 

 the fact that working a muscle makes it grow, and this is no 

 less true of the involuntary muscles of the internal organs 

 than of the skeletal muscle; but like most other familiar 

 responses of the body, we often take it for granted without 

 realizing how little we know how it comes about. Why the 

 uterine muscle grows when that organ is distended is a large 

 question of general physiology, beyond the scope of this book. 

 Reynolds has shown that if he distends the rabbit's uterus 

 by introducing pellets of wax, it will grow in thickness just as 

 it does in pregnancy. By this means he has been able to test 

 the eff*ects of the ovarian hormones upon the growth-response 

 to distention, and has found (among many other interesting 

 facts) that treatment with estrogenic hormone cuts down this 

 response. This hormone, then, which at first helps start the 

 growth of the pregnant uterus, afterward helps to control it. 

 In human pregnancy we know there is plenty of estrogen 

 available in the later months; in all probability this serves 

 to keep the growth of the uterus from going too far. With 

 this hint that the interplay of the hormones is indeed complex, 



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