THE HORMONES IN HUMAN REPRODUCTION 



is impeded, the capillary circulation is impaired, ischaemia of 

 the endometrium results and is followed by menstrual break- 

 down. As a variant of this idea, it has been thought that a 

 drop in estrogen occurring toward the end of the cycle 

 causes involution of the endometrium, with a reduction of its 

 thickness and consequent tighter coiling of the arteries. This 

 was supposed to cause damage to the tissues and conse- 

 quently to bring on menstrual bleeding. 



Another totally different supposition is now put forward 

 by the Danish investigators Schlegel, Dalgaard, and Okkels 

 (see, for instance, J. V. Schlegel, "Arteriovenous Anasto- 

 moses in the Endometrium in Man," Acta Anatomica, vol. 1, 

 1945-46). These workers, using very careful methods of 

 injecting the uterine blood vessels, have shown almost beyond 

 any doubt that in the human endometrium there are frequent 

 direct connections (anastomoses) between the terminal arteri- 

 oles and the venous spaces from which the uterine veins take 

 origin. Some of the blood flowing through the lining of th^ 

 uterus follows the pathway usual in other organs and tissues, 

 through the capillary blood vessels, thus serving the ordinary 

 metabolic functions of the blood. Some of the blood, however 

 (according to these investigators) passes directly through a 

 shunt, as it were, into the veins. Schlegel off^ers a theory of 

 menstruation based on this finding. He conjectures that as 

 the endometrium grows thicker in each cycle under the in- 

 fluence of estrogen, the number of short circuits between the 

 arterial and venous systems increases. The increasing pro- 

 portion of blood thus shunted must be compensated for by 

 an increased flow through the capillaries also. Such a flow, 

 it is well known, is eff'ected by the estrogenic hormone. The 

 time comes, however, it is thought, when the estrogenic stimu- 

 lus is not able to produce further capillary flow whereas the 

 shunts still divert much of the blood. The tissues nourished 

 by capillary blood suffer injury and menstruation is thus 

 initiated. 



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