EFFECTS OF EXTRACTS OF PARS NEURALIS 



of contraction is followed by a phase of inhibition of the 

 normal movements. This inhibitory phase is due to the vaso- 

 pressor hormone. The vasopressor hormone alone causes in- 

 hibition and may antagonize small doses of the oxytocic 

 hormone. Tachyphylaxis (of the inhibitory effect) appears 

 after repeated doses of the vasopressor principle (Reynolds, 

 1930, 1933; Weinstein and Friedman, 1935). Similarly, the 

 isolated uterus (of the rabbit), not responding to the oxytocic 

 principle because it has been obtained from an animal in 

 which there are actively secreting corpora lutea in the ovary, 

 may relax if posterior-lobe extract or the vasopressor prin- 

 ciple is added to the bath (Robson). 



The effect of ovarian secretion of hormones or of pregnancy 

 on the response of the isolated or intact uterus of man, the 

 cat, the guinea pig, the rabbit, the rat, and the mouse has 

 been studied by a number of investigators. Sahako (1925-26) 

 reported that the response of the isolated or intact uterus 

 of the rabbit is related to the condition of the ovaries. He 

 found that the uterine response to a posterior-lobe extract 

 was diminished or was inhibitory if actively secreting corpora 

 lutea were present in the ovary as in pregnancy. However, 

 late in pregnancy the uterine response was found to be in- 

 creased. Sahako's findings have been extended and con- 

 firmed by Knaus, Okazaki, Robson, and others. 



The most numerous experiments have been performed in 

 rabbits. Pregnancy can be limited to one horn, so that the 

 other horn is left free for testing its response to posterior- 

 lobe extract in vitro (Knaus, 1927-28). The response of the 

 uterus to the oxytoxic principle becomes markedly reduced 

 about 48 hours after coitus. If coitus is infertile and pseudo- 

 pregnancy appears, the oxytocin response is absent or 

 diminished for about 2 weeks. In the case of pregnancy this 

 is true for a period of 3 weeks. Thereafter, the uterine re- 

 sponse to the oxytocic principle increases, especially during 

 the last few days of pregnancy; the sensitivity of the uterus 



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