CHAPTER XI 



PARTURITION 



Numerous theories of the cause of parturition have been put 

 forward, and while many subsidiary factors (separation of the 

 placenta, etc.) no doubt play a part, it is fairly clear that the 

 physiological changes which initiate the act are due to 

 hormonic stimuli. 



(a) CORRELATION WITH OVARIAN CYCLE 



Conditions in pseudo-pregnancy . All the evidence tends to 

 show that parturition takes place when some particular stage 

 of the ovarian cycle is reached. Thus, the immediate forerunner 

 of parturition is the retrogression of the corpora lutea of preg- 

 nancy. The view that this retrogression of the corpora lutea is 

 the actual cause of parturition is supported by the fact that at 

 the end of pseudo-pregnancy some animals show certain 

 symptoms normally associated with parturition. Thus, the 

 rabbit plucks its fur and makes a nest, while Dasyurus also 

 makes a nest and cleans its marsupial pouch. It may be 

 assumed, therefore, that a certain phase of the Qvarian cycle is 

 correlated with the occurrence of parturition. No ovarian 

 extracts, however, have been shown to have any really signifi- 

 cant direct action upon uterine contraction, so it is probable that 

 some indirect mechanism is at work. 



The relation between oestrus and parturition. Since in many 

 animals a period of oestrus follows very soon after parturition, 

 it is necessary to discuss the relation between these two events. 

 In the mouse, rat, rabbit, and guinea-pig, oestrus follows within 

 a few hours of parturition. In other mammals, however, a 

 greater delay is found. ExperimentaHy, it has been shown by 

 injection of oestrin, or by the implantation of anterior pituitary 



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