1138 PARTURITION. [Book iv. 



at each recurring "pain" the shortening of the muscular 

 fibres starts so to speak from this more permanent shortening 

 instead of from complete relaxation, and the return is to it not 

 to complete relaxation. 



This more permanent tonic contraction or " retraction " does 

 not however affect the whole uterus ; it is, broadly speaking, 

 confined to the body and absent from the cervix. Indeed in 

 the latter region all contractions are wanting, the muscular 

 fibres appear to be inhibited, and the walls yielding to the 

 pressure exerted upon them become thinner instead of thicker ; 

 as the pressure increases the fibres possibly become lamed 

 or paralyzed. In this way a distinction is established between 

 an " upper segment " of the uterus corresponding to the body, 

 the walls of which become thicker and shorter through the 

 continued and progressive " retraction," and a "lower segment," 

 corresponding to the cervix but possibly including the lower 

 part of the body, the walls of which become stretched and 

 thinner, the line of demarcation between the two segments 

 being often called " the retraction ring." As the pressure 

 in the body of the uterus continues and waxes greater, the 

 mouth becomes wider and wider, until the head of the foetus 

 begins to pass through it into the vagina, the walls of which 

 like those of the "lower segment" have meanwhile become 

 stretched and thinner ; and as the foetus is thus leaving the 

 uterus the progressive tonic contraction adapts the uterine 

 walls to the lessening cavity. Sometimes the membranes are 

 ruptured, with escape of the " waters," before the head has left 

 the uterus, at other times they form a bulging cushion preceding 

 and making way for the foetus. 



When the os uteri has become fully expanded and is ready 

 to allow the head of the foetus to pass through it into the vagina, 

 the intrinsic contractions of the uterus begin to be assisted by an 

 extrinsic force, by contractions of the abdominal walls which 

 thus exert on the uterus and its contents a pressure very similar 

 to that exerted on the rectum in defiecation '(§ 225). These 

 contractions, which mark the onset of the "second stage" of 

 labour, are rhythmical in nature like those of the uterus itself, 

 and synchronous with them. The expulsive power of the uterus 

 is thus greatly increased, and the head of the foetus followed 

 by the rest of its body is driven through the vagina and then 

 through the vulva, these playing apparently a wholly passive 

 part in the matter, and the child is thus literally " thrust upon 

 the world." 



At the very beginning of labour there takes place at the 

 internal os a cleavage of the decidua vera between a deeper less 

 altered and a superficial more altered layer, so that the latter, 

 attached to the chorion and thus forming part of the "mem- 

 branes," separates from the uterine surface. This separation, 



