CHAP. IL] PREGNANCY AND BIRTH. 1535 



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 paralysed. In this way a distinction 

 is established between an " upper segment " of the uterus corre- 

 sponding 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 pro- 

 gressive 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 loose cushion precediug 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 defecation ( 275). These con- 

 tractions, which mark the onset of the " second stage " of labour, 

 are rhythmical in nature like those of the uterus itself, and syn- 

 chronous with them. The expulsive power of the uterus is thus 

 greatly increased, and the head of the fcetus 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 " membranes," 

 separates from the uterine surface. This separation, the beginning 

 of which is the cause of the " show " spoken of above, and which 

 is considered to be a mechanical effect of the uterine contractions 

 but which must be prepared for by histological changes, during the 

 early stages of labour extends upwards for two or three inches 

 only; but, at the last, it is carried right through the "decidual 

 layer " of the placenta. Hence, after the expulsion of the foetus, 

 the uterus contains within its cavity, separated from and now 



