PREGNANCY AND PARTURITION 1287 



pregnancy they have little or no effect on the contents of the uterus. During 

 the last weeks or days of pregnancy however, these contractions, which have 

 now become more marked, have a distinct physiological effect. Not only 

 do they, by pressing on the foetus, cause it in most instances to assume a 

 suitable position for its subsequent expulsion but, affecting the whole body 

 of the uterus including the longitudinal muscular fibres surrounding its 

 neck, they assist the general enlargement of the organ in dilating the 

 internal os uteri, so that the upper part of the cervix is obliterated and 

 drawn up into the body of the uterus some little time before labour has 

 commenced. 



With these changes in the uterus are associated' changes in the round 

 ligaments and in the vagina and vulva. The muscular fibres of the round 

 ligaments become much hypertrophied and lengthened, and these structures 

 can therefore aid appreciably the uterine contractions in the subsequent 

 expulsion of the foetus. The vaginal walls become thickened and of looser 

 texture, so as to afford less resistance to distension during the passage of 

 the foetal head. 



Considerable discussion has taken place as to the cause for the onset of 

 the processes comprised under the heading of labour or parturition at a nearly 

 constant period of two hundred and seventy-two days after conception. 

 Most of the explanations which have been suggested, such as the great irrita- 

 bility of the uterus at the termination of pregnancy, the loosening of the 

 foetal membranes, the return of the menstrual congestion after ten months, 

 thrombosis of the placental sinuses, simply replace one question by another. 

 According to Spiegelberg the phenomena accompanying the birth of twins, 

 which are often born at a considerable interval from each other, the onset of 

 contractions of the uterus at the right time in extra-uterine as well as in 

 normal foetation, the fact that the extra-uterine foetus dies when it has 

 become mature, all go to show that the reason why labour occurs at a 

 definite time must be sought for in foetal rather than in uterine changes . 

 This author suggests that some substances which had previously been used 

 up by the foetus gradually accumulate in the maternal blood as the foetus 

 becomes mature, and provoke, by their direct action on the uterus or spinal 

 cord, the uterine contractions which give rise to labour. 



Actual parturition in the woman is generally divided into two stages. 

 In the first stage the contractions are confined to the uterus, and chiefly act 

 in dilating the os uteri. In this dilatation two factors are involved, namely, 

 the active dilatation brought about by the contraction of the longitudinal 

 muscular fibres which form the chief constituent of the lower part of the 

 uterine wall ; and in the second place, a passive dilatation by the pressure 

 of the foetal bag of membranes, which is filled with amniotic fluid, and forced 

 down as a fluid wedge into the os by the contractions of the uterine fundus. 

 The uterine contractions are essentially rhythmical, being feeble at first, and 

 increasing gradually in intensity to a maximum which endures a certain 

 time, and then gradually subsides. The frequency and duration of the 

 contractions increase as labour advances. 



