1382 PHYSIOLOGY 



such as the great irritability of the uterus at the termination of preg- 

 nancy, the loosening of the fcetal membranes, the return of the men- 

 strual 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 normal as well as in extra-uterine 

 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 is in man 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 in- 

 volved, 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 fcetal 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. 



As soon as the os is fully dilated and the fcetal head has entered 

 the pelvis, the contractions change in character, being much more pro- 

 longed and frequent, and attended by more or less voluntary con- 

 tractions of the abdominal muscles. This action of the abdominal 

 muscles is associated with fixation of the diaphragm and closure of the 

 glottis, so that pressure is brought to bear on the whole contents of the 

 abdomen, including the uterus. No expelling force can be ascribed 

 to the vagina, since it is too greatly stretched by the advancing foetus. 

 In this way the foetus is gradually thrust through the pelvic canal, 

 dilating the soft parts which impede its progress, and is finally expelled 

 through the vulva, the head being born first. The membranes generally 

 rupture towards the end of the first stage of parturition. 



A third stage of labour is generally described. This consists in a 

 renewal of uterine contractions about twenty to thirty minutes after 

 the birth of the child, and results in the expulsion of the placenta and 

 decidual membranes. 



