534 THE PHYSIOLOGY OF REPRODUCTION 



is the projection. It follows also that it becomes progressively 

 more and more effective in proportion to the degree of dilatation 

 which has already been produced." ^ 



The second stage, which has been called the expulsive stage, 

 may be said to include the period from the complete dilatation 

 of the OS uteri to the dehvery of the foetus. When the os has 

 become fully expanded, and the membranes have ruptured, 

 there is generally a short lull in the pains of labour. At the 

 end of the lull the contractions of the uterus begin to recur with 

 increasing vigour and frequency, while the abdominal muscles 

 which are brought into play for the first time exert on the 

 uterus an additional extrinsic force similar to that exerted 

 on the rectum during defsecation. These abdominal contrac- 

 tions are synchronous with those of the uterus, and therefore, 

 Hke them, tend to occur rhythmically. At the commencement 

 of the process the patient is able to some extent to control 

 the contractions by an effort of the will, but later on they are 

 quite involuntary. The combined effect of the contractions is 

 to drive the child, usually head foremost, through the vagina 

 and then out through the vulva, these however playing a purely 

 passive part in the act of expulsion. Sometimes the membranes 

 do not rupture before birth, and the child is born surrounded 

 by a " caul." 



The third stage of labour comprises the expulsion of the 

 placenta. After the dehvery of the child the uterus ceases to 

 contract for a longer or shorter period, at the end of which its 

 activity is renewed once more. At this time the placenta becomes 

 completely separated from the wall of the uterus, and passes into 

 the upper part of the vagina. It is expelled thence through 

 the action of the muscles of the abdomen. During this stage 

 there is almost invariably a certain amount of hsemorrhage, 

 which is represented in normal cases by from three to four 

 hundred cubic centimetres of blood. 



The duration of labour shows considerable variation, but is 

 generally longer in primiparous women {i.e., those who have 

 never borne children before) than in muciparous ones. The 

 average for the former is rather more than eighteen hours, the 

 three stages respectively occupying sixteen, two, and from a 

 • Galabin, loc. cit. 



