INNERVATION OF THE GENERATIVE ORGANS 567 



of mechanics that the effect of any given pressure within the bag of 

 membranes in producing a tension of the edge, either of the internal 

 or external os, is directly proportional to the diameter of the os, and 

 therefore vanishes when the os is very small. Hence, if the os is. 

 closed to begin with, some dilatation by the stretching influence of 

 the longitudinal fibres must have taken place before the mechanism 

 of dilatation by the bag of membranes or parts of the fcstus can come 

 into play. The mechanical action of the dilating part, as it is 

 pressed into the cervix, is that of a wedge; a fluid and uniform 

 wedge, in the case of the bag of membranes ; a solid and irregular 

 wedge, in the case of the head or other part of the foetus. It follows 

 that the effect produced by the wedge varies according to the 

 acuteness of its angle at the points where it is in contact with the 

 edge of the os. . . . It follows that the dilating force vanishes when 

 there is no projection, and becomes greater the more complete 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 prodiiced." ^ 



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 delivery of the foetus. When the os has become fully 

 expanded, and th§ membranes have ruptured, there is generally a 

 short lull in the pains of labour. At the end of the lull the con- 

 tractions 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 contractions are synchronous \^ith those of the uterus, and 

 thereforej like them, tend to occur rhythmically; At-.the commence- 

 ment 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 delivery of the ,child the uterus ceases tp contract for a 

 longer or shorter period, at the end of which its activity is renewed 

 once niore. 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 



^ dralabin, I06. cit. 



