Normal Parturition 513 



When discussing the position of the fetus in the uterus toward 

 the end of gestation, we took occasion to note that it is not rare 

 to find a portion of it extending beyond the cervix and resting 

 in a cul-de-sac of the uterus alongside the vagina. When partu- 

 rition comes on, if the abdominal muscles alone should act on 

 the fetus, they would simply tend to push the fetus further on- 

 ward in this cul-de-sac and threaten finally to rupture the uterus. 



If, on the other hand, the uterus contracts first, and especially 

 the longitudinal fibers, their contraction would efface the cul-de-sac 

 of the uterus alongside the vagina. This brings the presenting 

 part of the fetus into a conical cavity which terminates with the 

 OS uteri and places the fetus in a position and direction which will 

 permit of its expulsion. 



The uterine contractions are essential for the dilation of the 

 OS uteri. Whenever these contractions take place, and conse- 

 quently increase the intra-uterine tension, the walls yield at the 

 point of least resistance. Normally this yielding occurs at the 

 cervix, causing it to become dilated, and later the cervix and os 

 uteri to become wholly effaced, so that the uterine and vaginal 

 cavities are merged into one common passage, without any line 

 of demarcation between them. 



The contractions of the uterus during the act of birth are 

 essential to the maintenance of the organ in its position in the 

 abdominal cavity. ' If the expulsive powers were dependent upon 

 the abdominal muscles alone, the uterus would tend to become 

 expelled along with the fetus and thus to induce a prolapse, but 

 the uterus, by its contraction, is able to maintain a natural posi- 

 tion throughout the act and to avoid being pushed back into the 

 vagina. 



The dilation of the cervical canal is dependent partly upon the 

 shortening of the longitudinal muscle fibers of the uterus, but 

 largely upon the pressure of the fetal membranes, with the con- 

 tained fluids, against its anterior opening. With the increase of 

 intra-abdominal tension, the fluids about the fetus tend to move 

 most readily toward the os uteri and push along with them the 

 membranes. The fetal fluids within the membranes serve as an 

 elastic dilator which, slowly becoming impacted in the os uteri, 

 gradually dilates the passage by exerting equal pressure upon 

 every part. 

 33 



