THE FEMALE GENERATIVE ORGANS 535 



quarter to half-an-hour. The average for multiparous women 

 is twelve hours, eleven of which are occupied by the first, and 

 one by the second stage. The duration of labour in primiparous 

 women depends also upon age, being usually more prolonged 

 in elderly subjects. 



Parturition in other Mammalia 



In animals the process of delivery varies somewhat in the 

 different animals. In the horse the foetus, which has been 

 lying on its back during intra-uterine life, preparatory to birth 



Fig. 131. — The first stage in the revolution of the equine foetus. The os is 

 dilated by the membranes, wliioh have not yet ruptured. (After Franck. 

 From Smith's Veterinary Physiology, Baillifere, Tindall & Cox.) 



changes on to its side and afterwards assumes the upright 

 position, with its muzzle and forelegs in the direction of the 

 pelvis. Dilatation of the passage follows, and the foal is de- 

 Uvered head first. In the cow and sheep the movements which 

 occur are essentially similar. It is stated that the alteration 

 in the position of the foetus is not brought about by its own 

 movements but by the uterine contractions. The revolution of 

 the foetus prior to birth in the mare and cow is apparently re- 

 sponsible for the torsion of the neck of the uterus and vagina 

 which often occurs in these animals. 



Parturition in the mare is accompanied by a complete 

 separation of the chorion from the uterine wall. As a conse- 



