CHAPTER IV. 

 PARTURITION. 



IN spite of the increasing distension of its cavity, the uterus remains 

 quiescent, as far as any marked muscular contractions are concerned, 

 until a certain time has been run. In the human subject the period 

 of gestation generally lasts from 275 to 280 days, i.e. about 40 

 weeks, the general custom being to expect parturition at about 

 280 days from the last menstruation. Seeing that, in many cases, 

 it is uncertain whether the ovum which developes into the embryo 

 left the ovary at the menstruation preceding or succeeding coitus, 

 or, as some have urged, independent of menstruation, by reason of 

 the coitus itself, an exact determination of the duration of preg- 

 nancy is impossible. 



In the cow the period of gestation is about 280 days, in the mare 

 about 350, sheep about 150 days, dog about 60 days, rabbit about 30 

 days. 



The extrusion of the foetus is brought about, partly by rhyth- 

 mical contractions of the uterus itself, and partly by a pressure 

 exerted by the contraction of the abdominal muscles, similar to that 

 described in defaecation. The contractions of the uterus are the 

 first to appear, and their first effect is to bring about a dilation of 

 the os uteri; it is not till the later stages of labour, while the foetus 

 is passing into the vagina, that the abdominal muscles are brought 

 into play. 



The whole process of parturition may be broadly considered as 

 a reflex act, the nervous centre being placed in the lumbar cord. 

 In a dog, whose dorsal cord had been completely severed, parturi- 

 tion took place as usual ; and the fact that, in the human subject, 



