PARTURITION. 853 



CHAPTER V. 



PARTURITION. 



795. IN spite of the increasing distention 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 develops 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 

 pregnancy is impossible. 



In the cow the period of gestation is aoout 280 days, in the mare about 350, 

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



796. The extrusion of the foetus is brought about partly by rhythmical 

 contractions of the uterus itself and partly by a pressure exerted by the 

 contraction of the abdominal muscles, similar to that described in defeca- 

 tion. The contractions of the uterus are the first to appear, and their first 

 effect is to bring about a dilatation of the os uteri ; it is not until the later 

 stages of labor, while the foatus is passing into the vagina, that the ab- 

 dominal muscles are brought into play. 



797. The whole process of parturition may be broadly considered a 

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

 whose dorsal cord had been completely severed, parturition took place as 

 usual ; and the fact that, in the human subject, labor will progress quite 

 naturally while the patient is unconscious from the administration of chloro- 

 form, shows that in woman also the whole matter is an involuntary action, 

 however much it may be assisted by direct volitional efforts. That the uterus 

 is capable of being thrown into contractions through reflex action, excited 

 by stimuli applied to various afferent nerves, is well known. The contraction 

 of the uterus, which is so necessary for the prevention of hemorrhage after 

 delivery, may frequently be brought about by exerting pressure or by dash- 

 ing cold water on the abdomen, by the introduction of foreign bodies into 

 the vagina, and especially by putting the child to the nipple. An'd we learn 

 from experiments on animals that rhythmic contractions of the uterus re- 

 sembling at least those of parturition, may be brought about in a reflex 

 manner by stimulating various afferent nerves. Similar movements may be 

 induced by direct stimulation of the spinal cord along its whole length, as 

 well as of various parts of the brain ; but there are reasons for thinking that 

 in these cases the impulses started in the brain and upper part of the spinal 

 cord produce their effects by working upon what may be called a " parturi- 

 tion " centre in the upper lumbar regions of the cord. And it would appear 

 that the uterine contractions which are induced by such drugs as ergot, as 

 well as those caused by asphyxia, are, at all events in part, brought about 

 by the agency of the same lumbar centre. From this centre the paths for 

 the efferent impulses appear (in the dog) to be two-fold ; one along sympa- 

 thetic tracts, by nerves passing from the inferior mesenteric ganglion to the 

 hypogastric plexus, and the other along spinal tracts by branches of the 

 sacral nerves to the same plexus. It is stated that the characters of the 



