PREGNANCY AND PARTURITION 1381 



With the first breath taken by the new-born child all the mechanical 

 conditions of the circulation are modified. The resistance to the blood- 

 flow through the lungs being diminished, the blood passes from the pul- 

 monary arteries through the lungs into the left auricle. The pressure 

 in the left auricle is thus raised, while that in the right auricle falls, 

 so that the foramen ovale is maintained closed. Even before birth 

 proliferation of the lining membrane may be seen both in the ductus 

 arteriosus and in the ductus venosus ; and with the mechanical 

 relief of the vessels afforded by respiration and the changed conditions 

 of the foetus, this proliferation goes on to complete obliteration of the 

 vessels. 



PARTURITION 



As the uterus increases in size and becomes more distended, its 

 irritability becomes greater, so that it is easily excited to contract. 

 The stimulus may be supplied from adjacent abdominal organs, 

 from the brain, as by emotions, or by direct excitation of the internal 

 surface of the uterus, in consequence of movements of the foetus. 

 In many cases no antecedent stimulus can be discovered, and the 

 automatic contraction of the uterus seems to be analogous to that which 

 occurs in the distended bladder. These contractions ordinarily give 

 rise to no sensations, and are only felt when they are augmented in 

 consequence of reflex stimulation. During the greater part of preg- 

 nancy they have little or no effect on the contents of the uterus. 

 During the last weeks or days of pregnancy, however, these contractions, 

 which have now become more marked, have a distinct physiological 

 effect. Not only do they, by pressing on the foetus, cause it, in most 

 instances, to assume a suitable position for its subsequent expulsion, 

 but, affecting the whole body of the uterus, including the longitudinal 

 muscular fibres surrounding its neck, they assist the general enlarge- 

 ment of the organ in dilating the internal os uteri, so that the upper 

 part of the cervix is obliterated and drawn up into the body of the 

 uterus some little time before labour has commenced. 



With these changes in the uterus are associated changes in the 

 round ligaments and in the vagina and vulva. The muscular fibres of 

 the round ligaments become much hypertrophied and lengthened, and 

 these structures can therefore aid appreciably the uterine contractions 

 in the subsequent expulsion of the foetus. The vaginal walls become 

 thickened and of looser texture, so as to afford less resistance to dis- 

 tension during the passage of the foetal head. 



Considerable discussion has taken place as to the cause for the onset 

 of the processes comprised under the heading of labour or parturition at 

 a nearly constant period of two hundred and seventy-two days after 

 conception. Most of the explanations which have been suggested, 



