540 THE PHYSIOLOGY OF REPRODUCTION 



slight rise of temperature during the process, and the pulse rate 

 is also affected, being much quicker during the pains, but slower 

 between them, the difference amounting to as much as thirty- 

 six beats a minute. The blood shows a marked leucocytosis, 

 and the blood pressure is higher. Urine is secreted in smaller 

 quantities, and is liable to contain traces of renal epithelium, 

 red and white blood corpuscles, and albumen. 



THE CAUSE OF BIRTH 



Foster in his Text-book of Physiology l has written as 

 follows : " We may be said to be in the dark as to why the 

 uterus, after remaining for months subject only to futile contrac- 

 tions, is suddenly thrown into powerful and efficient action, and 

 within it may be a few hours, or even less, gets rid of the burden 

 which it has borne with such tolerance for so long a time. None 

 of the various hypotheses which have been put forward can be 

 considered as satisfactory. We can only say that labour is 

 the culminating point of a series of events, and must come 

 sooner or later, though its immediate advent may sometimes be 

 decided by accident/'' What. Foster wrote about this question 

 is still true to-day, for no real progress has been made towards 

 the solution of the physiological problem as to the immediate 

 cause of parturition. 



Williams 2 has classified the various theories which have 

 been formulated under eleven heads. These may now be 

 briefly considered in the order adopted by him. 



(1) The increasing irritability of the uterus, as manifested by 

 its greater tendency to respond to stimulation in the later 

 stages of gestation, is probably a factor in determining the 

 time of birth. As already described, the uterine contractions 

 towards the close of pregnancy are not only more frequent, but 

 they are also much more intense. This growing irritability is 

 no doubt to be directly associated with the increase in the size 

 of the foetus. 



(2) It is suggested that the mere distension of the uterus 

 must, after a certain point, lead to a reaction, when the organ 



1 Foster, Text-Book of Physiology, 5th edition, vol. iv., London, 1891. 



2 Williams, loc. cit. 



