INNERVATION OF THE GENERATIVE ORGANS 573 



THE CAUSE OF UIRTH 



Foster in his " Text-Book of Physiology " 1 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 contractions, 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 problem is now only partly true, but although progress has been 

 made towards its solution no complete answer has yet been given to 

 the question 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 1>\ 

 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 associated in 

 part 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 attempts to 

 reduce itself to its former size, and so expels its contents. This 

 idea receives some support from the fact that twin pregnancies and 

 hydramnios (or the presence of an excessive quantity <>f liquor ainnii) 

 often result in premature labour. 



(->) It has been supposed from early times onwards that parturi- 

 tion might he brought about through the pressure of tin- t<itu> 

 producing a gradual dilatation of the cervix. Williams, however. 

 has pointed out that this condition of the cervix cannot be the 

 sole factor, since in a certain number of cases, especially in twin 

 pregnancies, a pronounced dilatation has heen known to occur for a 

 considerable period prior to the onset of labour. 



(4) Keilmann, 3 working upon the bat, came to the conclusion that 

 the onset of labour was caused by the increasing pressure set up 

 by the lower distended portion of the pregnant uterus (the lower 



1 Foster, Text-Book of Phytwlogy^ .">th Kdition, vol. iv., London, 1891. 

 - Williams, loc. cit. 



3 Keilmann, "Zur Kliinmg der Cervix fragp," Z>it*>-h. /'. <>'>/'. K. '//////.. 

 vol. xxii., 189). 



