1142 PAKTURITION. [BOOK iv. 



710. Though under normal circumstances efficient uterine 

 contractions do not set in until the full period of gestation is 

 completed, yet by reason of changes in the uterus or its con- 

 tents, occurring from natural causes or induced artificially, the 

 full swing of movements may, at almost any time, though at 

 some times more readily than at others, be brought about. On 

 the other hand it may be delayed for a considerable time beyond 

 the proper term. 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 effi- 

 cient 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. There is 

 no evidence for the view, based on the occurrence of contrac- 

 tions in consequence of an asphyxiated condition of the blood, 

 that the onset of labour is caused by a gradual diminution of 

 oxygen or accumulation of carbonic acid in the blood, reaching 

 at last to a climax. Nor are there sufficient facts to connect 

 parturition with any condition of the ovary resembling that 

 accompanying menstruation. Nor can much stress be laid on 

 the supposition that the real exciting cause is the separation of 

 the decidua from the permanent uterine wall, the separation 

 being the outcome of the preceding processes of growth, since 

 the actual separation itself seems to be caused by the initial con- 

 tractions of labour, and the histological changes which precede 

 it are only one set of changes among many others all having 

 their goal in labour. We can only say that labour is the cul- 

 minating point of a series of events, and must come sooner or 

 later, though its immediate advent may at times be decided by 

 accident ; but it would not be profitable to discuss this question 

 here. 



The action of the abdominal muscles in parturition, at least 

 so much as takes place independently of the will, is, in contrast 

 to that of the uterine muscles, obviously a reflex act of a more 

 ordinary kind carried out by means of the spinal cord ; and we 

 may suppose that, though the mere contractions of the uterus 

 may serve as a possible source, the necessary stimulus is sup- 

 plied by the pressure of the foetus in the vagina ; in support of 

 this it may be noted that the action becomes much intensified 

 towards the end of labour as the stress and strain caused by the 

 advancing head tell more and more on the external skin. 



711. Hence as we have said the whole act of parturition 

 may with reason be considered as a reflex one. Whether it be 

 wholly a reflex or in a certain sense an automatic one, the act 

 can readily be inhibited by other contemporary actions of the 

 central nervous system. Thus emotions very frequently become 

 a hindrance to the progress of parturition ; as is well known, 



