CHAP. IL] PREGNANCY AND BIRTH. 1539 



the movements may be induced or increased by asphyxia, the 

 venous blood acting, under these conditions, directly on the 

 muscular fibres ; and a like result has been obtained with certain 

 drugs. The same view moreover is not only indirectly supported 

 by the occurrence of the rhythmic but futile contractions, which 

 as we have said take place during a large part of the period of 

 pregnancy, but is strongly confirmed by the observation in animals 

 that rhythmic movements may take place in a uterus wholly 

 removed from the body, and that even in a uterus which is not 

 pregnant. We may therefore probably apply to the uterus argu- 

 ments similar to those which we used ( 429) in connection with 

 the movements of the bladder in micturition; and indeed many 

 analogies may be drawn between the two acts. We may regard 

 the efferent impulses which issue from the lumbar centre, not so 

 much of the nature of directly excitor impulses as of impulses of 

 an augmentor kind, increasing and developing the intrinsic beats 

 of the uterus itself. 



970. 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 contents, 

 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 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. There is no evidence for the view, based on the 

 occurrence of contractions in consequence of an asphyxiated con- 

 dition 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 contractions 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 culminating 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 



F. 98 



