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XLII . — On a Lower Limit to the Power exerted in the Function of Parturition. 

 By J. Matthews Duncan, M.D., &c. &c. 



(Read 29th April 1867.) 



The dynamics of natural labour have been the field of very little successful 

 study or investigation. The object of the present paper is to make a contribu- 

 tion to this subject. I purpose to show what amount of pressure per square inch 

 is sustained by the ovum in the easiest class of natural labours, and thence to 

 estimate the propelling power exerted in such cases. 



It is well known that natural births are ever and anon occurring, in which 

 the ovum is expelled whole, the membranes containing the liquor amnii con- 

 tinuing entire. Into this category many more cases would enter, were it not a 

 generally -followed rule for the attendant to rupture the bag should it advance 

 entire as far as the external parts. Again, as Dr Poppel has pointed out, the 

 attentive observer of a series of easy natural labours has no difficulty in arriving 

 at the conclusion, that in not a few cases the same force which ruptures the bag 

 of membranes is able to, and actually does, complete the delivery. 



In all such cases, the strength of the membranes to resist impending rupture 

 measures the force exerted in the process of parturition. When the bag is pro- 

 duced without laceration, its strength exceeds, certainly only to a small amount, 

 the power of the labour. When the bag is ruptured at a very advanced stage of 

 labour, as not rarely happens, its strength exceeds the power of labour exerted 

 up till the time of its rupture. When the bag is ruptured by pains, which, 

 without increasing in strength, rapidly and easily terminate the process, then 

 the power of labour is probably only a little greater than the estimate, founded on 

 the strength of the membranes, would indicate. 



The strength of the membranes is thus shown to give us a means of ascer- 

 taining the power of labour in the easiest class of natural cases. 



It might be suggested, that, in cases of persistent membranes, they were spe- 

 cially and unnaturally strong. My own experience lends no support to such a 

 notion. Besides, so far as I know, no obstetrician has used the only means of veri- 

 fying such a supposition — means such as are exemplified in the experiments to be 

 hereafter related. Obstetricians have judged of the strength of membranes to 

 resist a bursting force by their united thickness, or other less definite qualities, 

 which form no criterion. It is not uncommon to read of the bag being strength- 

 ened by decidua ; and that such thickening may be a source of strength is a 

 common opinion ; but as the decidua is far weaker and less extensible than the 

 other membranes, the opinion is merely a natural delusion. 



VOL. XXIV. PART III. 8 L 



