POWER EXERTED IN THE FUNCTION OF PARTURITION. 647 



In making such experiments, a small given error in the estimate of the depth of 

 the approximately spherical segment will be of least consequence, when the mem- 

 brane bursts in a nearly hemispherical form, for by (3) 



2 it 



= »( l -# 



and the error in the estimated radius vanishes, if h — a. Hence, also, the as- 

 sumption that, in nature, the rupture takes place when the protruded portion of 

 the membrane is hemispherical, gives a minimum value of the whole extruding- 

 force. 



For the purposes of this paper the greatest value of the Table lies in the 

 twenty-second column, which gives the power of the labour at the time of the 

 rupture of the membranes and evacuation of the liquor amnii, on the supposition 

 that the lumen of the passage opened up was circular, and of 4J inches in 

 diameter, and that the bulge was hemispherical at bursting. The first striking- 

 observation to be made, is the great variation in the strength of the bag of mem- 

 branes. The force required to rupture the weakest amnion showed that the 

 power of the labour was at least 4-08 lbs. ; that for the strongest, a power of 

 37*58 lbs. ; and the average power indicated by the experiments on the amnion, 

 was 16-73 lbs. The average tensile strength was 119 lbs. Next, it is to be 

 remarked, that in the cases whose membranes were tried, the power of labour 

 almost certainly exceeded the power required to burst the bag, for it is not pro- 

 bable that a particularly weak small portion, unlike the rest of the membrane, 

 was ruptured in the labours. 



In cases 5, 6, 10, 14, 16, 26, the labour did not last above half an hour after 

 the rupture of the membranes ; and the greatest power indicated experimentally 

 by rupturing the membranes was in each case respectively 37-58 lbs., 31-18, 408, 

 18-72, 2703, 12-23. 



In case 22, it was particularly observed by me that, so far as I could judge,* the 

 pain rupturing the bag was stronger than any that followed ; it may therefore 

 be supposed, that the power terminating labour little exceeded 21*37 lbs., the 

 greatest power indicated by the experiments as rupturing the membranes. 



It was only after conceiving the means above described for arriving at the 

 conclusions of this paper, and after the plan of the apparatus had been made by 

 Professor Tait, that I fell in with an interesting and valuable paper by Dr J. 

 Poppel of Munich — "Ueber die Resistenz der Eihaute, ein Beitrag zur Mechanik 



"O 



* The same contractile force of the uterus at different periods of labour, or, to he more exact, 

 at different dimensions of the uterus, will produce greater internal pressure, and, consequently, 

 greater expulsive force, as the uterus is smaller (vide equation (2), p. 646) ; and, the amount of mus- 

 cular contraction being supposed to be the same, there may be no sign to the attendant or patient of 

 the increase of power. On the other hand, the application of the same principle shows, that, when 

 the curvature of the extruded portion of the membranes is greatest, the difficulty of rupturing them 

 is also greatest. This occurs when the extruded portion is hemispherical ; and it is on this supposi- 

 tion that the numbers in column 22 are calculated. 



VOL. XXIV. PART III. 8 N 



