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DR MATTHEWS DUNCAN ON A LOWER LIMIT TO THE 



Experiments at once show that thickness of the membranes is no indication 

 of strength. They also at once show that, for the special purposes of this paper, 

 the amniotic membrane, being the strongest, alone requires to be observed. Long 

 before the amnion is burst, the decidua and chorion have generally given way, 

 and ceased to support the persistent amniotic membrane. The decidual mem- 

 brane generally gives way first, under a bursting pressure applied to all three 

 membranes. It sometimes does so with a sound as of a gentle fillip. Occa- 

 sionally it bursts simultaneously with the chorion ; and occasionally all three 

 membranes burst at once. The decidua has been found, in the experiments, to 

 burst at a tension of 35 lb. per linear inch, corresponding under the circumstances 

 supposed to exist in actual labour to a forward pressure of nearly 5 lbs. 



As a general statement, it may be said that the chorion behaves like the 

 decidua. It is of more uniform strength than the decidual membrane, and is only 



a little stronger, the average tensile strength 

 being -62 lbs. per linear inch, corresponding to 

 a propelling power in labour of nearly 9 lbs. 

 [In taking these averages, experiment 25 is 

 omitted, because its exceptional value indicates 

 almost certainly a mistake.] 



The strength of the fcetal membranes lies in 

 the innermost sac, in the amniotic membrane, 

 which appears the thinnest and most delicate 

 of all. To try the strength of it, as well as of 

 the others, I made numerous experiments in the 

 following manner : — They were all performed 

 in the laboratory, and with the apparatus, of Pro- 

 fessor Tait, to whose knowledge and skill I am 

 indebted for their value and accuracy. The 

 apparatus used was connected with a pipe in 

 the bottom of an open cistern aa. Into this 

 pipe b water,* under high pressure, of which 

 there was a convenient supply, could be admit- 

 ted gradually by a cock c. The apparatus ex- 

 panded upwards from the pipe to its mouth d. 

 In one apparatus used, this mouth had an ex- 

 ternal diameter of 3-35 inches, in the other it 

 had an external diameter of only 2-25 inches. Over the mouth of the apparatus 

 the membranes experimented on were placed, and tied on by a waxed hempen 

 cord, around a broad rim ee, immediately beneath the mouth. That the apparatus 



* Water is preferable in these experiments to air, because, when it is employed, there is less 

 violent action at the bursting of the membrane. 



