660 Proceedings of the Royal Society 
Before concluding the consideration of the great curve of the 
genital passage in the anteroposterior vertical mesial plane, it is 
necessary to point out an important difficulty introduced into its 
study by the change in the condition of the ovum when passing 
through it, as compared with the ordinary condition of the ovum 
when passing the pelvic brim. Hitherto I have spoken on the 
assumption that the ordinary view of the action of the power of 
labour holds good at all parts of the course of the child. This 
view is, that the power is uniformly applied by the concave surface 
of the approximately spheroidal uterus to the uniform surface of 
the approximately spheroidal ovum, in a direction corresponding 
to the axis of the uterus and of the developed genital passage. 
Now, this view is probably nearly correct so long as the mem- 
branes are unruptured, or while no special part of the foetus 
impinges on the uterus so as to injure its approximately spheroidal 
form, and provided no part of the foetus impinges on the passage 
so as to cause special friction or obstruction at the part impinging. 
But while the great anteroposterior vertical curvature of the genital 
passage is being permeated, this view is no longer tenable, although 
even then it may, in a confessedly inexact way, be advantageously 
kept in mind, if other more exact conditions are not stated. While 
the curve is being described, the membranes are generally ruptured 
and the waters more or less completely discharged; and conse- 
quently the foetus is in a variety of places impinging on and chang- 
ing the form of the propelling uterus, and meeting with frictional 
obstruction in the passage at special points more than at others. 
These changes introduce an amount of complication of the problem 
which damages greatly the value of such considerations as I have 
above adduced, and I see no means at present of overcoming it 
and of arriving at exactness, though there is probably no insuper- 
able difficulty in the matter. Another element of confusion is 
introduced by the want of uniformity which exists in the composi- 
tion of the foetus as a mechanical body. It is especially to be 
noted that it contains a longitudinally-placed elastic beam of con- 
nected vertebrae, which lies nearer the surface of the mass at one 
side than at the other. 
The ovum or foetus, in its passage through the developed genital 
canal, is subjected in various circumstances to various rotations on 
