THE FEMALE GENERATIVE ORGANS 533 



of mercury of twenty millimetres, during the pains it rose to a 

 height of from eighty to two hundred and fifty millimetres. 

 This difference is calculated to represent a force of from eight 

 and a half to twenty-seven and a half pounds. 



The clinical course of labour and the muscular forces con^ 

 cerned in the process are fully dealt with in the text-books on 

 Midwifery, 1 and it is not proposed in the present work to devote 

 more than a very brief space to the consideration of this subject. 

 It is customary to divide the period of labour into three stages. 



The first stage is characterised by the dilatation of the 

 cervix and os uteri. Galabin gives the following account of 

 the mechanical processes which take place in the uterus during 

 this stage of labour : ''' There are three elements in the 

 mechanism of dilatation of the cervix and os ; first, the 

 mechanical stretching by the bag of membranes ; secondly, 

 the contraction of the longitudinal fibres of the uterus, which 

 draw the cervix open ; and thirdly, the physiological relaxation 

 of the circular fibres, which [is always associated] with the con- 

 traction of the body of the uterus. It follows from the principles 

 of mechanics that the effect of any given pressure within the 

 bag of membranes in producing a tension of the edge, either of 

 the internal or external os, is directly proportional to the 

 diameter of the os, and therefore vanishes when the os is very 

 small. Hence, if the os is closed to begin with, some dilatation 

 by the stretching influence of the longitudinal fibres must have 

 taken place before the mechanism of dilatation by the bag of 

 membranes or parts of the foetus can come into play. The 

 mechanical action of the dilating part, as it is pressed into the 

 cervix, is that of a wedge ; a fluid and uniform wedge, in the 

 case of the bag of membranes ; a solid and irregular wedge, in 

 the case of the head or other part of the foetus. It follows that 

 the effect produced by the wedge varies according to the acute- 

 ness of its angle at the points where it is in contact with the edge 

 of the os. ... It follows that the dilating force vanishes when 

 there is no projection, and becomes greater the more complete 



1 See Williams, loc. cit. Galabin, Manual of Midwifery, 6th Edition, 

 London, 1904, and the other text-books on the subject. See also Sellheim, 

 ' Die Physiologic der Weiblichen Geschlechtsorgane," Nagel's Handbuch der 

 Physiologic des Menschen, vol. ii., Braunschweig, 1906. 



