40 A Study of the Variations in the Female Pelvis 
Pubic symphysis 
Maximum ; 
inter-tubers i 
14"66 cms. 
Diagram II. Outlet. 
Circles represent the average fetal head circumference, 9 - 5 cms. 
Average inter-tuberal diameter . 
Minimum ,, ,, 
Maximum ,, ,, indicated. 
This diagram shows the effect of the narrowing of the inter-tuberal diameter. 
A contracted outlet, as is clearly pictured in Williams' article, means a 
narrowing of the space between the tuberosities, accompanied usually by the 
so-called " male arch," combined with a short antero-posterior diameter. The 
obstruction caused by this narrowing may be greatly increased by a forward 
position of the tip of the sacrum, or may be decreased by a backward position 
of that prominence (see Diagram III). The determination of the- inter-tuberal 
diameter is, therefore, of prime importance, that of the " posterior-sagittal " 
(Diagram I, 8 — T), the distance from the tip of the sacrum to the inter-tuberal 
line, is secondary to it and correlated with it. 
One of the first difficulties met with in estimating the outlet was the 
determination of the points on the tuberosities from which to measure. Owing 
to the soft parts this difficulty is even greater in the living. While in some 
pelves the angle of the tuberosities was such that the points could easily be 
selected, in others the rounded and complex curves made this selection a mere 
guess. To overcome this difficulty and to obtain consistent results, a circular 
sheet of transparent celluloid was marked with concentric circles O'o centimetre 
apart (Plate VII, 8). This sheet was applied to the inner edge of the tip of the 
sacrum, as one point, and to the two tuberosities as the other essential points 
