A. B. Emmons 
41 
of resistance (cf. Diagrams II and III). The circle which just touched these 
three points was read off as representing the size of the fetal head which might 
pass that particular outlet, and the points of contact were marked as the "tuber- 
osities." Often the circle was tangent for a short distance to the tuberosities. 
The middle of this line of tangency was taken as the point of maximum resistance. 
Pubic symphysis 
Diagram III. Outlet. 
Circles represent tbe average fetal head circumference, 9'5 cms. 
Inter-tuberal diameter of 8 cms. requires a posterior-sagittal, 7-3 cms. or more ■ . 
Normal sacral movements lengthen the posterior-sagittal, 1-5 cms. 
Reduction of the posterior-sagittal to minimum, 6 cms. . 
The antero-posterior diameter of the outlet (Diagram I, P — 8) was measured 
from the nearest point of the inner surface of the pubic symphysis to the tip of the 
sacrum. The normal rotation of the sacrum on its axis, usually the second sacral 
segment, may during parturition lengthen this diameter, as well as the posterior 
sagittal, a variable amount, from 15 to 2 cms. (9). Such lengthening would allow 
the head to pass the tuberosities more posteriorly ; this in turn would bring the 
points of resistance on the tuberosities a variable distance back on those curved bony 
prominences, and, depending on this curve, the points of impingement of the 
head would fall a variable distance further out. Consequently we see that the 
moving backward of the tip of the sacrum for this circular passenger enlarges 
the available space not merely directly in proportion to the distance backward, 
but more nearly by the square of that distance. The figures of the measurements 
taken do not include such increase in the outlet space from the mobility of the 
sacrum (Diagrams I and II). 
Biometrika ix 6 
