Fig. 95.—The various diameters of the inlet with the lengths given in cubic centimeters. (Williams.)
Fig. 96.—Measuring the distance between the anterior superior spines of the pelvis. (Williams.)
The diagonal conjugate is the distance from the lower border of the symphysis to the promontory of the sacrum. It should measure 12.5 cm. The first and second fingers are passed into the vagina and pushed up until the tip of the second finger touches the promontory of the sacrum. The finger of the other hand marks the depth of the examining fingers just below the symphysis. The distance is measured when the finger is withdrawn, and 1.5 cm. is subtracted. The result is the true conjugate. These measurements carefully made and the deduction judicially estimated, give one a fairly approximate idea of size and shape of the pelvic inlet. The aim of nearly all the pelvic measurements is to get not only the size and shape of the inlet, but so far as possible, a working estimate of the anteroposterior diameter of the brim, which is the most important of all the diameters. In normal cases this should be 11 cm.
Fig. 97.—Measuring the external conjugate. (Williams.)
Thus, taking 9.5 cm. from the external conjugate (20.5 cm.) gives 11 cm.
Subtracting 1.5 cm. from the diagonal conjugate as obtained with the fingers as above described, (12.5 cm.) gives 11 cm. The subtraction is made to compensate for the thickness of the pubic bone and its inclination outwards.
