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Different estimates are made by different anatomists of the measurements of the brim of the pelvis. The following is nearly the correct size of the ordinary female pelvis:

Fig. 7. The bony pelvis.

Fig. 7—The pelvis seen from above. a a, The antero-posterior or sacro-pubic diameter; b b, the transverse diameter; c c, the two oblique diameters.

The circumference varies from thirteen to fifteen inches; the antero-posterior diameter, i.e., from the prominence of the sacrum to the upper edge of the symphasis pubis, (Fig. 6), is about four and a quarter inches; the transverse across the widest part of the brim, at right angles to the antero-posterior, is five and a quarter inches, and the oblique from the sacro-iliac synchondrosis of one side to the opposite of the brim, just above the acetabulum, is five inches. (Fig. 7).

The cavity of the pelvis, of which the fixed boundaries are the sacrum and the pubis, is of unequal depth. The height in front is one and a half inches; upon the sides, three and three-quarter inches, and it is four and a quarter inches if a straight line be drawn from the sacro-vertebral angle to the point of the coccyx, five and a quarter inches following the curve of the sacrum, and six inches if the coccyx be extended. (Fig. 6).

The antero-posterior diameter of the outlet from the arch of the pubis to the point of the coccyx is usually four and a quarter inches, but may increase to five inches during labor by the retrocession of the coccyx (Fig. 8); the transverse from one tuber ischii to another is four and a quarter inches, and the oblique about four and three-quarter inches.

Fig. 8—Position of the pelvis and the axis at the termination of labor.

Fig. 8—a b, Total axis of the excavation; c, the axis of the superior strait; d e, perineum as distended at the moment of the passage of the head.

It is important to notice that the diameters are entirely changed between the rim and the outlet, and that the change is effected gradually. The axes of the inlet and outlet form an obtuse angle with each other (this is illustrated in Figs. 6 and 8.) The three diameters taken at the center of the pelvis are very nearly equal—about four and three-quarter inches.