[Illustration: _Fig. 1. Fig. 2._ In the annexed representations of the superior and inferior aspects of the female pelvis are shown the three diameters of its brim and outlet; those of the former in _fig. 1._, and those of the latter in _fig. 2._ The same letters of reference are used in each figure to indicate the several diameters; thus _a p_ refers to the antero-posterior, _t t_ to the transverse, _o o_ to the right oblique, and _o' o'_ to the left oblique diameters. In _fig. 2._ the coccyx is represented in situ.] In the cavity these dimensions vary more or less. The straight diameter, measured from the centre of the hollow of the sacrum to that of the symphysis pubis, is 4·8 inches; the transverse, from the point corresponding to the lower margin of the acetabulum on one side to that of the other, 4·3; and the oblique, drawn from the centre of the free space formed by the sacro-ischiatic notch and ligaments on one side of the foramen ovale of the other, 5·2. At the inferior aperture or outlet the alteration is still more remarkable. The straight diameter, from the point of the coccyx to the lower edge of the symphysis pubis, measures only 3·8 inches; but from the mobility of the coccyx enabling it to be pushed back during labour to the extent of a whole inch, it is capable of being extended to 4·8 inches. The transverse diameter from one tuberosity of the ischium to the other, measures 4·3 inches: and the oblique, from the middle of the lower edge of the sacro-sciatic ligament of one side, to the point of union between the ischium and descending ramus of the pubes on the other 4·8 inches.