The coccyx is more moveable, more fre quently in several jointed pieces, less pro jected forwards, and less frequently ankylosed to the sacrum in the female.
The sacro-sciatic notches in the female are wider and uot so deep as in the male ; the dis tance from the ischiadic spine and tuberosity to the sacrum and coccyx being greater, and the sacro-sciatic ligaments longer and more slender.
The peculiarities above mentioned give to the female pelvis a wider, shallower, more open, and less massy appearance than that of the inale, and give rise to a still more im portant distinction derived from the measure ments from one point to another, and from the relative diameters of the cavity and out lets of the pelvis. Another distinction will be presently flituid in the relative angles which the sacrum and whole pelvis form _with the axis of the spinal column, and this again will influence the relative direction of the axes of the cavity and outlets.
The nwasurenzents of the The most evident distinctions between the adult pelves of the sexes are derived from their com parative diinensions, and result from the im portant bearing they have upon the me chanism of parturition in the female. For this purpose, an average is taken from the measurements of many well-formed pelves, and one with the average results is adopted as the standard pelvis.
The measurements referring to the width of the pelvis are commonly spoken of as the diameters of the pelvis. They are taken at the brim, in the cavity, and at the inferior outlet, and are usually an or conjugate, two diagonal or oblique, and a transverse.
At the brim of the pelvis, the antero-pos terior or conjugate diameter is the distance between the upper part of the posterior sur face of the symphysis pubis and the pro montory of the sacrum (a, b, AT. 83.); the oblique, betw een the point of the brim nearest the pectineal eminence and the sacro-iliac joint of the opposite side (c, ; and the transverse diameter is the distance between the ilia at a point halfway between the sacro iliac joint and pectineal eminence (e, f).
In the cavity, the antero-posterior diameter extends between the centre of the pubic sym physis, and the body of the third piece of the sacrum ; while the oblique and transverse correspond to those of the upper outlet, on the same plane.
At the inferior strait, the antero-posterior extends from the lower extremity of the symphysis pubis to the tip of the coccyx ; and the transverse, from the middle of the inner border of one ischiadic tuberosity to the other (g, h). An oblique diameter at the inferior outlet is not one commonly given by writers, although possessed of some hnport ance in certain cases of deformity. In the table on the next page, there is the average of six measurements taken on the recent subject, before the shrinking of the ligaments, from the centre or junction of the ischio-pubic ratni to the centre of the great sciatic liga ment opposite. The antero-posterior diameter of this strait is capable of much increase by the mobility of the coccyx, which will also affect, in some measure, the oblique diameters, in an opposite degree, from the stretching of the great sciatic ligaments, a point which I think has scarcely been sufficiently noticed by accoucheurs.
Besides these, the distances between many other points may be of great importance to the accoucheur. Such are those pointed out by Naegele, to be presently noticed ; the distances between the spines of the ischia, so much greater in the feinale ; and another, which I have not hitherto seen definitively given, viz. the distance between the lower edge of the symphysis pubis and the sacral promontory, a measurement of considerable importance in the use of pelvimeters, to ascertain the conjugate diameter of the brim. This may be called the lower or inclined conjugate diameter, and it will be found to be, in most instances, half an inch more than the direct or superior conjug,ate diameter, being, in fact, the longest side of a triangle, having the conjugate diameter, and the breadth of the pubic spnphysis for the other sides. The measurement of the circumference of the brim of the pelvis, and the proportion con tributed to it by the sacrum, ilia and pubes respectively, announce a manifest difference between the pelves of the two sexes.