3d. The two oblique diameters extend from the middle of the inferior edge of the greater sciatic ligament to the middle of the opposite ischio pubic ramus.
All these diameters are, on an average, 4.3 inches long. The coccy pubic diameter is, however, very often less than this measurement, but during labor the backward movement of the coccyx lengthens this diame ter by to 1 inch, and even more in some cases.
The circumference of the inferior strait, instead of being regular like that of the superior, is very rough. It has three corners and three notches. The three corners are formed by the coccyx and the two ischia tic tuberosities. The three notches are formed by the edge of the greater sciatic ligaments, and by the pubic arch.
Finally there is the sacro-sub-pubic or promonto-sub-pubic diameter, which extends from the middle of the promontory to the median and in ferior part of the triangular ligament of the pubic symphysis. It is, on an average, about 4.5 inches long.
Inclination, Direction, Planes and Axes of the Pelvis.
The pelvis articulates with the spinal column in such a way as to form an angle, the vertex pointing forward. This is the promontory or sacro vertebral angle. The result is that the pelvis is not in the axis of the abdominal cavity, but is inclined in the upright position from above downwards, and from behind forwards. In this position, the sacro vertebral angle is about 3.9 inches above a horizontal line, which passes from before backwards, and touches the superior edge of the symphysis. The coccyx is .7 of an inch above a horizontal line which touches the inferior edge of the symphysis. From this it results that the inclination of the plane of the superior strait is from 59 to 60 degrees, that of the inferior strait from 10 to 11 degrees (Naegele-Danyau). (Fig. 13.)
(By the planes of the straits are meant, in obstetrics, those planes which pass through the diameter A P, and touch similar points on each half of the pelvis). (Tarsier and Chantreuil.) axis of the superior strait is a line perpendicular to the plane of that strait at its middle point. Prolonged upwards and forwards this line meets the umbilicus; backwards and downwards it ends at the anterior surface of the coccyx.
The axis of the inferior strait, when prolonged upward and forward, meets the axis of the superior strait near the centre of the cavity, and ends at the sacro-vertebral angle, or a little above it, on the anterior face of the last lumbar vertebrae. When prolonged backward and downviard it meets the perineum a little in front of the anus.
The axis of the cavity is a curved line passing through the centre of the true pelvis, and everywhere equally distant from the sacrum and the pubes.
The fcetus passes along this line in entering, crossing, and leaving the cavity. (Fig. 14.) In the upright position, such is the direction and such are the axes of the pelvis; they vary of course with different attitudes. In the complete dorsal decubitus, which is the usual position of women in labor, in France, the plane of the superior strait faces from above downwards and from in front backwards, and its axis points from behind forwards, and from above downward. The plane of the inferior strait is thus inclined from above downward and from in front backwards. The axis of this strait then points downward and almost directly forward.