In walking, the human pelvis is thrown alternately, on each side, upwards, forwards, and sideways, as the leg on that side is lifted ; the trunk keeping its centre of gravity over the bearing leg by swaying regularly to that side, the pelvic hoop being at the same time drawn over the supporting leg by the powerful abducting muscles, the glutei.
On account of the greater width of the pelvis and trochanters in the female, the centre of gravity oscillates through a greater space, and takes longer time to pass over from one leg to the other, and hence the greater amount of undulation in their gait, especially when running.
Mechanism of the human pelvis in regard to parturition. — As a containing cavity, when completed by its muscular and fascial struc tures, the pelvis offers a basin-shaped struc ture with a somewhat triangular superior aperture, the sides of which are formed by the pose muscles, and the base by the pubes ; and with a moveable floor, formed by fasciae and the levator ani muscle, and perforated by the rectum and generative organs. Its walls are interrupted laterally by the sacro-sciatic and obturator foramina, which are filled by soft and yielding muscular and ligarnentous struc tures, and give way considerably to pressure from within, enlarging the pelvic diameters opposite to them. They afford, in common with the- superior and inferior openings, the outlets for the nerves and vessels passing from the lumbar and sacral plexuses and iliac trunks to the inferior extremities and peri neum. The inferior outlet also transmits the external communications of the pelvic viscera. These are, the bladder supported by the pubis ; the rectum, supported by the sacrum and coccyx ; and the internal organs of gene ration placed between them.
In the female these internal organs are more bulky than in the male, and consist of the uterus and its ovarian and vaginal appendages. That there is a relation between the greater size and the functions of these organs, and the greater extent of the female diameters, is evident from the consideration of their simul taneous development at the period of puberty, and corresponding increase afterwards.
This relative development of the pelvis seems to extend not only to sex, but to the varieties of mankind, either as an irrespective consequence of primitive formative type, or in regard to the adaptability of the foetal head to the pelvis in the processes of parturition. In
either case this adaptation is strikingly illus trated by the different pelvic forms prevailing in different races of men, which will be found, when considering that branch of our subject, to be markedly assimilated to the form of' the skull.
The pelvic bone, which is of the greatest importance in an obstetric point of view, entering as it does into the formation of both the pelvic brim, cavity, and inferior outlet, is the pubis ; and deformities of this bone produce the greatest obstacles to par turition. The sub-pubic arch is a peculiarity of the human species, it being only imper fectly developed in the lower animals ; and it has an important bearing upon human par turition, in being compensatory. for tbe great curve of the pelvic axis, and the change of direction forward of the inferior from the superior outlet. This curve is dependent upon, and follows the curve of, the sacro-coccygeal column, which, being more acute and di rected more forwards below, affords the chief support in the earlier months of pregnancy to the uterus and its contents, which lie in the axis of the superior outlet, which axis, as we have seen, impinges below upon the coccygeal bone. Now, the coccyx, being principally held up in its forward position by the elastic sciatic ligaments and by the resilient ischio coccygeus and levator ani muscles, affords a resisting support which is at once powerful and yielding, and acts like an elastic spring in supporting the uterus and its delicate contents under the effects of accidental shock. The female coccyx is much more mobile at every age than the male, and when ankylosed to the sacrum it is less favourable both to pregnancy and Jabour. In the more advanced stages of pregnancy, however, the uterus rises into the abdominal cavity, and rests mainly upon the smooth concave surfaces of the pubes and the soft muscular margins of the pelvic brim, being embraced and supported above and in front by the abdominal muscles. To allow the great expansion of the uterine contents, the broad ventral notch and expanding iliac wings are dispositions of great significance.