The pelvis is placed obliquely with regard to the trunk of the body, so that the inner surface of the ossa pubis is directed upward, and would support the superincumbent weight of the viscera. The base of the sacrum rises nearly four inches above the level of the upper border of the symphysis pubis and the apex of the coccyx, somewhat more than half an inch above its lower border. If a line were carried through the central axis of the inlet, it would impinge by one extremity against the umbilicus, and by the other against the middle of the coccyx. The axis of the inlet is therefore directed downward and backward, while that of the outlet points downward and forward, and corresponds with a line drawn from the upper part of the sacrum, through the center of the outlet. The axis of the cavity represents a curve, which corresponds very nearly with the curve of the sacrum, the extremities being indicated by the central points of the inlet and outlet. A knowledge of the direction of these axes is most important to the surgeon, as indicating the line in which instruments should be used in operations upon the viscera of the pelvis, and the direction of force in the removal of calculi from the bladder; and to the accoucher, as explaining the course taken by the fetus during parturition.
There are certain striking differences between the male and female pelvis. In the male the bones are thicker, stronger, and more solid, and the cavity deeper and narrower. In the female the bones are lighter and more delicate, the iliac fossæ are large, and the ilia expanded; the inlet, the outlet, and the cavity, are large, and the acetabula farther removed from each other; the cavity is shallow, the tuberosities widely separated, the obturator foramina triangular, and the span of the pubic arch greater. The precise diameter of the inlet and outlet, and the depth of the cavity, are important considerations to the accoucher.
The contents of the pelvis are, the bladder, vagina, uterus with its appendages, and the rectum. Some portion of the small intestines also occupies the upper part of its cavity.
THE BLADDER.
This is in relation with the os pubis in front, with the uterus behind, from which it is usually separated by a convolution of small intestine, and with the neck of the uterus and vagina beneath. The form of the female bladder corresponds with that of the pelvis, being broad from side to side, and often bulging more on one side than on the other. This is particularly evident after frequent parturition. The coats of the bladder are the same as those of the male.
THE URETHRA.
This is about an inch and a half in length, and is lodged in the upper wall of the vagina in its course downward and forward, beneath the arch of the os pubis, to the meatus urinarius. It is lined by mucous membrane, which is disposed in longitudinal folds, and is continuous internally with that of the bladder, and externally with the vulva; the mucous membrane is surrounded by a proper coat of elastic tissue, to which the muscular fibres of the detrusor urinæ are attached. It is to the elastic tissue that is due the remarkable dilatability of the female urethra, and its speedy return to its original diameter. The meatus is encircled by a ring of fibrous tissue, which prevents it from distending with the same facility as the rest of the canal; hence it is sometimes advantageous in performing this operation, to divide the margin of the meatus slightly with the knife.
THE VAGINA.
This is a membranous canal, leading from the vulva to the uterus, and corresponding in direction with the axis of the outlet of the pelvis. It is constricted at its commencement, but near the uterus becomes dilated, and is closed by the contact of the anterior with the posterior wall. Its length is variable; but it is always longer upon the posterior than upon the anterior wall, the former being usually about five or six inches in length, and the latter four or five. It is attached to the cervix of the uterus, which latter projects into the upper extremity of the canal.
In structure, the vagina is composed of a mucous lining, a layer of erectile tissue, and an external tunic of contractile fibrous tissue, resembling the dartos of the scrotum. The upper fourth of the posterior wall is covered, on its pelvic surface, by the peritoneum, while in front the peritoneum is reflected from the upper part of the cervix of the uterus to the posterior surface of the bladder. On each side it gives attachment, superiorly to the broad ligaments of the uterus, and inferiorly to the pelvic fascia and levatores ani.