THE PELVIS.
The pelvis, considered as a whole, is divisible into a false and true pelvis; the former is the expanded portion, bounded on each side by the ossa ilii, and separated from the true pelvis by the linea iliopectinea. The true pelvis is all that portion which is situated beneath the iliopectinea. This line forms the margin or brim of the true pelvis, while the included area is called the inlet. The form of the inlet is heart-shaped, obtusely pointed in front at the symphysis pubis, expanded on each side, and encroached upon behind by a projection of the upper part of the sacrum, which is named the promontory. The cavity is somewhat encroached upon at each side by a smooth quadrangular plane of bone, corresponding with the internal surface of the acetabulum, and leading to the spine of the ischium. In front are two fossæ around the obturator foramina, for lodging the obturator internus muscle, at each side. The inferior termination of the pelvis is very irregular, and is termed the outlet. It is bounded in front by the convergence of the rami of the ischium and pubes, which constitute the arch of the pubes; on each side by the tuberosity of the ischium, and by two irregular fissures formed by the greater and lesser sacro-ischiatic notches; and behind by the lateral borders of the sacrum, and by the coccyx.
A FEMALE PELVIS.
1. The last lumbar vertebra. 2, 2. The intervertebral substance connecting the last lumbar vertebra with the fourth and sacrum. 3. The promontory of the sacrum. 4. The anterior surface of the sacrum, on which its transverse lines and foramina are seen. 5. The tip of the coccyx. 6, 6. The iliac fossæ, forming the lateral boundaries of the false pelvis. 7. The anterior superior spinous process of the ilium; left side. 8. The anterior inferior spinous process. 9. The acetabulum. a. The notch of the acetabulum. b. The body of the ischium. c. Its tuberosity. d. The spine of the ischium seen through the obturator foramen. e. The os pubis. f. The symphysis pubis. g. The arch of the pubes. h. The angle of the os pubis. i. The spine of the pubes; the prominent ridge between h and i is the crest of the pubes. k, k. The pectineal line of the pubes. l, l. The ilio-pectineal line; m, m. The prolongation of this line to the promontory of the sacrum. The line represented by h, i, k, k, l, l, and m, m, is the brim of the true pelvis. n. The ilio-pectineal eminence. o. The smooth surface which supports the femoral vessels. p, p. The great sacro-ischiatic notch.
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.