The pelvis is composed of four bones: the two coxal or hip-bones, the sacrum, and the coccyx. The hip-bones form the lateral and anterior boundaries, meeting each other in front to form the pubic symphysis [symphysis ossium pubis]; posteriorly they are separated by the sacrum. The interior of the pelvis is divided into the major and minor pelvic cavity.
The major (or false) pelvis
The major (or false) pelvis is that part of the cavity which lies above the terminal (iliopectineal) lines and between the iliac fossa. This part belongs really to the abdomen, and is in relation with the hypogastric and iliac regions.
The minor (or true) pelvis is situated below the terminal (ilio-pectineal) lines. The upper circumference, known as the superior aperture (inlet or brim) of the pelvis, is bounded anteriorly by the tubercle and pecten of the pubis on each side, posteriorly by the anterior margin of the base of the sacrum, and laterally by the terminal lines. The inlet in normal pelvis is heart-shaped, being obtusely pointed in front; posteriorly it is encroached upon by the promontory of the sacrum. It has three principal diameters; of these, the antero-posterior, called the conjugate diameter [conjugata], is measured from the sacro-vertebral angle to the symphysis. The transverse diameter represents the greatest width of the pelvic cavity. The oblique diameter is measured from the sacro-iliac synchondrosis of one side to the ilio-pectineal eminence of the other.
The cavity of the minor (true) pelvis
The cavity of the minor (true) pelvis is bounded in front by the pubes, behind by the sacrum and coccyx, and laterally by a smooth wall of bone formed in part by the ilium and in part by the ischium. The cavity is shallow in front, where it is formed by the pubes, and is deepest posteriorly.
The inferior aperture, or outlet, of the minor pelvis is very irregular, and encroached upon by three bony processes: the posterior process is the coccyx, and the two lateral processes are the ischial tuberosities. They separate three notches. The anterior notch is the pubic arch, and is bounded on each side by the conjoined rami of the pubes and ischium. Each of the two remaining gaps, bounded by the ischium anteriorly, the sacrum and coccyx posteriorly, and the ilium above, corresponds to the greater and lesser sciatic notches. These are converted into foramina by the sacro-tuberous (great sacro-sciatic) and sacro-spinous (small sacro-sciatic) ligaments.
The position of the pelvis. - In the erect position of the skeleton the plane of the pelvic inlet forms an angle with the horizontal plane, which varies in individuals from 50° to 60°.
The base of the sacrum in an average pelvis lies nearly ten centimeters (four inches) above the upper margin of the symphysis pubis.
The axis of the pelvis. - This is an imaginary curved line drawn through the minor pelvis at right angles to the planes of the inlet, cavity, "and outlet through then central points.
As the posterior wall, formed by sacrum and coccyx, is nearly five inches long and concave, and the anterior wall at the symphysis pubis one, one and a half to two inches long, it follows that the axis must be curved.
The average measurements of the diameters of the minor pelvis
The average measurements of the diameters of the minor pelvis in the three planes are given below:
Conjugate or antero-posterior.
- Inlet 4 ¼ inches (10.6 cm.)
- Cavity 4 ¾ inches (11.8 cm.)
- Outlet 3 ¾ inches (9.0 cm.)
- Inlet 5 inches (12.5 cm.)
- Cavity 5 ¼ inches (13.0 cm.)
- Outlet 4 ½ inches (11.2 cm.)
- Inlet 5 ¼ inches (13.0 cm.)
- Cavity 4 ¾ inches (11.8 cm.)
- Outlet 4 ¼ inches (10.6 cm.)
There is, however, a difference between the sexes, the diameters of the male pelvis in general averaging slightly less, and those of the female slightly greater than the figures above given.
Differences according to sex
There is a marked difference in the size and form of the male and female pelvis, the peculiarities of the latter being necessary to qualify it for its functions in parturition. The various points of divergence may be tabulated as follows:
- Bones heavier and rougher.
- Ilia less vertical.
- Iliac fossae deeper.
- Major pelvis relatively wider.
- Minor pelvis deeper.
- Minor pelvis narrower.
- Superior aperture move heart-shaped.
- Symphysis deeper.
- Tuberosities of ischia inflexed.
- Pubic angle narrow and pointed.
- Margins of ischio-pubic rami more everted.
- Obturator foramen oval.
- Sacrum narrower and more curved.
- Capacity of minor pelvis less.
- Bones more slender.
- Ilia more vertical.
- Iliac fossae shallower.
- Major pelvis relatively narrower.
- Minor pelvis shallower.
- Minor pelvis wider.
- Superior aperture more oval.
- Symphysis shallower.
- Tuberosities of ischia everted.
- Pubic arch wider and more rounded.
- Margins of ischio-pubic rami less everted.
- Obturator foramen triangular.
- Sacrum wider and less curved.
- Capacity of minor pelvis greater.
The sexual characters of the pelvis as shown by A. Thomson are manifest as early as the
fourth month of fetal life.
Quite recently attention has been drawn by D. Derry to some special points in which the OS coxal differ in the two sexes, and two figures are shown here in which one of these points is clearly brought out. It will be seen that the great sciatic notch is larger in the female, and that the sacrum projects less forward at its apex. Moreover, the facies auricularis is smaller whilst below and in front of this surface, the sulcus preauricularis, a depression for the attachment of the ligamenta sacroiliaca anteriora, is usually more pronounced.
In comparison with the pelvis of lower animals, which, speaking generally, are elongated and narrow, the human pelvis is characterized by its breadth, shallowness, and great capacity. Differences are also to be recognized in the form of the pelvis in the various races of mankind, the most important being the relation of the antero-posterior to the transverse diameter, measured at the inlet. This is expressed by the pelvic index = 100 x conjugate diameter /transverse diameter transverse diameter In the average European male the index is about 80; in the lower races of mankind, 90 to 95. Pelvis with an index below 90 are platypellic, from 90 to 95 are mesatipellic, and above 95 dolichopellic. (Sir William Turner.)