The pelvis is composed of the two innominate bones, the sacrum, and the coccyx. It is constructed with a view to connecting the lower extremities with the trunk, to support the weight of the trunk and to promote locomotion, to act as a receptacle and protector of the pelvic viscera and to fulfil the function of parturition.

In infancy locomotion and parturition are in abeyance, hence the pelvis is undeveloped, the bladder and uterus are almost entirely in the abdomen, and the rectum is almost straight. As the child begins to use its lower limbs for locomotion the pelvis increases progressively with the growth of the lower extremities, and with the advent of puberty its development is completed. The structure of the pelvis in relation to the function of locomotion will be considered later in connection with the pelvic girdle and lower extremity; here we will consider it in relation to the pelvic viscera and their functions.

Fig. 438.   The male pelvis, front view.

Fig. 438. - The male pelvis, front view.

Fig. 439.   The male pelvis, back view.

Fig. 439. - The male pelvis, back view.

That part of the pelvis above the iliopectineal line has been called the false pelvis, while that below is the true pelvis. The inlet of the pelvis is formed anteriorly by the crest and spine of the pubes, the iliopectineal lines on the sides, and the base of the sacrum with its promontory posteriorly. The outlet is formed by the pubic arch anteriorly with the symphysis in the middle, the rami of the pubes and ischia on the sides, and the great sacrosciatic ligaments and coccyx posteriorly. The viscera above the inlet are abdominal, those below are pelvic. When the body is vertical the inlet forms an angle of 60 degrees with the horizon, and the promontory of the sacrum is 9 to 10 cm. (3 1/2 to 4 in.) above the upper edge of the symphysis.

The male pelvis is fashioned preeminently for locomotion: man is the more active, hence the pelvis in the male is both heavier and rougher; the false pelvis is broad and shallow, while the true pelvis is deep and narrow and its capacity is less. The inlet is heart-shaped, the tuberosities closer together, and the pubic arch narrower. The obturator foramen is oval (see Figs. 438 and 439).

The Female Pelvis

In addition to the functions of the pelvis and its contents common to the two sexes the female has the function of child-bearing. To fulfil this function the female pelvis is different from that of the male. Owing to the less muscular life of the female the pelvis is smoother, its bony prominences not being so marked (see Fig. 440). The extreme width of the pelvis does not differ much in the two sexes, some authorities giving them as of equal size and some stating that the female is slightly narrower. The main difference consists in the modification of the true pelvis to favor the birth of the child. Its cavity is larger and shallower than in the male. The symphysis pubis is shallower and the sacrum is shorter and less curved. The acetabula are set wider apart as are also the tuberosities of the ischia. This causes the thyroid foramen to be triangular in the female while it has a long diameter parallel with the long axis of the body in the male. It also causes the subpubic angle to be greater in the female, forming an angle of about 90 degrees as against 65 degrees to 70 degrees in the male. The inlet of the female pelvis is more oval and not so heart-shaped as in the male. The cavity is largest at a level between the second and third sacral vertebrae posteriorly and the middle of the symphysis anteriorly. It is smallest between the sacrococcygeal articulation behind and the lower third of the symphysis in front, and the spines of the ischia on the sides. There are three diameters of the pelvis used in obstetrics; an anteroposterior or conjugate, an oblique, and a transverse. The size of these varies according to different authors.

Superior view, inlet.

Superior view, inlet.

Inferior view, outlet.

Inferior view, outlet.

Fig. 440. - The female pelvis, superior and inferior views, with the diameters of the inlet and outlet.

Hirst ("Obstetrics," p. 22), gives the following:

Anteroposterior.................

11 cm

(4.33 in).

inlet.

Transverse ..................

13.5 cm

(5.32 in).

Oblique ....................

12.75 cm

(5.02 in).

Anteroposterior..............

11 cm

.(4.33 in).

outlet.

Transverse...................

11 cm

(4.33 in).

Dwierit, in "Piersol's Anatomy, gives them as follows:

Male.

FEM ALE.

Inlet. cm. (in).

Cavity, cm. (in).

Outlet. cm. (in).

Inlet. cm. (in).

Cavity.

cm. (in).

Outlet. cm. (in).

Anteroposterior

10.25 (4)

11.5 (4 1/2)

8.25 (3 1/4)

10.25 (4)

12.75 (5)

11.5 (4 1/2)

Transverse . .

12.75(5)

120 (4 3/4)

9.00 (3 1/2)

13.25 (5 1/4)

12.75 (5)

12.0 (4 3/4)

Oblique . . .

12.00 (4 3/4)

11.5 (4 1/2)

10.25 (4)

12.75 (5)

13.25 (5 1/4)

11.5 (4 1/2)

The cavity of the pelvis is narrowed somewhat by the soft parts on its sides. The blood-vessels, nerves, and obturator muscles are placed laterally and so usually escape injury. In pregnancy the venous flow is most often interfered with. The first evidence of this is the dusky hue of the vagina; hemorrhoids and varicosities of the veins of the external genitals and lower extremities are common. The rectum and bladder being placed more anteroposteriorly, interference with their functions is frequent. The peculiarities of the female pelvis are evident from birth and are not solely acquired with age.