233
CHAPTER SEVEN
Skeletal System
A portion of each pubis passes posteriorly and downward
to join an ischium. Between the bodies of these bones on either
side is a large opening, the
obturator foramen,
which is the
largest foramen in the skeleton. An obturator membrane cov-
ers and nearly closes this foramen (see F gs. 7.47 and 7.48).
Greater and Lesser Pelves
If a line were drawn along each side of the pelvis from the
sacral promontory downward and anteriorly to the upper
margin of the symphysis pubis, it would mark the
pelvic
brim
(linea terminalis). This margin separates the lower, or
lesser (true), pelvis from the upper, or greater (false), pelvis
(see F
g. 7.49).
The
greater pelvis
is bounded posteriorly by the lumbar
vertebrae, laterally by the fl
ared parts of the iliac bones, and
anteriorly by the abdominal wall. The greater pelvis helps
support the abdominal organs.
The
lesser pelvis
is bounded posteriorly by the sacrum and
coccyx and laterally and anteriorly by the lower ilium, ischium,
On the posterior border of the ilium is a
posterior supe-
rior iliac spine.
Below this spine is a deep indentation, the
greater sciatic notch,
through which a number of nerves and
blood vessels pass.
The
ischium
(is
ke-um), which forms the lowest portion
of the hip bone, is
L
-shaped, with its angle, the
ischial tuber-
osity,
pointing posteriorly and downward. This tuberosity has
a rough surface that provides attachments for ligaments and
lower limb muscles. It also supports the weight of the body
during sitting. Above the ischial tuberosity, near the junc-
tion of the ilium and ischium, is a sharp projection called the
ischial spine.
Like the sacral promontory, this spine, which
can be felt during a vaginal examination, is used as a guide
for determining pelvis size. The distance between the ischial
spines is the shortest diameter of the pelvic outlet.
The
pubis
(pu
bis) constitutes the anterior portion of the
hip bone. The two pubic bones come together at the midline
to form a joint called the
symphysis pubis
(sim
fı¯-sis pu
bis).
The angle these bones form below the symphysis is the
pubic
arch
(f g. 7.49)
.
Sacral promontory
Flared ilium
Pelvic brim
Symphysis pubis
Pubic arch
Pubic arch
(a)
Female pelvis
(b)
Male pelvis
Sacral promontory
Sacral curvature
FIGURE 7.49
The female pelvis is usually wider in all diameters and roomier than that of the male. (
a
) Female pelvis. (
b
) Male pelvis.
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