238
UNIT 2
Covering, Support, and Movement of the Body
7
The Lower Limb
Identify the lower limb bones and their important markings.
Te lower limbs carry the entire weight of the erect body and are
subjected to exceptional forces when we jump or run. Tus, it is
not surprising that the bones of the lower limbs are thicker and
stronger than comparable bones of the upper limbs. Te three
segments of each lower limb are the thigh, the leg, and the foot
(see
Table 7.5
on p. 243).
Thigh
Te
femur
(fe
9
mur; “thigh”), the single bone of the thigh
(Figure 7.32)
, is the largest, longest, strongest bone in the
body. Its durable structure reflects the fact that the stress on the
femur during vigorous jumping can reach 280 kg/cm
2
(about
2 tons per square inch)! Te femur is clothed by bulky muscles
that prevent us from palpating its course down the length of the
thigh. Its length is roughly one-quarter of a person’s height.
Proximally, the femur articulates with the hip bone and then
courses medially as it descends toward the knee. Tis arrange-
ment allows the knee joints to be closer to the body’s center of
gravity and provides for better balance. Te medial course of
the two femurs is more pronounced in women because of their
wider pelvis, a situation that may contribute to the greater inci-
dence of knee problems in female athletes.
Te ball-like
head
of the femur has a small central pit called the
fovea capitis
(fo
9
ve-ah ka˘
9
pĭ-tis; “pit of the head”). Te short
liga-
ment of the head of the femur
runs from this pit to the acetabulum,
where it helps secure the femur. Te head is carried on a
neck
that
angles
laterally
to join the shaF. Tis arrangement reflects the fact
that the femur articulates with the lateral aspect (rather than the in-
ferior region) of the pelvis. Te neck is the weakest part of the femur
and is oFen fractured, an injury commonly called a broken hip.
At the junction of the shaF and neck are the lateral
greater
trochanter
(tro-kan
9
ter) and posteromedial
lesser trochanter
.
Tese projections serve as sites of attachment for thigh and
buttock muscles. Te two trochanters are connected by the
in-
tertrochanteric line
anteriorly and by the prominent
intertro-
chanteric crest
posteriorly.
Inferior to the intertrochanteric crest on the posterior shaF is
the
gluteal tuberosity
, which blends into a long vertical ridge,
the
linea aspera
(lin
9
e-ah as
9
per-ah; “rough line”), inferiorly.
Distally, the linea aspera diverges, forming the
medial
and
lateral supracondylar lines
. All of these markings are sites of
muscle attachment. Except for the linea aspera, the femur shaF
is smooth and rounded.
Distally, the femur broadens and ends in the wheel-like
lat-
eral
and
medial condyles
, which articulate with the tibia of the
leg. Te
medial
and
lateral epicondyles
(sites of muscle attach-
ment) flank the condyles superiorly. On the superior part of
the medial epicondyle is a bump, the
adductor tubercle
. Te
smooth
patellar surface
, between the condyles on the ante-
rior femoral surface, articulates with the
patella
(pah-tel
9
ah),
or kneecap (see ±igure 7.32 and ²able 7.5). Between the con-
dyles on the posterior aspect of the femur is the deep, U-shaped
Pelvic Structure and Childbearing
Te deep, basinlike structure formed by the hip bones, sacrum,
and coccyx is called the
pelvis
or the
bony pelvis
. Te differences
between the male and female pelves are striking. Te female
pelvis is modified for childbearing: It tends to be wider, shal-
lower, lighter, and rounder than that of a male. Te female pelvis
not only accommodates a growing fetus, but it must be large
enough to allow the infant’s relatively large head to exit at birth.
Te major differences between the typical male and female pel-
ves are summarized and illustrated in ²able 7.4.
Te pelvis is said to consist of a false (greater) pelvis and
a true (lesser) pelvis separated by the
pelvic brim
, a con-
tinuous oval ridge that runs from the pubic crest through the
arcuate line and sacral promontory (±igure 7.30). Te
false
pelvis
, that portion superior to the pelvic brim, is bounded
by the alae of the ilia laterally and the lumbar vertebrae pos-
teriorly. Te false pelvis is really part of the abdomen and
helps support the abdominal viscera. It does not restrict
childbirth in any way.
Te
true pelvis
is the region inferior to the pelvic brim that
is almost entirely surrounded by bone. It forms a deep bowl
containing the pelvic organs. Its dimensions, particularly those
of its
inlet
and
outlet
, are critical to the uncomplicated delivery
of a baby, and they are carefully measured by an obstetrician.
Te
pelvic inlet
is
the pelvic brim, and its widest dimension
is from right to leF along the frontal plane. As labor begins, an
infant’s head typically enters the inlet with its forehead facing
one ilium and its occiput facing the other. A sacral promontory
that is particularly large can impair the infant’s entry into the
true pelvis.
Te
pelvic outlet
, illustrated in the photos at the bottom
of ²able 7.4, is the inferior margin of the true pelvis. It is
bounded anteriorly by the pubic arch, laterally by the ischia,
and posteriorly by the sacrum and coccyx. Both the coccyx
and the ischial spines protrude into the outlet opening, so a
sharply angled coccyx or unusually large spines can interfere
with delivery. Te largest dimension of the outlet is the antero-
posterior diameter.
Generally, aFer the baby’s head passes through the inlet, it
rotates so that the forehead faces posteriorly and the occiput
anteriorly, and this is the usual position of the baby’s head as
it leaves the mother’s body (see ±igure 28.18
2
). Tus, during
birth, the infant’s head makes a quarter turn to follow the widest
dimensions of the true pelvis.
Check Your Understanding
28.
The ilium and pubis help to form the os coxae. What other
bone is involved in forming the os coxae?
29.
The pelvic girdle is a heavy, strong girdle. How does its
structure reflect its function?
30.
Which of the following terms or phrases refer to the female
pelvis? Wider, shorter sacrum; cavity narrow and deep;
narrow heart-shaped inlet; more movable coccyx; long ischial
spines.
For answers, see Appendix H.
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