HUMAN BIOLOGY

the SKeLetaL SyStem 95

What are the parts of the appendicular

skeleton?

• The appendicular skeleton includes bones of the limbs, a pectoral
  girdle at the shoulders, and a pelvic girdle at the hips.


• The thighbone (femur) is the largest bone in the body and also
  one of the strongest.


• The wrists and hands and the ankles and feet have corresponding
  sets of bones known respectively as carpals and metacarpals and
  tarsals and metatarsals.

taKe-hoMe Message

In order to take up calcium from the diet—essential

for maintaining strong bones—the body requires an

adequate supply of vitamin D. Section 4.9 noted that

cells in skin make this “sunshine vitamin” when the

skin is exposed to sunlight, and animal-based foods

such as egg yolks and fortified milk also contain it.

Studies suggest that in the United States, more than

40 percent of adults are deficient in vitamin D, and

more than the health of their bones may be at risk.

On your own or with a classmate, investigate this

topic. What are common causes of vitamin D

deficiency? Who is most susceptible? Beyond its

role in bone health, what possible other health

benefits of vitamin D are researchers exploring?

Think ouTside The Book

pelvis

sacrum

femur

patella

tibia

fibula

metatarsals

phalanges

tarsals

pubic

symphysis

Figure 5.8 The pelvic girdle, the leg, and the
foot form the lower part of the appendicular
skeleton. (© Cengage Learning)

body’s entire weight when we are standing. The pelvic
girdle (Figure 5.8) is much more massive than the com-
bined pectoral girdles, and it is attached to the axial skel-
eton by extremely strong ligaments. It forms an open basin:
A pair of coxal bones attach to the lower spine (sacrum) in
back, then curve forward and meet at the pubic arch. (“Hip
bones” are actually the upper iliac regions of the coxal
bones.) This combined structure is the pelvis. In females
the pelvis is broader than in males, and it shows other
structural differences that are evolutionary adaptations for
childbearing. A forensic scientist or paleontologist exam-
ining skeletal remains can easily establish the sex of the
deceased if a pelvis is present.
The legs contain the body’s largest bones. In terms of
length, the thighbone, or femur, ranks number one. It is
also extremely strong. When you run or jump, your femurs
routinely withstand stresses of several tons per square inch
(aided by contracting leg muscles). The femur’s ball-like
upper end fits snugly into a deep socket in the coxal (hip)
bone. The other end connects with one of the bones of the
lower leg, the thick, load-bearing tibia on the inner (big toe)
side. A slender fibula parallels the tibia on the outer (little
toe) side. The tibia is your shin bone. A triangular kneecap,
the patella, helps protect the knee joint. As Susanna’s story
in the chapter introduction noted, however, athletes often
damage their knees.

The ankle and foot bones correspond closely to those

of the wrist and hand. Tars al bones make up the ankle and

heel, and the foot contains five long bones, the metatarsals.

The largest metatarsal, leading to the big toe, is thicker and

stronger than the others to support a great deal of body

weight. Like fingers, the toes contain phalanges.

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