238
UNIT TWO
with a loss of about 1/16 of an inch a year. In the later years,
compression fractures in the vertebrae may contribute signiF
-
cantly to loss of height
(f g. 7.55)
. Overall, as calcium levels
fall and bone material gradually vanishes, the skeleton loses
strength, and the bones become brittle and increasingly prone
to fracture. However, the continued ability of fractures to heal
reveals that the bone tissue is still alive and functional.
Components of the skeletal system and individual bones
change to different degrees and at different rates over a life-
time. Gradually, osteoclasts come to outnumber osteoblasts,
which means that bone is eaten away in the remodeling pro-
cess at a faster rate than it is replaced—resulting in more
spaces in bones. The bone thins, its strength waning. Bone
matrix changes, with the ratio of mineral to protein increasing,
making bones more brittle and prone to fracture. Beginning in
(F g. 7.54). The heads at the distal ends of these bones form
the ball of the foot. The tarsals and metatarsals are bound by
ligaments, forming the arches of the foot. A longitudinal arch
extends from the heel to the toe, and a transverse arch stretches
across the foot. These arches provide a stable, springy base for
the body. Sometimes, however, the tissues that bind the meta-
tarsals weaken, producing fallen arches, or fl at feet.
The
phalanges
of the toes are shorter but otherwise
similar to those of the F
ngers and align and articulate with
the metatarsals. Each toe has three phalanges—a proximal,
a middle, and a distal phalanx—except the great toe, which
has only two because it lacks the middle phalanx (F
g. 7.54).
Table 7.11
summarizes the bones of the pelvic girdle and
lower limbs.
An infant with two casts on her feet is probably being treated for club-
foot, a common birth defect in which the foot twists out of its normal
position, turning in, out, up, down, or some combination of these
directions. Clubfoot probably results from arrested development dur-
ing fetal existence, but the precise cause is not known. Clubfoot can
almost always be corrected with special shoes, or surgery, followed
by several months in casts to hold the feet in the correct position.
PRACTICE
37
Locate and name each of the bones of the lower limb.
38
Explain how the bones of the lower limb articulate with one
another.
39
Describe how the foot is adapted to support the body.
7.13
LIFE-SPAN CHANGES
Aging-associated changes in the skeletal system are appar-
ent at the cellular and whole-body levels. Most obvious is the
incremental decrease in height that begins at about age thirty,
FIGURE 7.55
The bones change to diF
erent degrees and at
diF
erent rates over a lifetime.
TABLE
7.11
|
Bones of the Pelvic Girdle and Lower Limbs
Name and Number
Location
Special Features
Hip bone (2)
Hip, articulating with the other hip bone
anteriorly and with the sacrum posteriorly
Ilium, iliac crest, anterior superior iliac spine, ischium, ischial tuberosity, ischial spine,
obturator foramen, acetabulum, pubis
±emur (2)
Thigh, between hip and knee
Head, fovea capitis, neck, greater trochanter, lesser trochanter, linea aspera, lateral condyle,
medial condyle, gluteal tuberosity, intercondylar fossa
Patella (2)
Anterior surface of knee
A ²
at sesamoid bone located within a tendon
Tibia (2)
Medial side of leg, between knee and ankle
Medial condyle, lateral condyle, tibial tuberosity, anterior crest, medial malleolus,
intercondylar eminence
±ibula (2)
Lateral side of leg, between knee and ankle
Head, lateral malleolus
Tarsal (14)
Ankle
±reely movable talus that articulates with leg bones; calcaneus that forms the base of the
heel; ³
ve other tarsal bones bound ³
rmly together
Metatarsal (10)
Instep
One in line with each toe, bound by ligaments to form arches
Phalanx (28)
Toe
Three in each toe, two in great toe
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