Chapter 7
The Skeleton
245
7
All bones, you will recall, lose mass with age. Cranial bones
lose less mass than most, but changes in facial contours with age
are common. As the bony tissue of the jaws declines, the jaws
look small and childlike once again. If the elderly person loses
his or her teeth, this loss of bone from the jaws is accelerated,
because the alveolar region bone is resorbed. As bones become
more porous, they are more likely to fracture, especially the ver-
tebrae and the neck of the femur.
Check Your Understanding
36.
What developmental events result in a dramatic enlargement
of the facial skeleton between the ages of 6 and 13?
37.
Under what conditions does the lumbar curvature of the
spine develop?
For answers, see Appendix H.
Our skeleton is a marvelous substructure, to be sure, but it
is much more than that. It is a protector and supporter of other
body systems, and without it (and the joints considered in Chap-
ter 8), our muscles would be almost useless. Te homeostatic re-
lationships between the skeletal system and other body systems
are illustrated in
System Connections
in Chapter 6 (p. 195).
preschool years, lordosis is oFen present, but this is usually rec-
tified as the abdominal muscles become stronger and the pelvis
tilts forward. Te thorax grows wider, but a true “military pos-
ture” (head erect, shoulders back, abdomen in, and chest out)
does not develop until adolescence.
Homeostatic Imbalance
7.10
Te appendicular skeleton can also suffer from a number of
congenital abnormalities. One that occurs in just over 1% of
infants and is quite severe is
dysplasia of the hip
(dis-pla
9
ze-ah;
“bad formation”). Te acetabulum forms incompletely or the
ligaments of the hip joint are loose, so the head of the femur
slips out of its socket. Early treatment (a splint or harness to
hold the femur in place or surgery to tighten hip ligaments) is
essential to prevent permanent crippling.
During youth, growth of the skeleton not only increases over-
all body height but also changes body proportions
(Figure 7.39)
.
At birth, the head and trunk are approximately 1½ times as long
as the lower limbs. Te lower limbs grow more rapidly than the
trunk from this time on, and by the age of 10, the head and trunk
are approximately the same height as the lower limbs, a condi-
tion that persists thereaFer. During puberty, the female pelvis
broadens in preparation for childbearing, and the entire male
skeleton becomes more robust. Once adult height is reached, a
healthy skeleton changes very little until late middle age.
Old age affects many parts of the skeleton, especially the
spine. As the discs become thinner, less hydrated, and less
elastic, the risk of disc herniation increases. By 55 years, a
loss of several centimeters in stature is common. ±urther
shortening can be produced by osteoporosis of the spine or
by kyphosis (called “dowager’s hump” in the elderly; see ±ig-
ure 7.17b). What was done during youth may be undone in
old age as the vertebral column gradually resumes its initial
arc shape.
Te thorax becomes more rigid with age, largely because the
costal cartilages ossify. Tis loss of rib cage elasticity causes shal-
low breathing, which leads to less efficient gas exchange.
Figure 7.38
The C-shaped spine of a newborn infant.
(a)
(b)
Human newborn
Newborn
2 yrs
5 yrs
15 yrs
Adult
Human adult
Figure 7.39
Different growth rates of body parts determine
body proportions.
(a)
Differential growth transforms the rounded,
foreshortened skull of a newborn to the sloping skull of an adult.
(b)
During growth of a human, the arms and legs grow faster than
the head and trunk, as seen in this conceptualization of different-
aged individuals all drawn at the same height.
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