HUMAN BIOLOGY

the SKeLetaL SyStem 93

What are the parts of the aXial skeleton?

• The axial skeleton consists of bones that make up the body’s
  vertical axis.


• The axial skeleton includes the skull and facial bones, the
  vertebral column (backbone), and the ribs and sternum.


• Portions of the axial skeleton, such as the skull and rib cage,
  protect key soft internal organs such as the brain and heart.


• Intervertebral disks absorb shocks and serve as flex points.

taKe-hoMe Message

frontal

sinus

sphenoid

sinus

ethmoid

sinus

maxillary

sinus

the ribs and sternum support and help

protect internal organs

In addition to protecting the spinal

cord, absorbing shocks, and pro-

viding flexibility, the vertebral col-

umn also serves as an attachment

point for twelve pairs of ribs, which

in turn serve as a scaffolding for

the thoracic cavity, the body cavity

of the upper torso. The upper ribs

also attach to the paddle-shaped

sternum (see Figure 5.4B). As you

will read in later chapters, this rib

cage helps protect the lungs, heart,

and other internal organs and is

vitally important in breathing.

Palatine bones make up part of the floor and side wall
of the nasal cavity. (Extensions of these bones, together
with the maxillary bones, form the back of the hard palate,
the “roof” of your mouth.) A vomer bone forms part of the
nasal septum, a thin “wall” that divides the nasal cavity
into two sections.

the vertebral column is the backbone

The flexible, curved vertebral column—your backbone or
spine—runs from the base of the skull to the hip bones
(pelvic girdle). This arrangement transmits the weight of a
person’s torso to the lower limbs. As a result, people who
gain too much weight may develop problems with their
knees and ankles because those joints are not designed
to bear such a heavy load. The vertebrae are stacked and
have bony projections that form a protected channel for the
delicate spinal cord. As sketched in Figure 5.6, humans have
seven cervical vertebrae in the neck, twelve thoracic vertebrae
in the chest area, and five lumbar vertebrae in the lower
back. During the course of human evolution, five other ver-
tebrae have become fused to form the sacrum, and several
more have become fused to form the coccyx, or “tailbone.”
Counting these, there are thirty-three vertebrae in all.
Roughly a quarter of your spine’s length consists of
intervertebral disks—compressible pads of fibrocarti-
lage sandwiched between vertebrae. The disks serve as
shock absorbers and flex points. They are thickest between
cervical vertebrae and between lumbar vertebrae. Severe
or rapid shocks, as well as changes due to aging, can cause
a disk to herniate or “slip.” If the slipped disk ruptures, its
jellylike core may squeeze out, making matters worse. And
if the changes compress neighboring nerves or the spinal
cord, the result can be excruciating pain and the loss of
mobility that often comes with pain. Depending on the
situation, treatment can range from bed rest and use of
painkilling drugs to surgery.

c

Figure 5.6 Vertebrae and intervertebral disks make

up the vertebral column (backbone). The cranium

balances on the column’s top vertebra.

sacrum

(5 fused)

cervical

vertebrae

(7)

thoracic

vertebrae

(12)

lumbar

vertebrae

(5)

intervertebral

disks

coccyx

(4 fused)

5

4

3

2

1

12

11

10

9

8

7

6

5

4

3

1

2

(^67)
(^45)
3
21
© Cengage Learning
© Cengage Learning
intervertebral disk Pad
of fibrocartilage between
vertebrae.
rib cage Portion of the axial
skeleton formed by the ribs
and sternum, which sup‑
ports and protects the heart,
lungs, and other organs.
sternum Breastbone.
vertebrae Irregular bones
that form the spinal column.
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