224
UNIT 2
Covering, Support, and Movement of the Body
7
Te characteristics of the regional vertebrae are summarized
in ±able 7.2.
Check Your Understanding
16.
What is the normal number of cervical vertebrae? Of thoracic
vertebrae?
17.
How would a complete fracture of the dens affect the
mobility of the vertebral column?
18.
How can you distinguish a lumbar vertebra from a thoracic
vertebra?
For answers, see Appendix H.
The Thoracic Cage
Name and describe the bones of the thoracic cage (bony
thorax).
Differentiate true from false ribs.
Anatomically, the thorax is the chest, and its bony underpinnings
are called the
thoracic cage
or
bony thorax
. Elements of the tho-
racic cage include the thoracic vertebrae posteriorly, the ribs lat-
erally, and the sternum and costal cartilages anteriorly. Te costal
cartilages secure the ribs to the sternum
(Figure 7.23a)
.
Roughly cone shaped with its broad dimension positioned in-
feriorly, the bony thorax forms a protective cage around the vital
organs of the thoracic cavity (heart, lungs, and great blood ves-
sels), supports the shoulder girdles and upper limbs, and provides
landmark. Four ridges, the
transverse ridges
, cross its concave
anterior aspect, marking the lines of fusion of the sacral ver-
tebrae. Te
anterior sacral foramina
lie at the lateral ends of
these ridges and transmit blood vessels and anterior rami of
the sacral spinal nerves. Te regions lateral to these foramina
expand superiorly as the winglike
alae
.
In its posterior midline the sacral surface is roughened by
the
median sacral crest
(the fused spinous processes of the sac-
ral vertebrae). Tis is flanked laterally by the
posterior sacral
foramina
, which transmit the posterior rami of the sacral spi-
nal nerves, and then the
lateral sacral crests
(remnants of the
transverse processes of S
1
–S
5
).
Te vertebral canal continues inside the sacrum as the
sacral
canal
. Since the laminae of the fi²h (and sometimes the fourth)
sacral vertebrae fail to fuse medially, an enlarged external open-
ing called the
sacral hiatus
(hi-a
9
tus; “gap”) is obvious at the
inferior end of the sacral canal.
Coccyx
Te coccyx, our tailbone, is a small triangular bone (Figure 7.22,
and see Figure 7.16). It consists of four (or in some cases three
or five) vertebrae fused together. Te coccyx articulates superi-
orly with the sacrum. (Te name
coccyx
is from the Greek word
meaning “cuckoo” and was so named because of its fancied re-
semblance to a bird’s beak.) Except for the slight support the
coccyx affords the pelvic organs, it is a nearly useless bone. Oc-
casionally, a baby is born with an unusually long coccyx, which
may need to be removed surgically.
■ ■ ■
Body of
first
sacral
vertebra
Transverse ridges
(sites of vertebral
fusion)
Coccyx
Coccyx
Anterior
sacral
foramina
Apex
Posterior
sacral
foramina
Median
sacral
crest
Sacral promontory
Sacral
canal
Sacral
hiatus
Body
Facet of superior
articular process
Lateral
sacral
crest
Auricular
surface
Ala
(a) Anterior view
(b) Posterior view
Figure 7.22
The sacrum and coccyx.
(For a related image, see
A Brief Atlas of the Human
Body
, Figure 22.)
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