226
UNIT 2
Covering, Support, and Movement of the Body
7
and at the level of the second pair of ribs. It is a handy reference
point for finding the second rib and thus for counting the ribs
during a physical examination and for listening to sounds made
by specific heart valves.
Te
xiphisternal joint
(zif
0
ĭ-ster
9
nul) is the point where the
sternal body and xiphoid process fuse. It lies at the level of the
ninth thoracic vertebra. Te heart lies on the diaphragm just
deep to this joint.
Ribs
±welve pairs of
ribs
form the flaring sides of the thoracic cage
(Figure 7.23a). All ribs attach posteriorly to the thoracic ver-
tebrae (bodies and transverse processes) and curve inferiorly
toward the anterior body surface. Te superior seven rib pairs
attach directly to the sternum by individual costal cartilages
(bars of hyaline cartilage). Tese are
true
or
vertebrosternal
ribs
(ver
0
tĕ-bro-ster
9
nal). (Notice that the anatomical name in-
dicates the two attachment points of a rib—the posterior attach-
ment given first.)
Te remaining five pairs of ribs are called
false ribs
because
they either attach indirectly to the sternum or entirely lack a
sternal attachment. Rib pairs 8–10 attach to the sternum indi-
rectly, each joining the costal cartilage immediately above it.
Tese ribs are also called
vertebrochondral ribs
(ver
0
tĕ-bro-
kon
9
dral). Te inferior margin of the rib cage, or
costal margin
,
is formed by the costal cartilages of ribs 7–10. Rib pairs 11 and
12 are called
vertebral ribs
or
floating ribs
because they have
no anterior attachments. Instead, their costal cartilages lie em-
bedded in the muscles of the lateral body wall.
Te ribs increase in length from pair 1 to pair 7, then de-
crease in length from pair 8 to pair 12. Except for the first rib,
which lies deep to the clavicle, the ribs are easily felt in people
of normal weight.
A typical rib is a bowed flat bone
(Figure 7.24)
. Te bulk of
a rib is simply called the
shaf
. Its superior border is smooth, but
its inferior border is sharp and thin and has a
costal groove
on its
inner face that lodges the intercostal nerves and blood vessels.
In addition to the sha², each rib has a head, neck, and tu-
bercle. Te wedge-shaped
head
, the posterior end, articulates
with the vertebral bodies by two facets: One joins the body of
the same-numbered thoracic vertebra, the other articulates with
the body of the vertebra immediately superior. Te
neck
is the
constricted portion of the rib just beyond the head. Lateral to
this, the knoblike
tubercle
articulates with the costal facet of the
transverse process of the same-numbered thoracic vertebra.
Beyond the tubercle, the sha² angles sharply forward (at the
angle of the rib) and then extends to attach to its costal cartilage
anteriorly. Te costal cartilages provide secure but flexible rib
attachments to the sternum.
Te first pair of ribs is quite atypical. Tey are flattened su-
periorly to inferiorly and are quite broad, forming a horizontal
table that supports the subclavian blood vessels that serve the
upper limbs. Tere are also other exceptions to the typical rib
pattern. Rib 1 and ribs 10–12 articulate with only one vertebral
body, and ribs 11 and 12 do not articulate with a vertebral trans-
verse process.
Te
sternal angle
is felt as a horizontal ridge across the front
of the sternum, where the manubrium joins the sternal body.
Tis cartilaginous joint acts like a hinge, allowing the sternal
body to swing anteriorly when we inhale. Te sternal angle is in
line with the disc between the fourth and fi²h thoracic vertebrae
Transverse costal facet
(for tubercle of rib)
Superior costal facet
(for head of rib)
Body of vertebra
Head of rib
Intervertebral disc
Tubercle of rib
Neck of rib
Shaft
Sternum
Angle
of rib
Cross-
section
of rib
Costal groove
Costal cartilage
(a) Vertebral and sternal articulations of a typical true rib
Spinous process
Articular facet
on tubercle of rib
Shaft
Ligaments
Neck of rib
Head of rib
Body of
thoracic
vertebra
Transverse
costal facet
(for tubercle
of rib)
Superior costal facet
(for head of rib)
(b) Superior view of the articulation between a rib and a
thoracic vertebra
Junction with
costal cartilage
Shaft
Head
Neck
Articular facet
on tubercle
Angle of rib
Costal groove
Facets for articulation
with vertebrae
(c) A typical rib (rib 6, right), posterior view
Figure 7.24
Ribs.
All ribs illustrated in this figure are right ribs. (For a
related image, see
A Brief Atlas of the Human Body,
Figure 23e and f.)
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