Skeleton of the
Thorax
The skeleton of the
thorax consists of 12 thoracic vertebrae, the 12 pairs of ribs and their costal
cartilages, and the sternum (Fig. 2.5). Structures in continuity between the root of
the neck and the upper part of the thoracic cavity pass through the superior
thoracic aperture (thoracic inlet), which is bounded by the first thoracic
vertebral body, the first pair of ribs and costal cartilages and the upper
border of the sternum. The inferior thoracic aperture (thoracic outlet),
through which structures pass between the thoracic and abdominal cavities, is
formed by the twelfth thoracic vertebral body, the twelfth and eleventh ribs
and the costal margin (the fused costal cartilages of the seventh to the tenth
ribs inclusive).
Although the ribs differ in size and
shape, most (2–9 inclusive) have features in common and are described as
typical ribs (Fig. 2.6). Each typical rib consists of a head, neck, tubercle,
shaft, upper and lower borders and inner and outer surfaces. The heads of the
ribs are those parts that articulate with the thoracic vertebral bodies. The lower part of the head forms a
synovial joint with its own vertebral body, while the upper part articulates
with the vertebra above. The intermediate part of the head lies against the
intervertebral disc. The neck of the rib connects the head and the tubercle and
lies in front of the transverse process. The tubercle of the rib faces
posteriorly and the medial part of its surface forms a synovial joint with the
articular facet on the transverse process of the corresponding vertebra. The
shaft forms the remainder of the rib and ends anteriorly at a shallow
depression, which receives the costal cartilage. Passing laterally from the
tubercle, the shaft slopes downwards and backwards before turning forwards
and outwards to form the angle. Lateral to the angle, the shaft possesses a
sharp lower border, which bounds the costal groove.
The first rib is atypical. Its head
possesses an articular facet solely for its own vertebral body. The shaft is
short and broad and has superior and inferior surfaces. In addition, its
superior surface carries a ridge that forms a projection on the inner border of
the rib, the scalene tubercle, to which is attached the scalenus anterior. Two
grooves lie across the shaft, one in front of the ridge (for the subclavian
vein) and the other behind (for the subclavian artery and lowest trunk of the
brachial plexus). The tenth, eleventh and twelfth ribs are also atypical, in
that each head pos- sesses a single facet and the rib is usually devoid of a
tubercle or an angle.
Costal cartilages
All ribs possess costal cartilages,
and those of the upper seven pairs (true ribs) articulate with the sides of the
sternum. Pairs 8–12 (false ribs) fall short of the sternum. These articulate
with the cartilage immediately above, while 11 and 12 (floating ribs) are
pointed and end freely in the muscle of the abdominal wall.
The sternum is a flat bone and
consists of the manubrium, the body (Fig. 2.7) and the xiphoid process. The manubrium articulates
with the medial end of each clavicle at the sternoclavicular joint and with the
first costal cartilage. Its upper margin includes the jugular notch, which forms part
of the superior
thoracic aperture. A palpable secondary cartilaginous joint (the
manubriosternal joint) unites the manubrium and body and forms a useful guide
to the second costal cartilage, which abuts the sternum at the lateral margin
of the joint. The lateral margins of the body of the sternum are indented by
the medial ends of the second to the seventh costal cartilages. The xiphoid
process lies in the subcostal angle and projects downwards and backwards from
the body of the sternum.
