Human Anatomy Vol 1

Level of upper border of body

of vertebral T1 or spine of C7

Sternal angle

Right pleural sac

Xiphisternal joint

Right xiphicostal angle

Level of eighth rib

Levdl oftenth rib

, Midclavicular line

Midaxillary line

INTRODUCTION

Left pleural sac

Level offourth costal cartilage

Thoracic wall

Level of sixth costal cartilage

Costal margin

Costo-

Twelfth rib

r of body of vertebra T12 or upper

border of spine of the same vertebra

Fig. 12.5: Surface marking of midclavicular and midaxillary lines

The skeleton of thorax is also known as the thoracic
cage. It is an osseocartilaginous, elastic cage which is
primarily designed for increasing and decreasing the
intrathoracic pressure, so that air is sucked into the
lungs during inspiration and expelled during
expiration.

FORMAIION

Anteriorly by the stemum (Greek chest) (Figs 72.1. and
12.2).
Posteriorly by the 12 thoracic vertebrae and the
intervening intervertebral discs (Fig. 12.3).
On each sideby 12 rlbs with their cartilages.
Each rib articulates posteriorly with the vertebral
column. Anteriorly, only the upper seven ribs articulate
with the sternum through their cartilages and these are
called true or oertbbrosternal ribs.
The costal cartilages of the next three ribs, i.e. the
eighth, ninth and tenth end by joining the next higher
costal cartilage. These ribs are, therefore, known as
zsertebrochondral ribs. The costal cartilages of the seventh,
eighth, ninth and tenth ribs form the sloping costal
margin.
The anterior ends of the eleventh and twelfth ribs
are free: These are called floating or aertebral ribs. The
vertebrochondral and vertebral ribs, i.e. the last five
ribs are also called false ribs because they do not
articulate with the stemum.
The costovertebral, costotransverse, manubrio-
sternal and chondrosternal joints permit movements
of the thoracic cage during breathing.

The chest wall of the child is highly elastic, and

fractures of the ribs are, therefore, rare. In adults,

the ribs may be fractured by direct or indirect

violence (Fig.72.6).In indirect violence, like crush

injury, the rib fractures at its weakest point located

at the angle. The upper two ribs which are protected

by the clavicle, and the lower two ribs which are free

to swing are least commonly injured.

Anterior curve

Angle

Fig. 12.6: Fracture of the rib at its angle

SHAPE

The thorax resembles a truncated cone which is narrow

above and broad below (Fig.12.7). The narrow uPPer

end is continuous with the root of the neck from which

it is partly separated by the suprapleural membrane or

Sibson's fascia. The broad or lower end is almost

completely separated from the abdomen by the

diaphragm which is deeply concave downwards. The

thoracic cavity is actually much smaller than what it

appears to be because the narrow uPPer part appears

broad due to the shoulders, and the lower part is greatly