The Thoracic Cage And Respiratory Muscles
Thoracic cage
The thoracic cage is composed of the sternum, ribs, intercostal
spaces and thoracic vertebral column, with the diaphragm dividing
the thorax from the abdomen.
The sternum
The dagger-shaped sternum has three parts. The manubrium,
with which the firs and upper parts of the second costal cartilage and the clavicle
articulate (Fig. 2a), lies at the level of the third and fourth thoracic
vertebrae (see Fig. 1b). The lower parts of the second and third to seventh
ribs articulate with the body of the sternum (level with T5-T8). The
angle between the manubrium and body at the cartilaginous manubriosternal
joint forms the sternal angle (angle of Louis), and this is a useful
anatomical reference point. The small xiphoid process (xiphisternum)
usually remains cartilaginous well into adult life.
The ribs and intercostal space
The first 7 (true or vertebrosternal) of the 12 pairs of
ribs are connected to the sternum by their costal cartilages. The hyaline
cartilages of the eighth, ninth and tenth (false or vertebrochondral)
ribs articulate with the cartilage above, and the eleventh and twelfth are free
(floating or vertebral ribs). A typical rib (Fig. 2b) has a head
with two facets for articulation with the corresponding vertebra,
the intervertebral disc and the vertebra above. The rib also articulates at the
tubercle with the transverse process of the corresponding vertebra. The
two articular regions act like a hinge, forcing the rib to move through an axis
passing through these areas. The flattene shaft of the rib is weakest at the angle
of the rib and this is where it tends to fracture in an adult. The upper
two ribs, protected by the clavicle and the two floatin ribs, are least likely
to fracture. There is a cervical rib attached to the transverse process of C7
in 0.5% of people, and the presence of this rib may cause paraesthesiae or
vascular problems, due to pressure on the brachial plexus or subclavian artery.
Intercostal spaces contain external intercostal muscles whose fibres
pass downwards and forwards between the ribs, internal intercostal muscles whose
fibres pass downwards and backwards and an incomplete innermost intercostal
layer (Fig. 2c). They are innervated by intercostal nerves, which
are the anterior primary rami of thoracic nerves. Intercostal veins,
arteries and nerves lie in grooves on the undersurface of the
corresponding rib, with the vein above, artery in the middle and nerve below.
The diaphragm
The dome-shaped diaphragm (Fig. 2d) separates the thorax and abdomen
and consists of a muscular peripheral part and a central tendon, which
is partly fused with the pericardium. The muscular diaphragm takes its origin
from the vertebrae and arcuate ligaments, the rib cage and the sternum. The right
crus arises from the upper three lumbar vertebrae and the left crus from
the upper two lumbar vertebrae. Their fibrous medial borders form the median
arcuate ligament over the front of the aorta. The medial and lateral
arcuate ligaments are thickenings of the fascia overlying the psoas
major and quadratus lumborum, respectively. The costal part of the
diaphragm is attached to the inner aspects of the seventh to twelfth ribs and
costal cartilages.
The sternal part originates as two slips from the back of the xiphis-
ternum. The phrenic nerves (C3, 4, 5) supply motor fibres Sensory
innervation of the central diaphragm also runs in the phrenic, and pain from
irritation of the diaphragm is often referred to the corresponding dermatome
for C4, the shoulder-tip. The lower intercostal nerves sup- ply sensory fibre
to the peripheral diaphragm. The aorta, thoracic duct and azygos vein pass
through the diaphragm at the aortic opening at the level of T12. The
oesophagus, branches of the left gastric artery and vein and both vagi pass
through the oesophageal opening at the level of T10, and the inferior vena cava
and right phrenic nerve pass through an opening at the level of T8.
Muscles of respiration
All inspiratory muscles act to increase thoracic volume, causing intrapleural
and alveolar pressure to fall to create an alveolar-mouth pressure gradient, drawing
air into the lungs. The expanded chest wall and lungs will recoil by themselves
and quiet breathing uses no expiratory muscles.
The main inspiratory muscle, the diaphragm, moves down when it
contracts, by about 1.5 cm during quiet breathing and 6-7 cm during deep
breathing. During quiet breathing, the first rib remains fairly still and the intercostal
muscles elevate and evert the succeeding ribs. The intercostal muscles also
stiffen the intercostal spaces preventing them from being sucked in during inspiration.
The scalene muscles, which insert into the first two ribs, are also
active in normal inspiration. Raising the upper ribs pushes the sternum forward
(the pump action) increasing the anterior-posterior diameter of the
chest, and as the sloping lower ribs rise, they move out (the bucket-handle
action) and the transverse diameter of the chest wall increases.
In quiet breathing in adults, ventilation is largely diaphragmatic. As
the diaphragm contracts, it squashes the abdominal contents and raises intra-abdominal
pressure, pushing out the abdominal wall and lower ribs. Consequently, in
normal breathing the chest wall and the abdominal wall move out together during
inspiration. If the diaphragm is paralysed, increased chest volume is produced
entirely by raising the ribs, and as intrathoracic pressure falls in
inspiration, the faccid diaphragm is sucked into the chest and the abdomen
moves in. This out-of-phase movement of the chest and abdominal walls is known
as paradoxical breathing. In a high cervical cord transection, all
respiratory muscles are paralysed, but when the damage is below the phrenic
nerve roots (C3, 4, 5) breathing continues via the diaphragm alone. In the
newborn, ribs are horizontal, so rib movements cannot increase the volume of the
chest and breathing is entirely by the up-and-down action of the diaphragm or
so-called abdominal breathing. As the ribs become more oblique with
increasing age, there is an increased contribution of thoracic breathing.
When ventilation or resistance to breathing is increased, accessory
inspiratory muscles aid inspiration. These include the scalene muscles,
sternomastoids and serratus anterior. If the arms are f xed by
grasping the edge of a table, contraction of the pectoralis major, which
normally adducts the arm, helps expand the chest. When ventilation exceeds
about 40 U min, there is activation of expiratory muscles, especially abdominal
muscles (rectus abdominis, external and internal oblique),
which speeds up recoil of the diaphragm by raising intra-abdominal pressure.
