The intercostal space (Fig. 2.1)

(Fig. 2.1)

Typically, each space contains three muscles comparable to those of

the abdominal wall. These include the:

•

External intercostal: this muscle fills the intercostal space from the

vertebra posteriorly to the costochondral junction anteriorly where it

becomes the thin anterior intercostal membrane. The fibres run downwards

and forwards from rib above to rib below.

•

Internal intercostal: this muscle fills the intercostal space from the

sternum anteriorly to the angles of the ribs posteriorly where it becomes

the posterior intercostal membrane which reaches as far back as the

vertebral bodies. The fibres run downwards and backwards.

•

Innermost intercostals: this group comprises the subcostal muscles

posteriorly, the

intercostales intimi laterally and the transversus thoracis

anteriorly. The fibres of these muscles span more than one intercostal

space.

The neurovascular space is the plane in which the neurovascular

bundle (intercostal vein, artery and nerve) courses. It lies between the

internal intercostal and innermost intercostal muscle layers.

The intercostal structures course under cover of the subcostal

groove.

Pleural aspiration should be performed close to the upper border

of a rib to minimize the risk of injury.

Vascular supply and venous drainage of the chest wall

The intercostal spaces receive their

arterial supply from the anterior

and posterior intercostal arteries.

• The

anterior intercostal arteries are branches of the internal thoracic

artery and its terminal branch the musculophrenic artery. The lowest

two spaces have no anterior intercostal supply (Fig. 2.2).

• The first 2–3

posterior intercostal arteries arise from the superior

intercostal branch of the costocervical trunk, a branch of the 2nd part of

the subclavian artery (see Fig. 60.1). The lower nine posterior intercostal

arteries are branches of the thoracic aorta. The posterior intercostal

arteries are much longer than the anterior intercostal arteries

(Fig. 2.2).

The anterior intercostal

veins drain anteriorly into the internal thoracic

and musculophrenic veins. The posterior intercostal veins drain

into the azygos and hemiazygos systems (see Fig. 4.2).

Lymphatic drainage of the chest wall

Lymph drainage from the:

•

Anterior chest wall: is to the anterior axillary nodes.

•

Posterior chest wall: is to the posterior axillary nodes.

•

Anterior intercostal spaces: is to the internal thoracic nodes.

•

Posterior intercostal spaces: is to the para-aortic nodes.

Nerve supply of the chest wall

(Fig. 2.2)

The intercostal nerves are the anterior primary rami of the thoracic segmental

nerves. Only the upper six intercostal nerves run in their intercostal

spaces, the remainder gaining access to the anterior abdominal

wall.

Branches of the intercostal nerves include:

•

Cutaneous anterior and lateral branches.

• A

collateral branch which supplies the muscles of the intercostal

space (also supplied by the main intercostal nerve).

•

Sensory branches from the pleura (upper nerves) and peritoneum

(lower nerves).

Exceptions include:

• The 1st intercostal nerve is joined to the brachial plexus and has no

anterior cutaneous branch.

• The 2nd intercostal nerve is joined to the medial cutaneous nerve of

the arm by the intercostobrachial nerve branch. The 2nd intercostal

nerve consequently supplies the skin of the armpit and medial side of

the arm.

The diaphragm

(Fig. 2.3)

The diaphragm separates the thoracic and abdominal cavities. It is composed

of a peripheral muscular portion which inserts into a central

aponeurosis

athe central tendon.

The muscular part has three component origins:

• A

vertebral part: this comprises the crura and arcuate ligaments.

The right crus arises from the front of the L1–3 vertebral bodies and

intervening discs. Some fibres from the right crus pass around the lower

oesophagus.

The left crus originates from L1 and L2 only.

The medial arcuate ligament is made up of thickened fascia which

overlies psoas major and is attached medially to the body of L1 and laterally

to the transverse process of L1. The lateral arcuate ligament is

made up of fascia which overlies quadratus lumborum from the transverse

process of L1 medially to the 12th rib laterally.

The median arcuate ligament is a fibrous arch which connects left

and right crura.

• A

costal part: attached to the inner aspects of the lower six ribs.

• A

sternal part: consists of two small slips arising from the deep surface

of the xiphoid process.

Openings in the diaphragm

Structures traverse the diaphragm at different levels to pass from

thoracic to abdominal cavities and vice versa. These levels are as

follows:

• T8, the

opening for the inferior vena cava: transmits the inferior vena

cava and right phrenic nerve.

• T10, the

oesophageal opening: transmits the oesophagus, vagi and

branches of the left gastric artery and vein.

• T12, the

aortic opening: transmits the aorta, thoracic duct and azygos

vein.

The left phrenic nerve passes into the diaphragm as a solitary

structure.

Nerve supply of the diaphragm

•

Motor supply: the entire motor supply arises from the phrenic nerves

(C3,4,5). Diaphragmatic contraction is the mainstay of inspiration.

•

Sensory supply: the periphery of the diaphragm receives sensory

fibres from the lower intercostal nerves. The sensory supply from the

central part is carried by the phrenic nerves

.