Thoracic Cage-Intercostal Spaces

The thoracic cage is formed by the vertebral column behind, the ribs and intercostal spaces on either side and the sternum and costal cartilages in front. Above, it communicates through the ‘thoracic inlet’ with the root (BASE) of the neck; below, it is separated from the abdominal cavity by the diaphragm (Fig. 1).

The intercostal spaces

There are slight variations between the different intercostal spaces, but typically each space contains three muscles, comparable to those of the abdominal wall, and an associated neurovascular bundle (Fig. 8).

 The muscles are:

1◊◊the external intercostal, the fibres of which pass downwards and forwards from the rib above to the rib below and reach from the vertebrae behind to the costochondral junction (RIB JOINT) in front, where muscle is replaced by the anterior intercostal membrane;

2◊◊the internal intercostal, which runs downwards and backwards from the sternum to the angles of the ribs where it becomes the posterior intercostal membrane;

3◊◊the innermost intercostal, which is only incompletely separated from the internal intercostal muscle by the neurovascular bundle. The fibres of this sheet cross more than one intercostal space and it may be incomplete. Anteriorly it has a more distinct portion which is fan-like in shape, termed the transversus thoracis (or sternocostalis), which spreads

upwards from the posterior aspect of the lower sternum to insert onto the inner surfaces of the second to the sixth costal cartilages. Just as in the abdomen, the nerves and vessels of the thoracic wall lie between the middle and innermost layers of muscles. This  eurovascular

bundle consists, from above downwards, of vein, artery and nerve, the vein lying in a groove on the undersurface of the corresponding rib (remember—v, a, n—vein, nerve, artery).

The vessels comprise the posterior and anterior intercostals. The posterior intercostal arteries of the lower nine spaces are branches of the thoracic aorta, while the first two are derived from the superior intercostal branch of the costocervical trunk, the only branch of the second part of the subclavian artery. Each runs forward in the subcostal groove to anastomose (COMMUNICATION BETWEEN VESSELS BY COLLATERAL CHANNELS )with the anterior intercostal artery.

Each has a number of branches to adjacent muscles, to the skin and to the spinal cord. The corresponding veins are mostly tributaries of the azygos and hemiazygos veins. The first posterior intercostal vein drains into the rachiocephalic or vertebral vein. On the left, the 2nd and 3rd veins often join to form a superior intercostal vein, which crosses the aortic arch to drain into the left brachiocephalic vein.

The anterior intercostal arteries are branches of the internal thoracic artery (1st–6th space) or of its musculophrenic branch (7th–9th spaces). The lowest two spaces have only posterior arteries. Perforating branches pierce the upper five or six intercostal spaces; those of the 2nd–4th spaces are large in the female and supply the breast.

The intercostal nerves are the anterior primary rami of the thoracic nerves, each of which gives off a collateral muscular branch and lateral and anterior cutaneous branches for the innervation of the thoracic and abdominal walls (Fig. 9).

Clinical features

1◊◊Local irritation of the intercostal nerves by such conditions as Pott’s disease of the thoracic vertebrae (tuberculosis) may give rise to pain which is referred to the front of the chest or abdomen in the region of the peripheral termination of the nerves.

2◊◊Local anaesthesia of an intercostal space is easily produced by infiltration around the intercostal nerve trunk and its collateral branch—a procedure known as intercostal nerve block.

3◊◊In a conventional posterolateral thoracotomy (e.g. for a pulmonary lobectomy) an incision is made along the line of the 5th or 6th rib; the periosteum over a segment of the rib is elevated, thus protecting the neurovascular bundle, and the rib is excised (REMOVED). Access to the lung or mediastinum is then gained though the intercostal space, which can be opened out considerably owing to the elasticity of the thoracic cage.
4◊◊Pus from the region of the vertebral column tends to track around the thorax along the course of the neurovascular bundle and to ‘point’ to the three sites of exit of the cutaneous branches of the intercostal nerves, which are lateral to erector spinae (sacrospinalis), in the midaxillary line and just lateral to the sternum ((Fig. 9)above).