External intercostal muscles

At a glance

English

External intercostal muscles

Latin (TA2)

Musculi intercostales externi

System

Muscle

Region

Trunk

Side

Paired, left and right

About the external intercostal muscles

Each gap between two ribs, an intercostal space, is closed by three thin layers of muscle. The external intercostals make up the outer layer. One muscle sits in every space on each side, and each spans from the rib above to the rib below, so that together they turn the rib cage into a continuous but flexible wall.

Each muscle starts on a rib's lower edge and ends on the upper border of the next rib down. Behind, the fibres reach as far back as the tubercles of the ribs, close to the vertebral column. In front they stop where the ribs meet their costal cartilages, and from there forward the space is bridged only by a thin sheet, the external intercostal membrane, that runs on to the sternum. The fibres slope downward and forward at the front of the chest, the same direction as the external oblique of the abdomen; at the back they run downward and outward. This layer is thicker than the internal intercostals beneath it, whose fibres cross it almost at right angles. Between the internal and innermost layers runs the intercostal nerve with its artery and vein, sheltered under the lower edge of the rib above.

When the external intercostals contract, they raise the ribs. Because the ribs slope downward from the spine, lifting them pushes the sternum forward and swings the ribs outward like bucket handles, widening the chest in both directions and drawing air into the lungs. They are most active in deeper or more forceful inspiration; in quiet breathing the diaphragm does most of the work. They also keep the spaces from being sucked in or blown out as pressure in the chest changes.

Each muscle is supplied by the intercostal nerve of its own space, the anterior ramus of a thoracic spinal nerve, and receives blood from the posterior and anterior intercostal arteries. When a needle is passed between the ribs, for example to drain fluid from the pleural cavity, it is placed just above a rib to avoid the nerve and vessels under the rib above.

Attachments, action and supply

Origin

Lower border of each rib, from the rib tubercle forward to the costochondral junction.

Insertion

Upper edge of the next rib down; continued forward to the sternum as the external intercostal membrane.

Action

Elevate the ribs to enlarge the chest in inspiration, and stiffen the intercostal spaces.

Nerve supply

Intercostal nerves (anterior rami of T1-T11).

Blood supply

Posterior intercostal arteries and anterior intercostal branches of the internal thoracic and musculophrenic arteries.