External intercostal membrane

At a glance

English

External intercostal membrane

Latin (TA2)

Membrana intercostalis externa

System

Ligament

Region

Thorax

Side

Paired, left and right

About the external intercostal membrane

Each space between two ribs is filled by three layers of muscle: the external, internal and innermost intercostals. The outermost of these, the external intercostal, does not stay fleshy across the whole width of the chest. Behind, it begins at the rib tubercles near the vertebral column; at the front it gives way, roughly at the junction of each rib with its costal cartilage, to a thin fibrous sheet. That sheet, filling the space between the costal cartilages as far as the edge of the sternum, is the external intercostal membrane.

The membrane keeps the direction of the muscle fibres it replaces, running downwards and forwards from one cartilage to the next, the same way as the hands in a coat pocket. Through it the deeper internal intercostal muscle can be seen, whose fibres run at right angles to it. Superficially it is covered by pectoralis major, and near the sternum the anterior cutaneous branches of the intercostal nerves and the perforating branches of the internal thoracic vessels pass through it to reach the skin of the front of the chest.

Functionally the membrane closes the outer layer of the intercostal space where the muscle is absent, so the wall stays airtight and does not suck in during inspiration. It also continues the pull of the external intercostals onto the cartilages. The external intercostals help raise the ribs when breathing in; between the cartilages, where the membrane takes their place, that work falls to the parasternal fibres of the internal intercostals.

In the clinic, the membrane is one of the layers crossed in parasternal procedures, such as some nerve blocks and access to the internal thoracic vessels for heart bypass grafts. Knowing that the outer layer is membranous here, rather than muscular, helps in reading ultrasound images of the anterior chest wall.

Attachments and function

Attaches from

The costochondral junction, where the fleshy external intercostal muscle ends.

Attaches to

The lateral margin of the sternum, spanning the space between adjacent costal cartilages.

Function

Closes the outer layer of the anterior intercostal space and carries the line of the external intercostal muscle onwards to the sternum.

Nerve supply

Lies in the territory of the intercostal nerves, whose anterior cutaneous branches pierce it.

Blood supply

Anterior intercostal branches of the internal thoracic artery.