Innermost intercostal muscles

At a glance

English

Innermost intercostal muscles

Latin (TA2)

Musculi intercostales intimi

System

Muscle

Region

Trunk

Side

Paired, left and right

About the innermost intercostal muscles

The wall of each intercostal space is made of three muscle layers. The innermost intercostals form the deepest of them, lying just outside the fascia that lines the chest and the pleura. They are often described as a split-off inner portion of the internal intercostals, because their fibres run in the same direction, but they are separated from that layer by the plane in which the intercostal nerve and vessels travel.

Each muscle passes from the inner surface of one rib to the inner surface of the rib below. The layer is incomplete: it is best developed in the middle, lateral part of each space and usually absent at the front and back, and it is often thin or missing in the upper spaces. The fibres slope downward and backward, like those of the internal intercostals. Two other muscle groups lie in the same deep plane and are sometimes grouped with them as the transversus thoracis group: the subcostal muscles at the back, which can cross more than one space, and transversus thoracis at the front, behind the sternum.

Because the intercostal nerve, artery and vein run between the internal and innermost layers, the innermost intercostal is a useful landmark: the neurovascular bundle lies on its outer surface, tucked under the costal groove of the rib above. The layer probably acts with the internal intercostals, helping to stiffen the spaces and to lower the ribs in forced breathing out, though its separate role is small.

Like the other intercostal layers, it is supplied by the intercostal nerve of its space, and its blood comes from the posterior and anterior intercostal arteries. Its position matters when instruments are passed through the chest wall: a needle or drain placed just above the upper border of a rib travels through all three layers while staying clear of the bundle that runs along the lower border of the rib above.

Attachments, action and supply

Origin

Inner surface of each rib, in the lateral part of the space.

Insertion

Inner surface of the rib below.

Action

Help the internal intercostals stiffen the spaces and lower the ribs in forced expiration.

Nerve supply

Intercostal nerves (anterior rami of thoracic spinal nerves).

Blood supply

Posterior and anterior intercostal arteries.