Innermost intercostal muscles

The innermost intercostal muscles are the deepest layer of the intercostal muscles located within the intercostal spaces of the thoracic wall.

Location and Relations

• The innermost intercostal muscles lie deep to the internal intercostal muscles. They are best developed in the middle part of the thoracic wall and are separated from the internal intercostal muscles by the neurovascular bundle (i.e. the intercostal vein, artery, and nerve).

Attachments

• Extend between adjacent ribs similarly to the internal intercostals.

• Attach from:

  • Inner surface of the rib above

  • To the inner surface of the rib below

Fibre Direction

• Fibres run infero-posteriorly (downwards and backwards). Fibre orientation is similar to the internal intercostal muscles.

Nerve Supply

• Supplied by the corresponding intercostal nerves (T1 to T11).

Blood Supply

• Supplied by:

  • Anterior intercostal arteries (branches of internal thoracic artery)

  • Posterior intercostal arteries (mainly arising from the descending thoracic aorta)

Functions

• Assist the internal intercostal muscles in stabilising intercostal spaces, and assisting forced expiration by depressing the ribs

Clinical correlates

• The intercostal neurovascular bundle lies within the costal groove; between the internal and innermost layers. This anatomical location is clinically significant to know when doing procedures, such as, thoracentesis and chest drain insertion.