Retractor costae muscle

The retractor costae is a slender skeletal muscle of the thoracic wall whose fibres originate from the lumbar region and insert on the last rib(s). Functionally, it assists with expiration (by retracting/bringing the last rib(s) caudally and medially, thus reducing thoracic cavity volume).

Origin: typically the transverse processes of the first few lumbar vertebrae and/or the thoracolumbar fascia.

Insertion: on the caudal face of the last rib (or last ribs).

Action: Inspiration.

Nerve: intercostal or lumbar spinal nerves (depending on species).