The iliopsoas muscle.

The iliocostalis muscle (m. iliocostalis) is the most lateral part of the muscle that straightens the spine. This muscle begins on the iliac crest, the inner surface of the superficial leaflet of the lumbosacral fascia. Muscle bundles pass upward along the back surface of the ribs laterally from their angles to the transverse processes of the lower (VII-IV) cervical vertebrae. According to the location of the individual parts of the muscle, it is divided into the iliocostalis lumborum muscle, the iliocostalis thoracic muscle, and the iliocostalis cervicis muscle.

The iliocostalis lumborum muscle (m. iliocostalis lumborum) originates on the iliac crest, the inner side of the superficial plate of the lumbothacral fascia; it is attached by separate flat tendons to the angles of the six lower ribs.

The iliocostalis thoracis muscle originates on the six lower ribs, medially from the attachment sites of the iliocostalis lumborum muscle; it is attached to the six upper ribs in the region of their angles and to the posterior surface of the transverse process of the 7th cervical vertebra.

The iliocostalis cervicis muscle (m. iliocostalis cervicis) begins in the area of the angles of the III, IV, V and VI ribs (inward from the attachment points of the iliocostalis thoracic muscle) and is attached to the posterior tubercles of the transverse processes of the VII-IV cervical vertebrae.

Function: the iliocostalis muscle, together with the other parts of the erector spinae muscle, extends the spine. With a unilateral contraction, it tilts the spine to its side, lowers the ribs. The lower bundles of this muscle, pulling and strengthening the ribs, create support for the diaphragm.

Innervation: posterior branches of the cervical, thoracic and lumbar spinal nerves (CIV-CIII).

Blood supply: posterior intercostal arteries, lumbar arteries.