Iliocostalis
Iliocostalis muscle is the muscle immediately lateral to the longissimus that is the nearest to the furrow that separates the hypaxial muscles|epaxial] muscles from the hypaxial. It lies very deep to the fleshy portion of the Serratus posterior [inferior muscle|serratus posterior muscle]. It laterally flexes the vertebral column to the same side.
Structure
Iliocostalis muscle has a common origin from the iliac crest, the sacrum, the thoracolumbar fascia, and the spinous processes of the vertebrae from T11 to L5.Iliocostalis cervicis arises from the angles of the third, fourth, fifth, and sixth ribs, and is inserted into the posterior [tubercle of cervical vertebra|posterior tubercles] of the transverse processes of the fourth, fifth, and sixth cervical vertebrae.
Iliocostalis thoracis arises by flattened tendons from the upper borders of the angles of the lower six ribs medial to the tendons of insertion of the iliocostalis lumborum; these become muscular, and are inserted into the upper borders of the angles of the upper six ribs and into the back of the transverse process of the seventh cervical vertebra.
Iliocostalis lumborum is inserted, by flattened tendons, into the inferior borders of the angles of the lower six to nine ribs.