Sacrospinalis is a very thick, lateral portion of an epaxial muscle in mammals which continues anteriorly up to the neck and divides into three muscles: Semispinalis, Longissimus, and Iliocostalis. Its origin is on the spines of the last four thoracic vertebrae, and its insertion is on both the spines of the most craniad thoracic vertebrae and the cervical vertebrae. Its action is to extend the vertebral column.

Semispinalis

The semispinalis is the muscle immediately lateral to the multifidus spinae and is the most medial of all three. It consists of diagonal fibers.

Longissimus

The longissimus is the muscle lateral to the semispinalis. It is the longest subdivision of the sacrospinalis that extends forward into the transverse processes of the posterior cervical vertebrae.

Iliocostalis

The iliocostalis is the muscle immediately lateral to the longissimus that is the nearest to the furrow that separates the epaxial msucles from the hypaxial. It lies very deep to the fleshy portion of the serratus ventralis .