Wiktionary
iliocostalis
n. the lateral division of the erector spinae, consisting of the iliocostalis lumborum musculus, iliocostalis thoracis musculus, and iliocostalis cervicis musculus
Wikipedia
Iliocostalis
The iliocostalis is the muscle immediately lateral to the longissimus that is the nearest to the furrow that separates the epaxial muscles from the hypaxial. It lies very deep to the fleshy portion of the serratus ventralis (serratus posterior).