Epaxial muscles

M. iliocostalis, M. longissimus et M. transversospinalis (musculi epaxiales)

Muscle mass dorsal to the transverse processes, along the vertebral column.

Draft, not yet reviewed by a veterinarian. Content may contain errors. It is published so the community can review it.

This piece is a schematic shape, not a scan: there is no freely licensed model for this structure yet.

3D model of this piece: Vet Atlas procedural musculature, CC BY-SA 4.0

Overview

The epaxial muscles form three parallel columns, from lateral to medial:

Iliocostalis (m. iliocostalis): from the ilium and lumbar vertebrae to the ribs.
Longissimus (m. longissimus): the largest, from the ilium to the skull, between the transverse and spinous processes.
Transversospinalis (m. transversospinalis): short bundles next to the spinous processes (spinalis, semispinalis, multifidus, rotatores).

The model shows the three columns as a single mass, from the ilium to the axis; in the neck it also stands for the deep dorsal neck muscles.

Ilium, sacrum, vertebral processes and ribs; each bundle spans one or several segments.

Extend and stabilise the vertebral column. Acting on one side only, they bend it laterally.

Dorsal branches of the spinal nerves.

The lumbar epaxial muscles are a common intramuscular injection site.
Their bulk is palpated for muscle condition scoring: atrophy is an early sign of chronic disease.
Painful spasm of these muscles often accompanies intervertebral disc disease.