The thoracic iliocostal muscle is the thoracic portion of the iliocostal muscle, that lies lateral to the latissimus dorsi and forms the cranial continuation of the lumbar portion of the iliocostal muscle. Its individual bundles originate with glistening tendons from the lumbar portion of iliocostal muscle and extend craniolaterally, spanning two to four intercostal spaces each.
After forming a common muscle belly, it inserts, with terminal serrations, on the caudal side of the first (tuberosity of iliocostal muscle) to the 12th ribs and to the transverse process of the seventh cervical vertebra (carnivores). In the horse, they insert on the caudal surface of the first to 15th ribs, with medial, deeper tendons of insertions of the cranial surface of the fourth to 18th ribs and to the transverse process of the seventh cervical vertebra (cervical portion of iliocostal muscle).
Origin: upper end of the cranial edge of the ribs
Insertion: caudal borders of the ribs and transverse processes of the last cervical vertebra
Action: bends the vertebral column sideways
Nerve: dorsal branches of the thoracic and lumbar nerves