Serratus posterior inferior

The serratus posterior inferior (Latin: musculus serratus posterior inferior) is a broad irregular-shaped muscle of the back that lies deep to the latissimus dorsi muscle. It stretches between the vertebrae of the spine and ribs of the thorax. The serratus posterior inferior is located in the lower back at the junction of the thoracic and lumbar regions. It is larger than the superiorly located serratus posterior superior. Serratus posterior muscles form the intermediate layer of the back muscles, and a wide interval separates both.

Origin

The serratus posterior inferior muscle originates from the spinous processes of the thoracic and lumbar vertebrae (usually Th11 - L2).

Insertion

Fibers of the serratus posterior inferior extend in a superolateral direction and insert on the inferior borders of ribs 9 to 12.

Action

The serratus posterior inferior muscle depresses ribs 9 to 12 and draws the ribs backward and downward (infero-posteriorly). Also, it is an accessory muscle of respiration and aids in expiration.

Innervation

The serratus posterior inferior is innervated by the 9th to 11th intercostal nerves and subcostal nerve (12th intercostal nerve).

Blood supply

The serratus posterior inferior muscle receives arterial blood supply from the posterior intercostal and subcostal arteries that arise from the thoracic aorta. Also, it is supplied by the lumbar arteries - branches of the abdominal aorta.