Serratus posterior inferior
The serratus posterior inferior (Latin: musculus serratus posterior inferior) is a broad muscle that lies underneath the latissimus dorsi. It is located at the junction of the thoracic and lumbar regions. The muscle has an irregular quadrilateral form, and it is larger than the serratus posterior superior. Both serratus posterior muscles are separated by a wide interval.
The muscle originates from the thoracic and lumbar portions of the spine (usually T11 - L3).
Fibers of the serratus posterior inferior extend in a superolateral direction and insert into the lower borders of ribs 9 to 12.
The main action of the serratus posterior inferior is the depression of the ribs 9 to 12. It draws the ribs backward and downward to assist in extension and rotation of the trunk. This action also aids in forced expiration.
The serratus posterior inferior is innervated by the intercostal nerves (9th through 12th).
Oxygenated blood is supplied to the serratus posterior inferior via the intercostal arteries.