Fig. The left levatores costorum, and right serratus posterior superior and inferior muscles, posterior view.
Lies deep to latissimus dorsi, arising from the spinous processes of Tl 1 to L2 and the associated supraspinous ligaments via the thoracolumbar fascia. The muscle fibres run horizontally to attach to the lower four ribs at their angles.
By the anterior primary rami of T9, 10 and 11.
Serratus posterior inferior helps to pull the lower four ribs downwards and backwards and so may assist expiration.