A flat, triangular muscle spread over the back and loins, where it commences in a broad aponeurotic tendon. It extends obliquely downward and forward over the side of the chest, and, gradually becoming narrower, passes between the fore-limb and the trunk to reach the humerus or upper arm bone.

Origin.- From the superior spines of all the lumbar and the fourteen posterior dorsal vertebrae.

Insertion. - By a short tendon into a small tubercle on the internal surface of the humerus or upper arm bone.

Action. - To flex the humerus on the scapula and incline the leg towards the trunk.