This is the largest muscle of the fore extremity.

It fills in the triangular space between the hinder edge of the shoulder and the point of the elbow. It is made up of three portions or heads, distinguished as the caput magnum or large head, the caput parvum or small head, and the caput medium or intermediate.

The caput magnum or large head arises from the posterior border of the scapula as high as the dorsal angle, and the caput medium arises from the upper and outer part of the humerus. The caput parvum arises from the inner face of the same bone.

Insertion. - All three divisions are inserted into the point of the elbow. A synovial bursa is interposed between the tendon of the caput magnum and the bone.

Action. - To extend the fore-arm on the humerus.