develops from pectoral muscle This currently groups the major and minor muscles (which have different attachment sites) as well as the AAO structure: Muscle which is comprised of three heads. The anterior head is the portio epicoracoidea that originates on the epicoracoid cartilage and inserts on the deltoid crest of the humerus. The next more posterior head, portio sternalis, originates on the sternum and inserts in the groove next to the deltoid crest of the humerus. The most posterior head, portio abdominalis, arises from the M. rectus abdominis and inserts on the deltoid crest of the humerus thoracic segment muscle muscle of pectoral girdle pectoral pre-muscle mass