The structure indicated is the teres major muscle.

The teres major muscle is located in the posterior scapular area. It originates on the inferior angle of the scapula and inserts anteriorly on the humerus on the intertubercular sulcus. Contraction of the teres major causes extension and medial rotation of the humerus at the glenohumeral joint.

Origin:  Inferior angle of scapula

Insertion: Medial portion of intertubercular sulcus of humerus

Action: Adduction, medial rotation, extension of humerus

Innervation: Lower subscapular nerve

The pectoralis major inserts on the lateral part of the intertubercular sulcus and the latissimus dorsi inserts on the floor of the intertubercular sulcus. A mnemonic for remembering this pattern of insertion is “The Lady Between Two Majors” – “Lady” referring to “Latissimus dorsi”, and the two majors being the pectoralis major and teres major.

Learn about the features of the scapula in this tutorial