The structure indicated is the infraspinatus muscle of the rotator cuff.
The infraspinatus muscle is one of the four muscles of the rotator cuff. The rotator cuff muscles are located in the posterior scapula region and serve to stabilise the glenohumeral joint. There are 4 rotator cuff muscles which can be remembered using the mnemonic SITS:
The supraspinatus and infraspinatus, as their names suggest originate in the fossae above and below the spine of the scapula respectively (supraspinatous and infraspinatous fossae). The supraspinatus, infraspinatus and teres minor muscles insert on to the greater tubercle of the humerus on the superior, middle and inferior facets, in that order.
Origin: Dorsal surface of scapula in infraspinatous fossa (below the spine of the scapula in)
Insertion: Greater tubercle of humerus (middle facet)
Action: Lateral (external) rotation of the humerus. Assists in stabilisation of the glenohumeral joint.
Innervation: Suprascapular nerve.
Learn more about the anatomy of the rotator cuff in this tutorial.