The structure indicated is the teres minor muscle.
The teres minor 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:
- Teres minor
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: Superolateral portion of scapula border
Insertion: Greater tubercle of humerus (inferior facet)
Action: Lateral rotation of arm. Assists in stabilisation of glenohumeral joint.
Innervation: Axillary nerve
Learn more about the anatomy of the rotator cuff in this tutorial.