The structure indicated is the long head of the biceps brachii muscle.

The biceps muscle is one of three muscles in the anterior compartment of the arm:

1. Biceps brachii
2. Brachialis
3. Coracobrachialis

The muscles of the anterior compartment are innervated by the musculocutaneous nerve and generally serve to flex the forearm at the elbow joint.

The biceps brachii muscle is comprised of two muscular heads:

1. Short head
2. Long head

The long head of the biceps brachii originates on the supraglenoid tubercle of the scapula, and traverses the superior aspect of the glenohumeral joint to descend in the intertubercular sulcus (bicipital groove) of the humerus to enter the arm. The long head of the biceps is the larger of the two muscular heads. Since the long head tendon crosses the glenohumeral joint, the biceps brachii also has action at the glenohumeral joint and is an accessory flexor of the humerus.

The muscle bellies of the long and short heads converge distally to form a tendon which inserts onto the radial tuberosity of the radius.

Learn more about the anatomy of the arm muscles in this tutorial.