The structure indicated is the latissimus dorsi muscle.

The latissimus dorsi muscle is one of the superficial muscles of the back. There are three groups of back mucsles:

1. Superficial
2. Intermediate
3. Deep

The superficial group of back muscles consists of 5 muscles:

1. Trapezius
2. Latissimus dorsi
3. Levator scapulae
4. Rhomboid major
5. Rhomboid minor

“Latissimus dorsi” translates literally from the Latin has the “broadest muscle of the back”, which reflects its large, flat structure. The latissimus dorsi muscle is involved in several movements of the upper limbs as well as movement at the lumbar spine and it also plays a role as an accessory muscle of inspiration.

Origin: Spinous processes of T7-L5, sacrum, iliac crest, ribs 10-12

Insertion: Bicipital groove of humerus

Action: extension, adduction and medial rotation of humerus

Innervation: thoracodorsal nerve

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