The structure indicated is the splenius capitis muscle.

The splenius capitis muscle is one of 8 muscles in the posterior triangle of the neck.

The posterior triangle of the neck is bounded as follows:

• Anteriorly: posterior border of sternocleidomastoid
• Posteriorly: anterior border of trapezius
• Apex: Occipital bone between attachment sites of trapezius and sternocleidmastoid
• Base: middle third of clavicle

Origin: Lower half of ligamentum nuchae and spinous processes of C7-T4

Insertion: Mastoid process, and part of skull inferior to lateral superior nuchal line.

Action: Contraction together – extension of the neck. Contraction individually – Lateral flexion of neck and rotation of head to same side.

Innervation: Posterior rami C3-C6

Learn more about the muscles of the posterior triangle of the neck in this anatomy tutorial.