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.