Which muscle has its insertion at the mastoid process and the superior nuchal line?

The muscle that inserts at the mastoid process and the superior nuchal line is the sternocleidomastoid. This muscle has two heads: the sternal head and the clavicular head, and its primary functions include rotation and flexion of the neck. The mastoid process is a prominent bony structure located just behind the ear, providing an attachment point for the sternocleidomastoid, which facilitates its role in head movement.

The superior nuchal line is a ridge on the occipital bone of the skull, and the insertion of the sternocleidomastoid here helps in stabilizing the head and neck. The anatomical significance of these insertion points underscores the muscle's importance in movements like turning the head, tilting, and extending the neck, as well as its role in respiratory function by aiding in forced inhalation when the head is fixed.

In contrast, other muscles listed do not have insertion points at the mastoid process or the superior nuchal line. For instance, while the trapezius does have attachments at the superior nuchal line, it also includes other points of origin and insertion that do not align with just the mastoid process. The scalenes, on the other hand