Which muscle inserts into the mastoid process?

The muscle that inserts into the mastoid process is the sternocleidomastoid. This key muscle plays a significant role in head movement and neck flexion. It originates from two points: the clavicle and the sternum, and it traverses upward to insert on the mastoid process of the temporal bone behind the ear.

The sternocleidomastoid’s primary functions include turning the head to the opposite side and tilting the head toward the shoulder on the same side. Because of its pivotal anatomical position and role in various movements, the sternocleidomastoid is a prominent muscle in both anatomy and clinical practice.

Other muscles listed, such as the trapezius, splenius, and scalenes, have different points of insertion and functions. The trapezius primarily affects shoulder movement, the splenius contributes to the extension and rotation of the head, and the scalenes are involved in neck flexion and assist with breathing. None of these muscles insert into the mastoid process, making the sternocleidomastoid the correct answer in this context.