Which of the following is NOT one of the major muscles of mastication?

The deltoid muscle is not one of the major muscles of mastication. The primary role of the muscles of mastication is to facilitate the movements necessary for chewing and grinding food, which are essential functions associated with the jaw.

The masseter, temporalis, and medial pterygoid are all specifically involved in these activities. The masseter is responsible for elevating the mandible, which allows for the powerful closing of the jaw. The temporalis muscle also assists in elevating the mandible, while contributing to the retraction of the jaw. The medial pterygoid works alongside these muscles to help in both the elevation and side-to-side movements of the jaw during chewing.

In contrast, the deltoid muscle is primarily associated with the shoulder and upper arm. Its primary function is to enable arm movements such as lifting and overhead reaching, making it unrelated to the mastication process. Thus, it is clearly the muscle that does not belong to the group responsible for chewing.