Which muscle is NOT one of the four muscles of mastication?

The orbicularis oris is not one of the four muscles of mastication. The muscles responsible for mastication include the medial pterygoid, temporalis, lateral pterygoid, and masseter, all of which are involved in the movement of the jaw for chewing.

The orbicularis oris, on the other hand, is a muscle that surrounds the mouth and is primarily responsible for movements related to facial expression and actions such as puckering the lips or closing the mouth. Its functions are not associated with the mechanics of chewing. Understanding the role of the individual muscles in mastication helps clarify why the orbicularis oris is excluded from this group, emphasizing its distinct anatomical and functional characteristics.