The temporalis muscle is a large, fan-shaped muscle located in the temporal region of the skull. It originates from the temporal fossa, a depression on the side of the skull, and inserts into the coronoid process of the mandible (lower jawbone). The temporalis muscle is one of the muscles of mastication, and it is responsible for closing the jaw and retracting the mandible. The muscle is innervated by the mandibular branch of the trigeminal nerve (cranial nerve V) and works in conjunction with other muscles of mastication, such as the masseter and medial pterygoid muscles, to produce efficient jaw movements during mastication. Dysfunction or weakness of the temporalis muscle can lead to difficulties with chewing and opening the mouth. The temporalis muscle is also involved in facial expressions, as it works with other muscles of the face to produce certain expressions, such as clenching the teeth or furrowing the brow.