Final answer:
The medial pterygoid is the deep counterpart to the masseter muscle in the muscles of mastication, assisting in side-to-side mandible movements during chewing.
Step-by-step explanation:
The deep counterpart to the masseter muscle in the muscles of mastication is the medial pterygoid. While the masseter and temporalis are primarily responsible for elevating and closing the jaw to break down food, the medial pterygoid aids in the side-to-side movement of the mandible, which is essential for the grinding aspect of mastication. This is important as it allows for the chewing and manipulation of food within the mouth.