The masseter muscle is a facial muscle that plays a major role in the chewing of solid foods. The muscle is shaped similar to a parallelogram, connecting to the mandible (lower jawbone) and the cheekbone.
The masseter is divided into two distinct sections, the "superficial" and "deep" portions. The superficial portion of the masseter is the thick and tendon-like portion of the muscle that connects to the cheekbone, while the deep portion is the smaller and more muscular portion of the muscle that connects to the mandible.
During chewing, the masseter is assisted by three other muscles: the temporalis, medial pterygoid, and lateral pterygoid. The four muscles work together to pull the jaw down and back up again. The masseter is the key muscle that pulls the mandible upward. All four muscles are connected to a single division of the trigeminal nerve.
Because of the sheer bulk of the masseter muscle, portions of it are sometimes removed by plastic surgeons performing jaw reduction surgery. Individuals who grind their teeth while sleeping may develop squared jaws as a result of the masseter growing due to the additional exercise it receives over time.