The masseter is a large muscle with two heads (although some researcher recently described a third, small head this will need to be vetted before it is accepted) that originate from the zygomatic arch and the adjacent maxillary process of the zygomatic bone. It inserts along the lateral border of the mandible in the area of the angle and up the ramus. Its main action is to elevate the mandible and it is capable of exerting great force doing this. The two heads run in slightly different directions. The smaller superficial head arises more anteriorly and is oriented so that it angles posteriorly as it descends. Given this angle this superficial head is capable of assisting in mandibular protrusion in addition to its role in elevation. The deep head is larger and oriented more vertically so is confined to elevating the mandible. As both heads of the muscle are superficial to the mandible, it is easy to place your fingers on the masseter and feel it tense as one does either of those movements. If you compare the tension of the muscle when clenching or chewing you will find that the pattern is different in protrusion due to the inactivity of the deep head in protrusion.