The masseter muscle originates from the zygomatic arch, the bony ridge you can feel along your cheekbone. More specifically, its superficial portion attaches to the temporal process of the zygomatic bone and the front two-thirds of the zygomatic arch’s lower border, while its deep portion originates from the inner surface and back third of the same arch. These two layers work together to create one of the strongest muscles in the human body, responsible for closing the jaw and grinding food.
Superficial and Deep Layers
The masseter has long been described as a two-layered muscle, though recent research has added a third. The superficial layer is the larger, more visible portion. It starts from a thick sheet of connective tissue (called an aponeurosis) on the temporal process of the zygomatic bone and runs along the front two-thirds of the zygomatic arch’s lower edge. From there, its fibers angle downward and backward to attach to the outer surface of the mandible near its angle and lower border.
The deep layer sits underneath. It originates from the inner surface of the zygomatic arch, particularly its posterior third, and its fibers run more vertically downward to insert on the upper half of the mandibular ramus and the coronoid process. Because of this steeper angle, the deep layer is better positioned for stabilizing the jaw during chewing rather than generating raw closing force.
The Recently Discovered Third Layer
In 2021, a team led by researchers at the University of Basel described a previously unrecognized third layer of the masseter. This anatomically distinct portion runs from the inner (medial) surface of the zygomatic process of the temporal bone to the root and back edge of the coronoid process, a bony projection on the upper mandible. The researchers named it the “pars coronoidea,” or coronoid part. It was found consistently across dissected specimens, suggesting it’s a standard feature of human anatomy that had simply been overlooked, likely because of its deep position and close relationship to the temporalis muscle.
Embryological Development
The masseter develops from the first pharyngeal arch, one of a series of tissue folds that form in the embryo during the third and fourth weeks of gestation. By the fifth week, this first arch divides into the mandibular and maxillary processes, which go on to shape the lower and upper jaw. All the major chewing muscles arise from this same arch: the masseter, the temporalis, the medial and lateral pterygoids, and the front belly of the digastric muscle. They all share the same nerve supply (the mandibular branch of the trigeminal nerve) precisely because of this common embryological origin.
Neighboring Structures
The masseter sits in a busy part of the face. The parotid gland, your largest salivary gland, essentially wraps around it. The superficial part of the gland lies behind and alongside the muscle, while the deep part of the gland tucks behind it. The parotid duct, which carries saliva forward to the inside of your cheek, crosses directly over the muscle’s outer surface on its way to the mouth. This close relationship matters clinically: swelling or lumps in the masseter region can sometimes be confused with parotid gland problems, and vice versa.
How Strong the Masseter Is
The masseter consistently provides more leverage than any other jaw-closing muscle at every tooth position. In bite force studies, the highest recorded force at the first molar was about 576 newtons, roughly equivalent to 130 pounds of force, measured in a 20-year-old. Bite force drops significantly toward the front teeth, where the same individual generated only about 128 newtons at the central incisors. This gradient exists because the molars sit closer to the jaw joint and the masseter’s line of pull, giving them a mechanical advantage.
Bite force builds steadily through childhood and adolescence. The lowest recorded measurement in one study was just 5.4 newtons at a front tooth in a 9-year-old. By adulthood, forces at the back teeth can be 50 to 100 times greater, reflecting both muscle growth and changes in skull geometry as the face matures.
When the Masseter Gets Too Large
Because the masseter responds to workload like any other muscle, chronic overuse can cause it to enlarge noticeably. This condition, called benign masseter hypertrophy, produces a squared-off appearance to the lower face. Most people who seek treatment are bothered by the cosmetic change, though some experience jaw pain, headaches, difficulty opening the mouth, or a fatigued feeling while chewing.
The causes aren’t always clear, but the condition has been linked to teeth grinding (bruxism), habitual jaw clenching, frequent gum chewing, and temporomandibular joint disorders. Stress and emotional tension also play a role, since people under psychological strain often clench without realizing it, gradually increasing the muscle’s bulk over months or years.