The gross anatomy and internal architecture of the jaw musculature inRattus norvegicushas been studied using a combination of gross dissection, thick sectioning and histological techniques. As the basis of an examination of the mechanics of the muscles, the following are described:temporal, masseter, internal and external pterygoids, digastric, mylohyoid, geniohyoid and transverse mandibular.On the basis of their anatomy, it is concluded that these muscles form a series of slings around the jaw:a vertical sling (anterior temporal and deep masseter in particular) suspending the jaw from the cranium; a horizontal sling (posterior temporal and superficial masseter) initiating and controlling horizontal movements. The pterygoids and the suprazygomatic slip of the posterior temporal probably form a subsidiary sling and act to stabilize the jaw and particularly the condyle during movement.