_Structure._—The superficial face of the muscle in its upper part is covered by a strong, glistening aponeurosis, and several tendinous intersections partially divide the muscle into layers. The fibers of the superficial layer take origin from the malar and maxilla only, and diverge somewhat to their insertion close to the thick ventral border of the lower jaw. The fibers of the deep layer arise from the entire area of origin, and pass straight to the border of the mandible; it will be noted that a small part, near the temporo-maxillary joint, is not covered by the superficial layer. The two layers are separable only above and behind; elsewhere they are fused. _Relations._—Superficially, the skin and panniculus, the parotid gland, the transverse facial and masseteric vessels, and the facial nerve; deeply, the ramus of the mandible, the buccinator, depressor labii inferioris, and mylo-hyoideus muscles, the superior buccal glands, the buccinator nerve, and two large varicose veins which join the facial vein at the anterior edge of the muscle. The facial vessels and parotid duct run along the anterior edge of the muscle; the duct, however, bends forward about the middle of the border and leaves the muscle. _Blood-supply._—Transverse facial and masseteric arteries. _Nerve-supply._—Mandibular nerve. 2. =Temporalis.=—This muscle occupies the temporal fossa. _Origin._—The rough part of the temporal fossa and the crests which surround it. _Insertion._—The coronoid process of the mandible, which it envelops. _Action._—Chiefly to raise the lower jaw, acting with the masseter and internal pterygoid muscles. _Structure._—The surface of the muscle is covered with a glistening aponeurosis, and strong tendinous intersections are found in its substance. The inner edge of the muscle is quite thin, but as the fibers converge toward the much smaller area of insertion, the muscle becomes nearly an inch thick. It fuses somewhat with the masseter. _Relations._—Superficially, the scutiform cartilage and anterior muscles of the external ear and the orbital fat; deeply, the temporal fossa and the deep temporal vessels and nerves. _Blood-supply._—Superficial and deep temporal, and mastoid arteries. _Nerve-supply._—Mandibular nerve. 3. =Pterygoideus internus= (s. medialis).—This muscle occupies a position on the inner surface of the ramus of the mandible similar to that of the masseter on the outside. _Origin._—The crest formed by the pterygoid process of the sphenoid and the palate bone. _Insertion._—The concave inner surface of the broad portion of the ramus of the mandible, and the inner lip of the lower border. _Action._—Acting together, to raise the lower jaw; acting singly, to produce also lateral movement of the jaw.