MANDIBULAR MUSCLES 261 



1. Masseter. — This muscle extends from the zygomatic arch and facial crest 

 over the broad part of the mandibular ramus. It is semi-elliptical in outline. 



Origin. — By a strong tendon from the zygomatic arch and the facial crest. 



Insertion. — The lateral surface of the broad part of the ramus of the mandible. 



Action. — Its action is to bring the jaws together. Acting singly, it also carries 

 the lower jaw toward the side of the contracting muscle. 



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-mandibular 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 cutaneus, the parotid gland, the 

 transverse facial and masseteric vessels, and the facial nerve; deeply, the ramus of 

 the mandible, the buccinator and depressor labii inferioris muscles, the superior 

 buccal glands, the buccinator vessels and nerve, and the vena reflexa, which joins 

 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 for- 

 ward 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 limit it. 



Insertion. — The coronoid process of the mandible, which it envelops. 



Action. — Chiefly to raise the lower jaw, acting with the masseter and medial 

 pterygoid muscles. 



Structure. — The surface of the muscle is covered with a glistening aponeurosis, 

 and strong tendinous intersections are found in its substance. The medial 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 partly with the 

 masseter. 



Relations. — Superficially, the scutiform cartilage, the anterior muscles of the 

 external ear, and the auricular and orbital fat ; deeply, the temporal fossa and the 

 deep temporal vessels and nerves. 



Blood-supply. — Superficial and deep temporal, and posterior meningeal arteries. 



Nerve-supply. — Mandibular nerve. 



3. Pterygoideus medialis (s. internus). — This muscle occupies a position on 

 the medial surface of the ramus of the mandible similar to that of the masseter 

 laterally. 



Origin. — The crest formed by the pterygoid process of the sphenoid and the 

 palatine bone. 



Insertion. — The concave medial surface of the broad part of the ramus of the 

 mandible and the medial lip of the ventral border. 



Action. — Acting together, to raise the lower jaw; acting singly, to produce also 

 lateral movement of the jaw. 



Structure. — The muscle is capable of division into two parts. The principal 

 part is superficial (medial), and its fibers are, for the most part, vertical in direc- 

 tion. It contains much tendinous tissue (septa). The smaller portion is lateral 

 to the foregoing, and its fibers are directed downward and backward. 



Relations. — Laterally, the ramus of the mandible, the lateral pterygoid muscle, 

 the inferior alveolar vessels and nerve, and the lingual and mylo-hyoid nerves; 



