| Masseter | Temporalis |
| Pterygoideus laterally (lateral pterygoid) | Pterygoideus medialis (medial pterygoid) |
Covering the Masseter, and firmly connected with it, is a strong layer of fascia derived from the deep cervical fascia and named the parotid fascia. It is attached to the lower border of the zygomatic arch, and invests the parotid gland.
The Masseter (fig. 591) is a quadrilateral muscle, consisting of two portions, superficial and deep. The superficial portion, the larger, arises by a thick aponeurosis from the zygomatic process of the maxilla, and from the anterior two-thirds of the lower border of the zygomatic arch; its fibers pass downwards and backwards, to be inserted into the angle and lower one-half of the lateral surface of the ramus of the mandible. The deep portion is much smaller and is partly concealed by the superficial portion; it arises from the posterior one-third of the lower border and from the whole of the medial surface of the zygomatic arch; its fibers pass almost vertically downwards to be inserted into the lateral surfaces of the coronoid process and the upper one-half of the ramus of the mandible.
Relations.-Superficial to the muscle are the integument, the Platysma, the Risorius, the Zygomaticus major, and the parotid gland; the parotid duct, branches of the facial nerve, and the transverse facial vessels cross the muscle. The deep surface is in relation with the insertion of the Temporal muscle and the ramus of the mandible; a mass of fat separates it from the Buccinator muscle and the buccal nerve. The masseteric nerve and artery reach the deep surface of the muscle bypassing through the posterior part of the mandibular notch. The posterior margin is overlapped by the parotid gland; the anterior margin projects over the Buccinator and is crossed below by the anterior facial vein.
Nerve-supply.-The Masseter is supplied by a branch of the anterior trunk of the mandibular nerve.
Actions.-The Masseter pulls the mandible towards and against the maxillae; from its relation to the axis of movement it can act with very great force.
The temporal fascia covers the Temporal muscle. It is a strong, fibrous investment, covered, laterally, by the Auriculares anterior et superior, the epicrainal aponeurosis, and part of the Orbicularis oculi. The superficial temporal vessels and the auriculotemporal nerve cross it from below upwards. Above, it is a single layer, attached to the entire extent of the superior temporal line; below, it consists of two layers, one of which is attached to the lateral, and the other to the medial border of the zygomatic arch. A small quantity of fat, the zygomatic branch of the superficial temporal artery, and the zygomaticotemporal branch of the maxillary nerve, are contained between these two layers. The deep surface of the fascia affords attachment to the superficial fibers of the Temporalis.
The Temporalis (fig. 585) is a fan-shaped muscle, situated at the side of the head. It arises from the whole of the temporal fossa (except the portion formed by the zygomatic bone) and from the deep surface of the temporal fascia. Its fibers converge as they descend, and end in a tendon which passes through the gap between the zygomatic arch and the side of the skull, and is inserted into the medial surface; apex, and anterior border of the coronoid process, and the anterior border of the ramus of the mandible nearly as far forwards as the last molar tooth.
Relations.—Superficial to the muscle are the skin, the Auriculares anterior et superior, the temporal fascia, the superficial temporal vessels, the auriculotemporal nerve, the temporal branches of the facial nerve the zygomaticotemporal nerve, the epicranial aponeurosis, the zygomatic arch, and the Masseter. The deep surface is in relation with the temporal fossa, the Lateral pterygoid and part of the Buccinator the maxillary artery and its deep temporal branches, the deep temporal nerves, and the buccal vessels and nerve. Behind the tendon of the muscle the vessels and nerve to the Masseter traverse the mandibular notch. The anterior border is separated from the zygomatic bone by a mass of fat.
Actions.-The Temporal muscle elevates the mandible and so closes the mouth. This movement requires both the upward pull of the anterior fibers and the backward pull of the posterior fibers, because the head of the mandible rests on the articular eminence when the mouth is open. The posterior fibers draw the mandible backwards after it has been protruded.
The Pterygoideus lateralis (Pterygoideus externus) (fig. 586) is a short, thick muscle, somewhat conical in form. It arises by two heads: an upper from the infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone; and a lower from the lateral surface of the lateral pterygoid plate. Its fibers pass backwards and laterally, to be inserted into a depression on the front of the neck of the mandible, and into the articular capsule and disc of the mandibular articulation.
Relations.-Its superficial surface is in relation with the ramus of the mandible, the maxillary artery, which crosses it, the tendon of the Temporal muscle and the Masseter. Its deep surface rests against the upper part of the Pterygoideus medialis, the sphenomandibular ligament, the middle meningeal artery, and the mandibular nerve; its upper border is in relation with the temporal and masseteric branches of the mandibular nerve; its lower border with the lingual and inferior dental (alveolar) nerves. The buccal (buccinator) nerve and the maxillary artery pass between the heads of the muscle.
The artery often lies deep to the muscle, as shown in Fig. 587.
Nerve-supply.-The Lateral pterygoid muscle is supplied by a branch from the anterior trunk of the mandibular nerve.
Actions.-The Lateral pterygoid muscle assists ia opening the mouth, by pulling forward the condyloid process of the mandible and the articular disc, while the body of the mandible is being depressed by the suprahyoid muscles. Acting with the Medial pterygoid muscle it protrudes the mandible so that the lower incisors are projected in front of the upper.
The Pterygoideus medialis (Pterygoideus internus) (fig. 586), a thick, quadrilateral muscle, arises from the medial surface of the lateral pterygoid plate, and from the grooved surface of the tubercle (pyramidal process) of the palatine bone; it has also a more superficial slip of origin which arises from the lateral surfaces of the tubercle of the palatine bone and tuberosity of the maxilla, and lies at first on the surface of the lower part of the lower head of the Lateral pterygoid muscle. Its fibers pass downwards, laterally, and backwards, and are inserted, by a strong tendinous lamina, into the lower and back part of the medial surfaces of the ramus and angle of the mandible, as high as the mandibular foramen.
Nerve-supply.—The Medial pterygoid muscle is supplied by a branch from the mandibular nerve.
Actions-The Medial pterygoid muscle assists in approximating the mandible to the maxillae. Acting with the Lateral pterygoid it protrudes the mandible, When the two Pterygoid muscles of one side are in action, the corresponding side of the mandible is drawn forwards, while the head of the mandible on the opposite side remains comparatively fixed; by an alternating action of the muscles of the two sides, the side-to-side movements, such as take place during trituration of the food, are effected.
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