ATLAS OF HUMAN ANATOMY - G.L. Bilich - 2014

Muscular System

MUSCLES AND FASCIAE OF THE HEAD

Muscles of Mastication

The Muscles of Mastication, developing from the mesenchyme of the first visceral (mandibular) arch, originate on the bones of The Skull and insert into the Mandible, providing its movements at the temporomandibular joint. All muscles of mastication are innervated by Branches of the Trigeminal nerve. They include the masseter, temporalis, medial pterygoid, and lateral pterygoid muscles (Table 38).

Class="center">Table 38. Muscles of Mastication (Innervated by Branches of the Trigeminal Nerve)

The masseter muscle (m. masseter) is a thick, quadrilateral plate located in the parotid region, covering the ramus of the mandible externally (Fig. 229). The muscle originates by a broad, thick tendon from the lower border of the anterior two-thirds of the zygomatic arch, as well as by muscle fibers from the lower border and internal surface of the posterior third of the zygomatic arch. Based on fiber orientation, the muscle is divided into two parts: superficial (larger) and deep (smaller).

The fibers of the superficial part run downwards and backwards, while those of the deep part run almost vertically from top to bottom. The muscle inserts into the masseteric tuberosity and the lateral surface of the coronoid process of the mandible. The outer surface of the masseter is covered by its dense fascia, and posteriorly by the parotid gland. The excretory duct of this gland crosses the anterior part of the muscle in a posteroanterior direction at the level between its upper and middle thirds.

Function: The masseter elevates the mandible, primarily the mandibular angle (pressing the lower molar Teeth against the upper molars), generating significant force ("crushing muscle"). The superficial part of the muscle assists in protruding the mandible forward.

Fig. 229. Masseter and temporalis muscles, right view:

1 — Lateral ligament; 2 — Styloid process; 3 — Joint Capsule; Articular capsule; 4 — Mastoid process; 5 — External acoustic meatus; 6 — Temporalis; Temporal musclc; 7— Parietal bone; 8 — Frontal bone; 9— Zygomatic arch; 10 — Masseter, deep part; 11 — Masseter, superficial part

Blood supply: masseteric artery, transverse facial artery.

The temporalis muscle (m. temporalis) is thick, broad superiorly and narrow inferiorly, occupying the entire temporal fossa. It originates from the temporal surface of the parietal bone and the squamous part of the Temporal bone, as well as the inner surface of the temporal fascia. The muscle fibers run downwards, converging into a short, thick, and narrow tendon that inserts into the coronoid process of the mandible (see Fig. 229).

Function: The temporalis elevates the mandible, acting primarily on the anterior teeth ("biting muscle"). The posterior fibers retract a protruded mandible.

Blood supply: superficial and anterior deep temporal Arteries.

The medial pterygoid muscle (m. pterygoideus medialis) is a thick, quadrilateral plate located in the infratemporal and pterygoid fossae, lying against the Medial surface of the mandibular ramus (Figs. 230, 231). The muscle originates within the pterygoid fossa of the pterygoid process of the sphenoid bone. Its fibers run downwards, laterally, and backwards in the same direction as the fibers of the masseter. The muscle inserts into the pterygoid tuberosity on the inner surface of the mandibular angle.

Function: The medial pterygoid elevates the mandible and protracts it forward.

Blood supply: maxillary and facial arteries.

The lateral pterygoid muscle (m. pterygoideus lateralis) is a thick, short plate located in the infratemporal and pterygoid fossae. Its fibers are oriented anteroposteriorly. The muscle has two heads: superior and inferior. The superior HEAD originates from the infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone; the inferior head originates from the lateral surface of the lateral plate of the pterygoid process of the sphenoid bone. Converging together, the two heads form a muscle belly that courses posteriorly and laterally to insert into the anterior surface of the neck of the mandible, the articular capsule of the temporomandibular joint, and the articular disc. The buccal nerve and buccal artery pass between the two heads of the muscle (see Figs. 230, 231).

Function: Upon unilateral contraction, the lateral pterygoid shifts the mandible to the opposite side; upon bilateral contraction, it protrudes the mandible forward. The muscle pulls the articular capsule and articular disc of the temporomandibular joint forward, protecting the capsule from being pinched and displacing the disc along with the condylar process of the mandible.

Blood supply: maxillary and facial arteries.

The Nature of mandibular movements driven by the masticatory muscles is illustrated in Fig. 232.

Fig. 230. Muscles of mastication, right lateral view (the zygomatic arch and coronoid process of the mandible are sectioned and removed, the tendon of the temporalis muscle is transected and reflected upward):

1 — Medial pterygoid; 2 — Lateral pterygoid, lower head; inferior head; 3 — Lateral pterygoid, upper head; superior head; 4 — Temporalis; Temporal muscle; 5 — Temporal muscle; Tendon; 6 — Articular disc

Fig. 231. Muscles of mastication, posterior view from the inside:

1 — Head of mandible; Articular surface; 2 — Articular disc; 3 — Lateral pterygoid, upper head; superior head: 4 — Temporalis; Temporal muscle; 5 — Lateral pterygoid, lower head; inferior head; 6 — Masseter, deep part; 7— Masseter, superficial part; 8— Medial pterygoid

Fig. 232. Directions of mandibular movements in the temporomandibular joint, right lateral view (A — diagram of overall mandibular movements, B — showing Movements of the mandibular head within the joint):

1— Lateral pterygoid; 2 — Articular tubercle; 3 — Articular disc; 4 — Mandibular fossa