Deep part of masseter

At a glance

English

Deep part of masseter

Latin (TA2)

Pars profunda masseteris

System

Muscle

Region

Head

Side

Paired, left and right

About the deep part of masseter

The masseter is the thick, four-sided chewing muscle at the side of the face, easy to feel bulging at the angle of the jaw when the teeth are clenched. It has two layers that fuse at their lower ends. The deep part is the smaller and more fleshy of the two, lying under the superficial part in front and under the parotid gland behind.

It takes origin from the zygomatic arch: from the rear third of its lower edge and from its entire inner surface. Its fibres run almost vertically downwards, with a slight forward slope, and attach to the upper half of the lateral surface of the mandibular ramus, reaching up to the base of the coronoid process. Because its fibres are more upright than those of the superficial part, which slope down and back, the two layers pull in slightly different directions.

Together with the temporalis and the medial pterygoid, the masseter raises the mandible and closes the mouth, and it supplies much of the force of the bite. The near-vertical deep fibres are the chief closing fibres; their posterior portion also helps to pull the mandible backwards after it has been protruded, and they help to steady the condyle in the temporomandibular joint. Like all the muscles of mastication, it is supplied by the mandibular division of the trigeminal nerve, through the masseteric nerve, which reaches its deep surface through the mandibular notch with the masseteric artery.

The muscle is examined by placing the fingers over both cheeks and asking the patient to clench several times; the two sides should harden equally. People who habitually clench or grind their teeth, or chew gum constantly, may develop painless enlargement of the masseter, usually on both sides but sometimes unequal. This masseteric hypertrophy can be mistaken for a swelling of the parotid gland, a dental infection or a tumour, but its outline matches the muscle. Spasm of the masseter, or trismus, can also be an early sign in malignant hyperthermia under anaesthesia.

Attachments, action and supply

Origin

Rear third of the lower edge of the zygomatic arch, plus its entire inner surface.

Insertion

Upper half of the lateral surface of the mandibular ramus, up to the coronoid process.

Action

Elevates the mandible to close the jaw; its posterior fibres help retract the mandible and steady the temporomandibular joint.

Nerve supply

Masseteric nerve, a branch of the mandibular nerve (V3).

Blood supply

Masseteric artery from the maxillary artery, with branches of the facial and transverse facial arteries.