Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

2 Muscles of the head

2.1 Muscles of Facial Expression, Overview

A Muscles of facia I expression

Anterior view. The superficial layer of muscles is shown on the right half of the face, the deep layer on the left half. The muscles of facial expression represent the superficial muscle layer in the face and vary greatly in their development among different individuals. They arise either directly from the periosteum or from adjacent muscles to which they are connected, and they insert either onto other facial muscles or directly into the connective tissue of the skin. The classic scheme of classifying the other somatic muscles by their origins and insertions is not so easily adapted to the facial muscles. Because the muscles of facial expression terminate directly in the subcutaneous fat and because the superficial body fascia is absent in the face, the surgeon must be particularly careful when dissecting in this region. Because of their cutaneous attachments, the facial muscles are able to move the facial skin (e.g., they can wrinkle the skin, an action temporarily abolished by botulinum toxin injection) and produce a variety of facial expressions. They also serve a protective function (especially for the eyes) and are active during food ingestion (closing the mouth for swallowing). All of the facial muscles are innervated by branches of the facial nerve, while the muscles of mastication (see p. 48) are supplied by motor fibers from the trigeminal nerve (the masseter muscle has been left in place to represent these muscles). A thorough understanding of muscular anatomy in this region is facilitated by dividing the muscles into different groups (see p. 47).

B Muscles of facial expression

Left lateral view. The superficial muscles of the ear and neck are particularly well displayed from this perspective. A tough tendinous sheet, the galea aponeurotica, stretches over the calvaria and is loosely attached to the periosteum. The muscles of the calvaria that arise from the galea aponeurotica are known collectively as the “epicranial muscle.” The two bellies of the occipitofrontalis (frontal and occipital) can be clearly identified. The temporoparietalis, whose posterior part is called the auricularis superior muscle, arises from the lateral part of the galea aponeurotica.

A Muscles of facial expression: palpebral fissure and nose

a Anterior view. The most functionally important muscle is the orbicularis oculi, which closes the palpebral fissure (protective reflex against foreign matter). If the action of the orbicularis oculi is lost because of facial nerve paralysis (see also D), the loss of this protective reflex will be accompanied by drying of the eye from prolonged exposure to the air. The function of the orbicularis oculi is tested by asking the patient to squeeze the eyelids tightly shut.

b The orbicularis oculi has been dissected from the left orbit to the medial canthus of the eye and reflected anteriorly to demonstrate its lacrimal part (called the Horner muscle). This part of the orbicularis oculi arises mainly from the posterior lacrimal crest, and its action is a subject of debate (expand or empty the lacrimal sac).

В Muscles of facial expression: mouth

a Anterior view, b left lateral view, c left lateral view of the deeper lateral layer.

The orbicularis oris forms the muscular foundation of the lips, and its contraction closes the oral aperture. Its function can be tested by asking the patient to whistle. Facial nerve paralysis may lead to drinking difficulties because the liquid will trickle back out of the unclosed mouth during swallowing. The buccinator lies at a deeper level and forms the foundation of the cheek. During mastication, this muscle moves food in between the dental arches from the oral vestibule.

2.2 Muscles of Facial Expression, Actions

C Changes of facial expression

a Contraction of the orbicularis oculi at the lateral canthus of the eye expresses concern.

b Contraction of the corrugator supercilii occurs in response to bright sunlight: “thoughtful brow.”

c Contraction of the nasalis constricts the naris and produces a cheery or lustful facial expression.

d Forceful contraction of the levator labii superioris alaeque nasi on both sides is a sign of disapproval.

e Contraction of the orbicularis oris expresses determination.

f Contraction of the buccinator signals satisfaction.

g The zygomaticus major contracts during smiling.

h Contraction of the risorius reflects purposeful action.

I Contraction of the levator anguli oris signals self-satisfaction.

j Contraction of the depressor anguli oris signals sadness.

к Contraction of the depressor labii inferioris depresses the lower lip and expresses perseverence.

I Contraction of the mentalis expresses indecision.

D Muscles of facial expression: functional groups

The various mimetic muscles are easier to learn when they are studied by regions. It is useful clinically to distinguish between the muscles of the forehead and palpebral fissure and the rest of the mimetic muscles. The muscles of the forehead and palpebral fissure are innervated by the superior branch of the facial nerve, while all the other mimetic muscles are supplied by other facial nerve branches. As a result, patients with central facial nerve paralysis can still close their eyes while patients with peripheral facial nerve paralysis cannot (see p. 79 for further details).

* Letters refer to sub-entries in C.

2.3 Muscles of Mastication, Overview and Superficial Muscles

Overview of the muscles of mastication

The muscles of mastication in the strict sense consist of four muscles: the masseter, temporalis, medial pterygoid, and lateral pterygoid.

The primary function of all these muscles is to close the mouth and move the upper teeth againstthe lower teeth in a grinding action during mastication. The lateral pterygoid muscle assists in opening the mouth. The two pterygoid muscles are also active during mastication (for the individual muscle actions, see A-C).

The mouth is opened primarily by the suprahyoid muscles and the force of gravity. The masseter and medial pterygoid form a muscular sling in which the mandible is suspended (see p. 50).

Note: all muscles of mastication are innervated by the mandibular nerve (third division of the trigeminal nerve), while the muscles of facial expression are innervated by the facial nerve.

D Temporalis and masseter

Left lateral view, a Superficial layer, b deep layer. The masseter and zygomatic arch have been partially removed. The full extent of the temporalis is shown in b. It is the most powerful muscle of mastication and does approximately half the work of mastication. The masseter consists of a superficial part and a deep part. The temporalis and masseter act powerfully in raising the mandible and closing the mouth. A small portion of the lateral pterygoid is visible in b.

2.4 Muscles of Mastication: Deep Muscles

A Lateral and medial pterygoid muscles

Left lateral views.

a The coronoid process of the mandible has been removed here along with the lower part of the temporalis so that both pterygoid muscles can be seen (see p.49 Db).

b Here the temporalis has been completely removed, and the inferior part of the lateral pterygoid has been windowed. The lateral pterygoid initiates mouth opening, which is then continued by the suprahyoid muscles. With the temporomandibular joint opened, we can see that fibers from the lateral pterygoid blend with the articular disk. The lateral pterygoid functions as the guide muscle of the temporomandibular joint. Because its various parts (superior and inferior) are active during all movements, its actions are more complex than those of the other muscles of mastication. The medial pterygoid runs almost perpendicular to the lateral pterygoid and contributes to the formation of a muscular sling that partially encompasses the mandible (see B).

C Muscles of mastication, coronal section at the level of the sphenoid sinus

Posterior view. The topography of the muscles of mastication and neighboring structures is particularly well displayed in this section.

2.5 Muscles of the Head, Origins and Insertions

A Muscle origins and insertions on the skull

a Left lateral view, b view of the inner surface of the right hemimandible, c inferior view of the base of the skull.

The origins and insertions of the muscles are indicated by color shading (origin: red, insertion: blue).