Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Musculoskeletal System
Muscular System
Muscles of the head

Muscles of Mastication. The muscles of mastication include the temporal, masseter, medial pterygoid, and lateral pterygoid muscles. They develop from the mesoderm of the first visceral (mandibular) arch. The coordinated and diverse actions of these muscles produce complex chewing movements.

The temporal Muscle (m. temporalis) originates in a fan-like shape from the temporal fossa (Atlas, Figs. 26, 37, 38). Converging downwards, the muscle fibers pass beneath the zygomatic arch and insert into the coronoid process of the Mandible.

The masseter muscle (m. masseter) originates from the zygomatic arch and inserts into the outer roughness of the angle of the mandible (Atlas, Figs. 26, 37, 38, 40).

Both the temporal and masseter muscles possess dense fasciae which, attaching to the bones surrounding these muscles, form osteofibrous sheaths for them.

The medial pterygoid muscle (m. pterygoideus medialis) originates from the pterygoid fossa of the Sphenoid bone and inserts into the inner roughness of the angle of the mandible (Atlas, Fig. 37B).

All three described muscles of mastication elevate the mandible. In addition, the masseter and medial pterygoid muscles project the jaw slightly forward, while the posterior bundles of the temporal muscles pull it backward. Upon unilateral contraction, the medial pterygoid muscle shifts the mandible to the opposite side.

The lateral pterygoid muscle (m. pterygoideus lateralis) lies in a horizontal plane, originates from the outer plate of the pterygoid process of the sphenoid bone, and, running backward, inserts into the neck of the mandible (Atlas, Fig. 37B). Upon unilateral contraction, the muscle pulls the mandible to the opposite side; upon bilateral contraction, it protrudes it forward.

Facial muscles. The facial muscles develop from the mesoderm of the second visceral (hyoid) arch. By one end, they attach to the BONES OF THE Skull, and by the other, to the facial Skin (Atlas, Figs. 38, 39). These muscles lack fasciae. Through their contractions, they shift the skin and produce facial expressions, i.e., expressive Movements of the face.

The facial muscles are grouped around the natural openings of the face, with one of them covering the cranial vault. This topography has a historical basis. In the animal ancestors of humans, the precursors of the facial muscles participated in grasping and holding food, protected Sensory Organs from excessive stimuli, or, conversely, facilitated their perception. During The process of anthropogenesis, these muscles gradually began to express sensations and emotions, especially with The Emergence of articulate speech in early humans. Participation in the act of speech caused the differentiation of muscles in the Regions of the Mouth and eyes. Since the SENSE OF SMELL is of secondary importance in humans, a reduction of muscles occurred in the Nose region; the absence of the need to prick up the ears led to the reduction of muscles associated with the auricle.

The facial muscles include: the epicranius (with frontal and occipital bellies); procerus; orbicularis oculi; corrugator supercilii; orbicularis oris; levator anguli oris; depressor anguli oris; buccinator; levator labii superioris; zygomaticus; risorius; depressor labii inferioris; mentalis; nasalis, and auricular muscles.

The epicranius muscle (m. epicranius) is primarily represented by an aponeurosis (galea aponeurotica) covering the cranial vault like a helmet. The aponeurosis transitions into small muscle bellies: posteriorly, the occipital bellies (venter occipitales), attaching to the highest nuchal line; anteriorly, the more developed frontal bellies (venter frontales), blending into the skin of the superciliary arches. When the aponeurotic helmet is fixed by the occipital bellies, the contraction of the frontal bellies creates horizontal wrinkles on the forehead and raises the eyebrows (Fig. 1.56). With sufficient Development of the epicranial bellies, their contraction sets the scalp in motion.

The procerus muscle (m. procerus) originates from the dorsum of the nose and inserts into the skin over the bridge of the nose. When contracting, the muscle forms vertical wrinkles in this area (Fig. 1.56).

The orbicularis oculi muscle (m. orbicularis oculi) is located in the orbital region and is divided into three parts: orbital, palpebral, and lacrimal. The orbital part is formed by the most peripheral fibers of the muscle; upon contraction, they tightly close the eye (Fig. 1.56). The palpebral part consists of fibers embedded beneath the skin of the eyelids; upon contraction, they close the eyes. The lacrimal part is represented by fibers surrounding the lacrimal sac; upon contraction, they dilate it, which facilitates the outflow of tear fluid into the nasolacrimal duct.

The corrugator supercilii muscle (m. corrugator supercilii) originates from the nasal part of the Frontal bone, runs laterally, and, piercing the frontal belly of the epicranius, inserts into the skin of the forehead in the region of the superciliary arches. Upon contraction, the muscle creates horizontal wrinkles on the forehead (Fig. 1.56).

The orbicularis oris muscle (m. orbicularis oris) is a complex of muscle fibers that form part of the upper and lower Lips. It consists primarily of circular fibers and, upon contraction, narrows the mouth. Several other facial muscles blend into the orbicularis oris.

The levator anguli oris muscle (m. levator anguli oris) originates from the canine fossa of the Maxilla. Descending toward the angle of the mouth, it inserts into the skin and mucosa, and blends into the orbicularis oris in the region of the lower lip.

The depressor anguli oris muscle (m. depressor anguli oris) originates from the Base of the mandible. Converging with its bundles toward the angle of the mouth, it inserts into the skin and blends into the orbicularis oris in the region of the upper lip.

The latter two muscles, when contracting simultaneously, close the lips.

The buccinator muscle (m. buccinator) lies deep within the cheek. With its upper bundles, it originates from the maxilla above its alveolar process; with its lower bundles, from the body of the mandible below the alveoli; and with its middle bundles, from the pterygomandibular raphe, a tendinous band connecting the skull base to the mandible (see Atlas, 30). Heading toward the angle of the mouth, the upper bundles of the buccinator blend into the lower lip, the lower bundles into the upper lip, and the middle bundles are distributed within the orbicularis oris. The primary function of the buccinator is to counteract intraoral pressure. By pressing the Cheeks and lips against the Teeth, it helps keep food between the masticatory surfaces of the teeth.

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Fig. 1.56. Anatomical analysis of facial expressions (drawn by V.G. Ostroumov): Bold lines indicate contracted muscles

1 and 3 — expression of surprise. The contraction of the frontal bellies of the epicranius raises the eyebrows, causing the eyes to open wide and transverse wrinkles to form on the forehead. The depressor anguli oris is contracted. The chin is elevated by the contraction of the mentalist muscle; 2 and 4 — expression of good nature. The contraction of the levator labii superioris deepens the nasolabial sulcus sharply, stretches the upper lip, and dilates the nostrils. The contraction of the nasalis forms vertical wrinkles on the nose. The contraction of the orbital part of the orbicularis oculi closes the eye tightly, and fan-like skin wrinkles form near its outer corner. In the region of the bridge of the nose, vertical wrinkles are caused by the contraction of the corrugator supercilii, while transverse wrinkles are caused by the contraction of the procerus

Adipose tissue (the buccal fat pad) accumulates on the buccinator muscle, especially in childhood (responsible for the roundness of children's cheeks).

The levator labii superioris muscle (m. levator labii superioris) originates with three heads from the frontal process and infraorbital margin of the maxilla and from the Zygomatic bone. The fibers run inferiorly and blend into the skin of the nasolabial fold. Upon contraction, they deepen this fold, raising and stretching the upper lip and widening the nostrils (Fig. 1.56).

The zygomaticus major muscle (m. zygmaticus maior) runs from the zygomatic bone to the angle of the mouth, which it pulls upward and laterally upon contraction (Fig. 1.56).

The risorius muscle (m. risorius) is inconsistent, extending as a thin bundle between the angle of the mouth and the cheek skin. When contracted, the muscle forms a dimple on the cheek.

The depressor labii inferioris muscle (m. depressor labii interioris) originates from the body of the mandible, deeper and more medially than the depressor anguli oris; it inserts into the skin of the lower lip, pulling it downward upon contraction.

The mentalis muscle (m. mentalis) originates from the alveolar hubs of the lower incisors, running downwards and medially; it attaches to the skin of the chin. Upon contraction, the muscle elevates and wrinkles the skin of the chin, causing dimples to form, and presses the lower lip against the upper lip (Fig. 1.56).

The nasalis muscle (m. nasalis) originates from the alveolus of the upper canine and lateral incisor. It consists of two parts: one that narrows the nostril and one that dilates it. The former ascends to the cartilaginous bridge of the nose, where it blends into a common aponeurosis with the muscle of the opposite side. The latter attaches to the Cartilage and skin of the nasal ala, pulling it downwards.

The anterior, superior, and posterior auricular muscles approach the auricle and the cartilaginous part of the external acoustic meatus. These muscles are rarely developed enough to move the auricle.