Human Anatomy (with the Fundamentals of Dynamic and Sports Morphology) - Ivanitsky M. F. 2008

Myology
Muscles of the head

All the Muscles of the HEAD are generally divided into two groups: the Muscles of Mastication (masticatory muscles) and the Muscles of facial expression (mimetic muscles). The first group is involved in the Movements of the Mandible, while the second group is responsible for facial expressions.

Muscles of Mastication

These muscles are capable of producing the following movements of the mandible:

1) elevation and depression;

2) forward and backward movements;

3) lateral movements (to the right and left).

These movements are performed by six functional Muscle groups.

Elevation of the Mandible

The Muscles responsible for elevating the mandible include:

1) the temporalis;

2) the masseter;

3) the medial pterygoid.

The temporalis muscle (Fig. 67) fills the entire temporal fossa, having a broad origin at its floor and a restricted insertion onto the coronoid process of the mandible. This muscle has a fan-shaped Structure. Its anterior fibers run vertically downward, the middle fibers obliquely forward, and the posterior fibers almost horizontally forward. The muscle is easily palpated beneath the Skin, especially upon contraction.

The primary function of the muscle is to elevate the mandible. In addition, its posterior fibers can retract the mandible.

The masseter muscle originates from the zygomatic arch and inserts into the lateral surface of the mandibular ramus—at its masseteric tuberosity—reaching as far as the Base of the coronoid process. It is a very powerful muscle. The direction of its fibers runs inferiorly and anteriorly. The contours of the muscle are easily visible and palpable in a living subject, especially when contracted. Its function is to elevate the mandible.

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Fig. 67. Muscles of mastication and facial expression:

1 — levator anguli oris m.; 2 — orbicularis oris m.; 3 — depressor labii inferioris m.; 4 — depressor anguli oris m.; 5 — buccinator m.; 6 — parotid duct (cut); 7 — lateral pterygoid m.; 8 — temporomandibular joint; 9 — temporalis m. (after G. F. Ivanov)

The medial pterygoid muscle originates from the pterygoid process of the Sphenoid bone and partly from the body of the Maxilla; it runs inferiorly and posteriorly to insert onto the Medial surface of the angle of the mandible. Both in its shape and fiber orientation, it corresponds to the masseter muscle. The medial pterygoid muscle elevates the mandible.

Depression of the Mandible

The muscles responsible for depressing the mandible include:

1) the digastric (see p. 206);

2) the Infrahyoid muscles (sternohyoid, thyrohyoid, etc.), which exert an indirect influence on the depression of the mandible (see p. 206);

3) the platysma (see p. 205).

The action of all these muscles is assisted by the force of gravity acting on the jaw itself.

Forward movement of the mandible

Forward movement of the mandible is driven by both the right and left lateral pterygoid muscles.

The lateral pterygoid muscle (see Fig. 67) is located in the infratemporal fossa, originating from the greater wing of the sphenoid bone and the lateral plate of its pterygoid process, and inserts into the neck of the mandible, while also sending fibers to the articular capsule of the temporomandibular joint. The function of the muscle is to protrude the mandible forward.

Backward movement of the mandible

This movement is produced by the contraction of the posteroinferior part of the temporalis muscle, with the participation of the digastric, stylohyoid, and geniohyoid muscles.

Lateral movement of the mandible

Lateral movements of the mandible are performed by the lateral pterygoid muscle; contraction of this muscle on the right side shifts the jaw to the left, and vice versa.

The bulk of the aforementioned muscles (the masseter, temporalis, and medial pterygoid) are involved in elevating the mandible. In contrast, the muscles that depress the mandible are relatively weak (the digastric muscle, as well as those attached to the mandible and the Hyoid bone). It should be noted that during mandibular depression, the axis of rotation passes not through the mandibular condyle, but slightly inferior and anterior to it; the digastric and lateral pterygoid muscles thus form a force couple, with the moment arm passing through the mandible (see Fig. 49). Elevation of the mandible is likewise the result of a force couple acting between the masseter and medial pterygoid muscles on one side and the temporalis on the other. During protrusive and retrusive movements of the mandible, the lateral pterygoid and the posterior part of the temporalis, together with the digastric muscle, act as antagonists.

If the mandible is immobilized by resting it against a rigid surface (for example, by supporting the chin with the hand), opening and closing the Mouth still remain possible. In these movements, it is not the mandible that moves relative to the rest of the head, but rather the entire head that moves relative to the mandible. During forced mouth opening, a concomitant backward movement of the head is typically observed, which increases the range of opening. This movement utilizes A number of powerful muscles of the nuchal region.

The masticatory muscles serve as a connecting link between the torso, the anterior Neck Muscles located above and below the hyoid bone, and the occipital muscles. Not only do they drive the mandible, but they also perform a static function. This function consists in helping to maintain the head in an elevated position. The ability to perform the circus feat of "hanging by the Teeth" demonstrates the immense lifting power possessed by the mandibular elevators. In a relaxed state, when these muscles are at rest, the mandible (even when the mouth is closed) is usually slightly lowered due to gravity, leaving a space between the upper and lower teeth.

Facial muscles

Facial expressions, as is well known, form part of body movements used to express various emotions and thoughts (pantomime). All muscles can participate in pantomimic movements. However, There is a specific group of muscles particularly adapted to expressing various emotional states (Fig. 68). These are muscles that attach to the skin of the face and lie mostly just beneath the skin. Some of them originate on bones and insert into the skin, while others are situated entirely within the soft Tissues of the face.

The primary function of the facial muscles is to open and close the facial apertures.

The Muscles surrounding the palpebral fissure and the oral aperture, as well as partly the nasal aperture, can be divided into two main categories: sphincters, which circularly surround and constrict a given opening, and dilators, which are arranged radially and serve to widen it.

The epicranial muscle has a unique structure compared to other muscles. Its central part is formed by the epicranial aponeurosis (or galea aponeurotica), which is a robust fibrous sheet covering the entire cranial vault. This sheet is firmly attached to the scalp and loosely connected to the periosteum of the Skull. Two bellies of this muscle attach to the aponeurosis: the frontal belly anteriorly and the occipital belly posteriorly.

Fig. 68. Diagram of potential changes in THE POSITION OF skin folds around the mouth, Nose, and eyes, which impart various expressions to the face:

a — sadness; b — calmness; c — joy; d — attention; e — contemplation, concern; f — questioning attention (after G.F. Ivanov)

The frontal belly (Fig. 69) is a thin muscle with vertically oriented muscle fibers. It attaches superiorly to the epicranial aponeurosis and inferiorly to the facial skin in the eyebrow region, covering nearly the entire forehead. The occipital belly acts as an antagonist to the frontal belly in movements of the scalp.

The occipital belly is located in the occipital region. It originates from the superior nuchal line and the base of the mastoid process, courses upward and laterally, and inserts into the epicranial aponeurosis.

The function of the occipital belly is to pull the epicranial aponeurosis posteriorly, along with the scalp. When the aponeurosis is fixed by the tension of this belly, contraction of the frontal belly elevates the eyebrows. Alternating contraction of the anterior and posterior PARTS OF THE epicranial muscle shifts the scalp anteriorly and posteriorly. In some individuals, this movement can be quite pronounced.

The orbicularis oculi muscle (see Fig. 69) is the sole muscle responsible for closing the palpebral fissure. It is divided into three parts: the orbital part, which is the widest, encircling the orbital margin and extending into the temporal and zygomatic regions; the palpebral part, located within the upper and lower eyelids; and the lacrimal part, a small muscle situated posterior and lateral to the lacrimal sac.

The Functions of the orbicularis oculi are as follows: the orbital part pulls the eyebrow downward, acting as an antagonist to the frontal belly of the epicranial muscle, and assists in squinting and bringing the eyelids together; the palpebral part facilitates eyelid closure; and the lacrimal part dilates the lacrimal sac while also aiding in eyelid closure.

The main antagonist of the orbicularis oculi is the levator palpebrae superioris, which is located within the orbital cavity and typically described alongside the extraocular muscles. It arises from the depth of the Orbit and inserts into the substance of the upper eyelid. Because no analogous muscle exists for the lower eyelid, the upper eyelid possesses significantly greater mobility.

Deep to the orbital part of the orbicularis oculi lies the corrugator supercilii muscle, the contraction of which produces one or two vertical folds between the eyebrows. The procerus muscle is situated over the bridge and ROOT of the nose; its contraction creates a horizontal skin fold. Located laterally and inferiorly to the alae of the nose is the nasalis muscle, which narrows the nostrils and depresses the nasal alae.

The most differentiated part of the facial musculature comprises the muscles surrounding the oral aperture. Among these, only one—the orbicularis oris—features circularly arranged fibers. The remaining muscles extend radially relative to the oral fissure.

The orbicularis oris (see Fig. 69) forms a well-developed muscular layer within the upper and lower Lips. Near the angle of the mouth, it partially attaches to the skin. The muscle has the shape of an irregular ring and, upon contraction, serves to close the oral aperture.

The muscles radiating from the oral fissure can be divided into two groups: one group elevates the upper lip and the angle of the mouth while also pulling it laterally, whereas the other depresses the lower lip and the angle of the mouth. The first group includes the levator labii superioris, the zygomaticus Major and minor, and the risorius. The second group comprises the depressor labii inferioris and the depressor anguli oris.

The zygomaticus major (see Fig. 69) extends from the Zygomatic bone toward the skin at the angle of the mouth, pulling it upward and laterally.

The levator labii superioris alaeque nasi (see Fig. 69) originates from the frontal process of the maxilla and inserts into the skin of the nasal ala and upper lip, pulling them upward and helping to dilate the nostril.

In some cases, when the muscles surrounding the oral aperture and radiating from it insert superficially into the dermis, their contraction forms depressions or dimples in the skin. This applies specifically to the risorius muscle.

Fig. 69. Muscles of facial expression:

1 — epicranius m. (frontal belly); 2 — corrugator supercilii m.; 3 — procerus m.; 4 — orbicularis oculi m.; 5 — zygomaticus major m.; 6 — levator anguli oris m.; 7 — orbicularis oris m.; 8 — depressor anguli oris m.; 9 — depressor labii inferioris m.; 10 — mentalis m.; 11 — platysma m.; 12 — risorius m.; 13 — levator labii superioris alaeque nasi m.; 14 — anterior auricular m.; 15 — superior auricular m.

The buccinator (see Fig. 69) is quadrangular in shape and forms the main structural layer of the cheek. It lies beneath the mucous membrane, positioned between it and the skin. This muscle originates from the outer surface of the alveolar process of the maxilla, the body and ramus of the mandible, and the pterygomandibular raphe—a fibrous band extending between the pterygoid process of the sphenoid bone and the posterior end of the alveolar arch of the mandible. Its muscle bundles run anteriorly in a nearly horizontal direction and terminate in the mucous membrane of the cheek, partially extending into the upper and lower lips to blend with the orbicularis oris. A distinctive feature of the buccinator is that it is pierced by the parotid duct.

The function of the muscle is to pull the angle of the mouth posteriorly, counteract intraoral pressure, and press the cheek and lips against the teeth and Gums of the upper and lower jaws.

The muscles located around the External ear can also be divided into two groups: extrinsic muscles that move the auricle, radiating outward from the external acoustic meatus, and intrinsic muscles located upon the auricle itself, running approximately circularly relative to this opening. In humans, all of these muscles are very poorly developed, especially the intrinsic muscles of the auricle.

Connective Tissue structures of the head

The arrangement of fasciae on the head exhibits certain peculiarities compared to those in all other Regions of the body. The superficial Fascia of the head is absent. The deep fascia of the head is well-defined only in specific areas (temporal, masseteric, and buccopharyngeal fasciae); in the region of the facial muscles and certain masticatory muscles (lateral and medial pterygoids), the deep fascia is lacking, and these muscles are covered only by the perimysium.

The temporal fascia covers the temporalis muscle, extending from its origin and attaching directly to the periosteum. As it descends, it reaches the zygomatic arch. This fascia consists of two layers: superficial and deep.

The masseteric fascia covers the masseter muscle and the parotid gland, extending inferiorly and posteriorly from the zygomatic arch to merge with the deep Cervical fascia. The masseteric fascia forms a sheath for the parotid gland.

The buccopharyngeal fascia covers the buccinator muscle and the Pharynx. This fascia is relatively poorly developed.