Orbicularis oculi

The orbicularis oculi is a large muscle that completely surrounds each orbital orifice and extends into each eyelid (Fig. 8.51). It closes the eyelids. It has two major parts:

the outer orbital part is a broad ring that encircles the orbital orifice and extends outward beyond the orbital rim;

the inner palpebral part is in the eyelids and consists of muscle fibers originating in the medial corner of the eye that arch across each lid to attach laterally.

Fig. 8.51 Orbital group of facial muscles.

The orbital and palpebral parts have specific roles to play during eyelid closure. The palpebral part closes the eye gently, whereas the orbital part closes the eye more forcefully and produces some wrinkling on the forehead.

An additional small lacrimal part of the orbicularis oculi muscle is deep, medial in position, and attaches to bone posterior to the lacrimal sac of the lacrimal apparatus in the orbit.

Corrugator supercilii

The second muscle in the orbital group is the much smaller corrugator supercilii (Fig. 8.51), which is deep to the eyebrows and the orbicularis oculi muscle and is active when frowning. It arises from the medial end of the superciliary arch, passing upward and laterally to insert into the skin of the medial half of the eyebrow. It draws the eyebrows toward the midline, causing vertical wrinkles above the nose.

Procerus

The procerus is a small muscle superficial to the nasal bone and is active when an individual frowns (Fig. 8.52). It arises from the nasal bone and the upper part of the lateral nasal cartilage and inserts into the skin over the lower part of the forehead between the eyebrows. It may be continuous with the frontal belly of the occipitofrontalis muscle of the scalp.

Procerus draws the medial border of the eyebrows downward to produce transverse wrinkles over the bridge of the nose.

Depressor septi nasi

The final muscle in the nasal group is the depressor septi nasi, another muscle that assists in widening the nares (Fig. 8.52). Its fibers arise from the maxilla above the central incisor tooth and ascend to insert into the lower part of the nasal septum.

Depressor septi nasi pulls the nose inferiorly, so assisting the alar part of the nasalis in opening the nares.

Orbicularis oris

The orbicularis oris is a complex muscle consisting of fibers that completely encircle the mouth (Fig. 8.53). Its function is apparent when pursing the lips, as occurs during whistling. Some of its fibers originate near the midline from the maxilla superiorly and the mandible inferiorly, whereas other fibers are derived from both the buccinator, in the cheek, and the numerous other muscles acting on the lips. It inserts into the skin and mucous membrane of the lips, and into itself.

Fig. 8.53 Oral group of facial muscles.

Contraction of the orbicularis oris narrows the mouth and closes the lips.

Buccinator

The buccinator forms the muscular component of the cheek and is used every time air expanding the cheeks is forcefully expelled (Figs. 8.53 and 8.54). It is in the space between the mandible and the maxilla, deep to the other facial muscles in the area.

Fig. 8.54 Buccinator muscle.

The buccinator arises from the posterior part of the maxilla and mandible opposite the molar teeth and the pterygomandibular raphe, which is a tendinous band between the pterygoid hamulus superiorly and the mandible inferiorly and is a point of attachment for the buccinator and superior pharyngeal constrictor muscles.

The fibers of the buccinator pass toward the corner of the mouth to insert into the lips, blending with fibers from the orbicularis oris in a unique fashion. Central fibers of the buccinator cross so that lower fibers enter the upper lip and upper fibers enter the lower lip (Fig. 8.54). The highest and lowest fibers of the buccinator do not cross and enter the upper and lower lips, respectively.

Contraction of the buccinator presses the cheek against the teeth. This keeps the cheek taut and aids in mastication by preventing food from accumulating between the teeth and the cheek. It also assists in the forceful expulsion of air from the cheeks.

Lower group of oral muscles

The muscles in the lower group consist of the depressor anguli oris, depressor labii inferioris. and mentalis (Fig. 8.53).

Depressor anguli oris is active during frowning. It arises along the side of the mandible below the canine, premolar, and first molar teeth and inserts into skin and the upper part of the orbicularis oris near the corner of the mouth. It depresses the corner of the mouth.

Depressor labii inferioris arises from the front of the mandible, deep to depressor anguli oris. Its fibers move superiorly and medially, some merging with fibers from the same muscle on the opposite side and fibers from the orbicularis oris before inserting into the lower lip. It depresses the lower lip and moves it laterally.

Mentalis helps position the lip when drinking from a cup or when pouting. It is the deepest muscle of the lower group arising from the mandible just inferior to the incisor teeth, with its fibers passing downward and medially to insert into the skin of the chin. It raises and protrudes the lower lip as it wrinkles the skin of the chin.

Upper group of oral muscles

The muscles of the upper group of oral muscles consist of risorius, zygomaticus major, zygomaticus minor, levator labii superioris, levator labii superioris alaeque nasi, and levator anguli oris (Fig. 8.53).

Risorius helps produce a grin (Fig. 8.53). It is a thin, superficial muscle that extends laterally from the corner of the mouth in a slightly upward direction. Contraction of its fibers pulls the corner of the mouth laterally and upward.

Zygomaticus major and zygomaticus minor help produce a smile (Fig. 8.53). Zygomaticus major is a superficial muscle that arises deep to the orbicularis oculi along the posterior part of the lateral surface of the zygomatic bone, and passes downward and forward, blending with the orbicularis oris and inserting into skin at the corner of the mouth. Zygomaticus minor arises from the zygomatic bone anterior to the origin of zygomaticus major, parallels the path of zygomaticus major, and inserts into the upper lip medial to the corner of the mouth. Both zygomaticus muscles raise the corner of the mouth and move it laterally.

Levator labii superioris deepens the furrow between the nose and the corner of the mouth during sadness (Fig. 8.53). It arises from the maxilla just superior to the infra-orbital foramen, and its fibers pass downward and medially to blend with the orbicularis oris and insert into the skin of the upper lip.

Levator labii superioris alaeque nasi is medial to the levator labii superioris, arises from the maxilla next to the nose, and inserts into both the alar cartilage of the nose and skin of the upper lip (Fig. 8.53). It may assist in flaring the nares.

Levator anguli oris is more deeply placed and covered by the other two levators and the zygomaticus muscles (Fig. 8.53). It arises from the maxilla, just inferior to the infra-orbital foramen and inserts into the skin at the corner of the mouth. It elevates the corner of the mouth and may help deepen the furrow between the nose and the corner of the mouth during sadness.

Platysma

Platysma is a large, thin sheet of muscle in the superficial fascia of the neck. It arises below the clavicle in the upper part of the thorax and ascends through the neck to the mandible. At this point, the more medial fibers insert on the mandible, whereas the lateral fibers join with muscles around the mouth.

Platysma tenses the skin of the neck and can move the lower lip and corners of the mouth down.

Auricular muscles

Three of these muscles, “other muscles of facial expression,” are associated with the ear—the anterior, superior, and posterior auricular muscles (Fig. 8.55):

the anterior muscle is anterolateral and pulls the ear upward and forward;

the superior muscle is superior and elevates the ear;

the posterior muscle is posterior and retracts and elevates the ear.

Fig. 8.55 Auricular muscles.

Occipitofrontalis

Occipitofrontalis is the final muscle in this category of “other muscles of facial expression” and is associated with the scalp (Fig. 8.50). It consists of a frontal belly anteriorly and an occipital belly posteriorly. An aponeurotic tendon connects the two:

the frontal belly covers the forehead and is attached to the skin of the eyebrows;

the occipital belly arises from the posterior aspect of the skull and is smaller than the frontal belly.

The occipitofrontalis muscles move the scalp and wrinkle the forehead.

Facial nerve

The facial nerve [VII] exits the skull through the stylomastoid foramen and then passes into the parotid gland, where it usually divides into upper and lower trunks. These pass through the substance of the parotid gland, where there may be further branching and anastomosing of the nerves.

Five terminal groups of branches of the facial nerve [VII]—the temporal, zygomatic, buccal, marginal mandibular, and cervical branches—emerge from the upper, anterior, and lower borders of the parotid gland (Fig. 8.56).

External carotid artery and its branches

The external carotid artery enters into or passes deep to the inferior border of the parotid gland (Fig. 8.56). As it continues in a superior direction, it gives off the posterior auricular artery before dividing into its two terminal branches (the maxillary and superficial temporal arteries) near the lower border of the ear:

the maxillary artery passes horizontally, deep to the mandible;

the superficial temporal artery continues in a superior direction and emerges from the upper border of the gland after giving off the transverse facial artery.

Retromandibular vein and its tributaries

The retromandibular vein is formed in the substance of the parotid gland when the superficial temporal and maxillary veins join together (Fig. 8.56), and passes inferiorly in the substance of the parotid gland. It usually divides into anterior and posterior branches just below the inferior border of the gland.

Sensory innervation

Because the face is derived developmentally from a number of structures originating from the first pharyngeal arch, cutaneous innervation of the face is by branches of the trigeminal nerve [V].

The trigeminal nerve [V] divides into three major divisions—the ophthalmic [V₁], maxillary [V₂], and mandibular [V₃] nerves—before leaving the middle cranial fossa (Fig. 8.58). Each of these divisions passes out of the cranial cavity to innervate a part of the face, so most of the skin covering the face is innervated by branches of the trigeminal nerve [V]. The exception is a small area covering the angle and lower border of the ramus of mandible and parts of the ear, which are innervated by the trigeminal [V], facial [VII], vagus [X], and cervical nerves.

Fig. 8.58 Trigeminal nerve [V] leaving the skull.

Ophthalmic nerve [V₁]

The ophthalmic nerve [V₁] exits the skull through the superior orbital fissure and enters the orbit. Its branches (Fig. 8.58) that innervate the face include:

the supra-orbital and supratrochlear nerves, which leave the orbit superiorly and innervate the upper eyelid, forehead, and scalp;

the infratrochlear nerve, which exits the orbit in the medial angle to innervate the medial half of the upper eyelid, the skin in the area of the medial angle, and the side of the nose;

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the lacrimal nerve, which exits the orbit in the lateral angle to innervate the lateral half of the upper eyelid and the skin in the area of the lateral angle; and

the external nasal nerve, which supplies the anterior part of the nose (Fig. 8.59).

Fig. 8.59 Cutaneous distribution of the trigeminal nerve [V].

Maxillary nerve [V₂]

The maxillary nerve [V₂] exits the skull through the foramen rotundum. Branches (Fig. 8.58) that innervate the face include:

a small zygomaticotemporal branch, which exits the zygomatic bone and supplies a small area of the anterior temple above the zygomatic arch;

a small zygomaticofacial branch, which exits the zygomatic bone and supplies a small area of skin over the zygomatic bone; and

the large infra-orbital nerve, which exits the maxilla through the infra-orbital foramen and immediately divides into multiple branches to supply the lower eyelid, cheek, side of the nose, and upper lip (Fig. 8.59).

Mandibular nerve [V₃]

The mandibular nerve [V₃] exits the skull through the foramen ovale. Branches (Fig. 8.58) innervating the face include:

the auriculotemporal nerve, which enters the face just posterior to the temporomandibular joint, passes through the parotid gland, and ascends just anterior to the ear to supply the external acoustic meatus, the surface of the tympanic membrane (eardrum), and a large area of the temple;

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the buccal nerve, which is on the surface of the buccinator muscle supplying the cheek; and

the mental nerve, which exits the mandible through the mental foramen and immediately divides into multiple branches to supply the skin and mucous membrane of the lower lip and skin of the chin (Fig. 8.59).

Motor innervation

The muscles of the face, as well as those associated with the ear and the scalp, are derived from the second pharyngeal arch. The cranial nerve associated with this arch is the facial nerve [VII] and therefore branches of the facial nerve [VII] innervate all these muscles.

The facial nerve [VII] exits the posterior cranial fossa through the internal acoustic meatus. It passes through the temporal bone, giving off several branches, and emerges from the base of the skull through the stylomastoid foramen (Fig. 8.60). At this point it gives off the posterior auricular nerve. This branch passes upward, behind the ear, to supply the occipital belly of the occipitofrontalis muscle of the scalp and the posterior auricular muscle of the ear.

Fig. 8.60 Facial nerve [VII] on the face. A. Terminal branches. B. Branches before entering the parotid gland.

The main stem of the facial nerve [VII] then gives off another branch, which innervates the posterior belly of the digastric muscle and the stylohyoid muscle. At this point, the facial nerve [VII] enters the deep surface of the parotid gland (Fig. 8.60B).

Once in the parotid gland, the main stem of the facial nerve [VII] usually divides into upper (temporofacial) and lower (cervicofacial) branches. As these branches pass through the substance of the parotid gland they may branch further or take part in an anastomotic network (the parotid plexus).

Whatever types of interconnections occur, five terminal groups of branches of the facial nerve [VII]—the temporal, zygomatic, buccal, marginal mandibular, and cervical branches—emerge from the parotid gland (Fig. 8.60A).

Although there are variations in the pattern of distribution of the five terminal groups of branches, the basic pattern is as follows:

temporal branches exit from the superior border of the parotid gland to supply muscles in the area of the temple, forehead, and supra-orbital area;

zygomatic branches emerge from the anterosuperior border of the parotid gland to supply muscles in the infra-orbital area, the lateral nasal area, and the upper lip;

buccal branches emerge from the anterior border of the parotid gland to supply muscles in the cheek, the upper lip, and the corner of the mouth;

marginal mandibular branches emerge from the anteroinferior border of the parotid gland to supply muscles of the lower lip and chin;

cervical branches emerge from the inferior border of the parotid gland to supply the platysma.

Facial artery

The facial artery is the major vessel supplying the face (Fig. 8.61). It branches from the anterior surface of the external carotid artery, passes up through the deep structures of the neck and appears at the lower border of the mandible after passing posterior to the submandibular gland.

Fig. 8.61 Vasculature of the face. A. Lateral view. B. Branches of the maxillary artery.

From this point the facial artery runs upward and medially in a tortuous course. It passes along the side of the nose and terminates as the angular artery at the medial corner of the eye.

Along its path the facial artery is deep to the platysma, risorius, and zygomaticus major and minor, superficial to the buccinator and levator anguli oris, and may pass superficially to or through the levator labii superioris.

Branches of the facial artery include the superior and inferior labial branches and the lateral nasal branch (Fig. 8.61).

The labial branches arise near the corner of the mouth:

the inferior labial branch supplies the lower lip;

the superior labial branch supplies the upper lip, and also provides a branch to the nasal septum.

Near the midline, the superior and inferior labial branches anastomose with their companion arteries from the opposite side of the face. This provides an important connection between the facial arteries and the external carotid arteries of opposite sides.

The lateral nasal branch is a small branch arising from the facial artery as it passes along the side of the nose. It supplies the lateral surface and dorsum of the nose.

Transverse facial artery

Another contributor to the vascular supply of the face is the transverse facial artery (Fig. 8.61), which is a branch of the superficial temporal artery (the smaller of the two terminal branches of the external carotid artery).

The transverse facial artery arises from the superficial temporal artery within the substance of the parotid gland, passes through the gland, and crosses the face in a transverse direction. Lying on the superficial surface of the masseter muscle, it is between the zygomatic arch and the parotid duct.

Branches of the maxillary artery

The maxillary artery, the larger of the two terminal branches of the external carotid artery, gives off several small branches which contribute to the arterial supply to the face:

the infra-orbital artery enters the face through the infra-orbital foramen and supplies the lower eyelid, upper lip, and the area between these structures.

the buccal artery enters the face on the superficial surface of the buccinator muscle and supplies structures in this area;

the mental artery enters the face through the mental foramen and supplies the chin.

Branches of the ophthalmic artery

Three small arteries from the internal carotid artery also contribute to the arterial supply of the face. These vessels arise from the ophthalmic artery, a branch of the internal carotid artery, after the ophthalmic artery enters the orbit:

the zygomaticofacial and zygomaticotemporal arteries come from the lacrimal branch of the ophthalmic artery (Fig. 8.61), enter the face through the zygomaticofacial and zygomaticotemporal foramina, and supply the area of the face over the zygomatic bone;

the dorsal nasal artery, a terminal branch of the ophthalmic artery, exits the orbit in the medial corner, and supplies the dorsum of the nose.

Other branches of the ophthalmic artery (the supra-orbital and supratrochlear arteries) supply the anterior scalp.

Facial vein

The facial vein is the major vein draining the face (Fig. 8.61). Its point of origin is near the medial corner of the orbit as the supratrochlear and supra-orbital veins come together to form the angular vein. This vein becomes the facial vein as it proceeds inferiorly and assumes a position just posterior to the facial artery. The facial vein descends across the face with the facial artery until it reaches the inferior border of the mandible. Here the artery and vein part company and the facial vein passes superficial to the submandibular gland to enter the internal jugular vein.

Throughout its course the facial vein receives tributaries from veins draining the eyelids, external nose, lips, cheek, and chin that accompany the various branches of the facial artery.

Transverse facial vein

The transverse facial vein is a small vein that accompanies the transverse facial artery in its journey across the face (Fig. 8.61). It empties into the superficial temporal vein within the substance of the parotid gland.

Intracranial venous connections

As it crosses the face, the facial vein has numerous connections with venous channels passing into deeper regions of the head (Fig. 8.62):

near the medial corner of the orbit, it communicates with ophthalmic veins;

in the area of the cheek it communicates with veins passing into the infra-orbital foramen;

it communicates with veins passing into deeper regions of the face (i.e., the deep facial vein connecting with the pterygoid plexus of veins).

Fig. 8.62 Intracranial venous connections.

All these venous channels have interconnections with the intracranial cavernous sinus through emissary veins that connect intracranial with extracranial veins. There are no valves in the facial vein or any other venous channels in the head, so blood can move in any direction. Because of the interconnections between the veins, infections of the face, primarily above the mouth (i.e., the “danger area”) should be handled with great care to prevent the dissemination of infectious material in an intracranial direction.

Skin

The skin is the outer layer of the scalp (Figs. 8.64 and 8.65). It is similar structurally to skin throughout the body with the exception that hair is present on a large amount of it.

Fig. 8.65 Layers of the scalp.

Connective tissue (dense)

Deep to the skin is dense connective tissue. This layer anchors the skin to the third layer and contains the arteries, veins, and nerves supplying the scalp.

Aponeurotic layer

The deepest layer of the first three layers is the aponeurotic layer. Firmly attached to the skin by the dense connective tissue of the second layer, this layer consists of the occipitofrontalis muscle, which has a frontal belly anteriorly, an occipital belly posteriorly, and an aponeurotic tendon—the epicranial aponeurosis (galea aponeurotica)—connecting the two (Fig. 8.66).

Fig. 8.66 Occipitofrontalis muscle. A. Frontal belly. B. Occipital belly.

The frontal belly of occipitofrontalis begins anteriorly where it is attached to the skin of the eyebrows. It passes upward, across the forehead, to become continuous with the aponeurotic tendon.

Posteriorly, each occipital belly of occipitofrontalis arises from the lateral part of the superior nuchal line of the occipital bone and the mastoid process of the temporal bone. It also passes superiorly to attach to the aponeurotic tendon.

The occipitofrontalis muscles move the scalp, wrinkle the forehead, and raise the eyebrows. The frontal belly is innervated by temporal branches of the facial nerve [VII] and the posterior belly by the posterior auricular branch.

Loose connective tissue

A layer of loose connective tissue separates the aponeurotic layer from the pericranium and facilitates movement of the scalp proper over the calvaria (Figs. 8.64 and 8.65).

Pericranium

The pericranium is the deepest layer of the scalp and is the periosteum on the outer surface of the calvaria. It is attached to the bones of the calvaria, but is removable, except in the area of the sutures.

Arteries

Arteries supplying the scalp (Fig. 8.68) are branches of either the external carotid artery or the ophthalmic artery, which is a branch of the internal carotid artery.

Fig. 8.68 Vasculature of the scalp.

Branches from the external carotid artery

Three branches of the external carotid artery supply the largest part of the scalp—the superficial temporal, posterior auricular, and occipital arteries supply the lateral and posterior aspects of the scalp (Fig. 8.68):

the smallest branch (the posterior auricular artery) leaves the posterior aspect of the external carotid artery, passes through deeper structures, and emerges to supply an area of the scalp posterior to the ear;

also arising from the posterior aspect of the external carotid artery is the occipital artery, which ascends in a posterior direction, passes through several layers of back musculature, and emerges to supply a large part of the posterior aspect of the scalp;

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the third arterial branch supplying the scalp is the superficial temporal artery, a terminal branch of the external carotid artery that passes superiorly, just anterior to the ear, divides into anterior and posterior branches, and supplies almost the entire lateral aspect of the scalp.

Veins

Veins draining the scalp follow a pattern similar to the arteries:

the supratrochlear and supra-orbital veins drain the anterior part of the scalp from the superciliary arches to the vertex of the head (Fig. 8.68), pass inferior to the superciliary arches, communicate with the ophthalmic veins in the orbit, and continue inferiorly to participate in the formation of the angular vein, which is the upper tributary to the facial vein;

the superficial temporal vein drains the entire lateral area of the scalp before passing inferiorly to join in the formation of the retromandibular vein;

the posterior auricular vein drains the area of the scalp posterior to the ear and eventually empties into a tributary of the retromandibular vein;

the occipital vein drains the posterior aspect of the scalp from the external occipital protuberance and superior nuchal lines to the vertex of the head; deeper, it passes through the musculature in the posterior neck to join in the formation of the plexus of veins in the suboccipital triangle.

Sympathetic innervation

Sympathetic innervation of the lacrimal gland follows a similar path as parasympathetic innervation. Postganglionic sympathetic fibers originating in the superior cervical ganglion travel along the plexus surrounding the internal carotid artery (Fig. 8.81). They leave this plexus as the deep petrosal nerve and join the parasympathetic fibers in the nerve of the pterygoid canal. Passing through the pteryatine ganglion, the sympathetic fibers from this point onward follow the same path as the parasympathetic fibers to the lacrimal gland.

Periorbita

The periosteum lining the bones that form the orbit is the periorbita (Fig. 8.84). It is continuous at the margins of the orbit with the periosteum on the outer surface of the skull and sends extensions into the upper and lower eyelids (the orbital septa).

Fig. 8.84 Periorbita. A. Lateral view. B. Common tendinous ring.

At the various openings where the orbit communicates with the cranial cavity the periorbita is continuous with the periosteal layer of dura mater. In the posterior part of the orbit, the periorbita thickens around the optic canal and the central part of the superior orbital fissure. This is the point of origin of the four rectus muscles and is the common tendinous ring.

Fascial sheath of the eyeball

The fascial sheath of the eyeball (bulbar sheath) is a layer of fascia that encloses a major part of the eyeball (Figs. 8.85 and 8.86):

posteriorly, it is firmly attached to the sclera (the white part of the eyeball) around the point of entrance of the optic nerve into the eyeball;

anteriorly, it is firmly attached to the sclera near the edge of the cornea (the clear part of the eyeball);

additionally, as the muscles approach the eyeball, the investing fascia surrounding each muscle blends with the fascial sheath of the eyeball as the muscles pass through and continue to their point of attachment.

Fig. 8.85 Fascial sheath of the eyeball.

Fig. 8.86 Check ligaments. A. Anterior view. B. Superior view.

A specialized lower part of the fascial sheath of the eyeball is the suspensory ligament (Figs. 8.85 and 8.86), which supports the eyeball. This “sling-like” structure is made up of the fascial sheath of the eyeball and contributions from the two inferior ocular muscles and the medial and lateral ocular muscles.

Check ligaments of the medial and lateral rectus muscles

Other fascial specialization in the orbit are the check ligaments (Fig. 8.86). These are expansions of the investing fascia covering the medial and lateral rectus muscles, which attach to the medial and lateral walls of the bony orbit:

the medial check ligament is an extension from the fascia covering the medial rectus muscle and attaches immediately posterior to the posterior lacrimal crest of the lacrimal bone;

the lateral check ligament is an extension from the fascia covering the lateral rectus muscle and is attached to the orbital tubercle of the zygomatic bone.

Functionally, the positioning of these ligaments seems to restrict the medial and lateral rectus muscles, thus the names of the fascial specializations.

Extrinsic muscles

Of the seven muscles in the extrinsic group of muscles, one raises the eyelids, whereas the other six move the eyeball itself (Table 8.8).

Table 8.8 Extrinsic (extra-ocular) muscles

The movements of the eyeball, in three dimensions, (Fig. 8.87) are:

elevation—moving the pupil superiorly;

depression—moving the pupil inferiorly;

abduction—moving the pupil laterally;

adduction—moving the pupil medially;

internal rotation (intorsion)—rotating the upper part of the pupil medially (or toward the nose); and

external rotation (extorsion)—rotating the upper part of the pupil laterally (or toward the temple).

Fig. 8.87 Movements of the eyeball.

The axis of each orbit is directed slightly laterally from back to front, but each eyeball is directed anteriorly (Fig. 8.88). Therefore the pull of some muscles has multiple effects on the movement of the eyeball, whereas that of others has a single effect.

Fig. 8.88 Axes of the eyeball and orbit.

Levator palpebrae superioris

Levator palpebrae superioris raises the upper eyelid (Table 8.8). It is the most superior muscle in the orbit, originating from the roof, just anterior to the optic canal on the inferior surface of the lesser wing of the sphenoid (Fig. 8.89B). Its primary point of insertion is into the anterior surface of the superior tarsus, but a few fibers also attach to the skin of the upper eyelid and the superior conjunctival fornix.

Fig. 8.89 Muscles of the eyeball. A. Superior view. B. Lateral view.

C. Coronal magnetic resonance image through the eye.

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Innervation is by the superior branch of the oculomotor nerve [III].

Contraction of the levator palpebrae superioris raises the upper eyelid.

A unique feature of levator palpebrae superioris is that a collection of smooth muscle fibers passes from its inferior surface to the upper edge of the superior tarsus (Fig. 8.71). This group of smooth muscle fibers (the superior tarsal muscle) help maintain eyelid elevation and are innervated by postganglionic sympathetic fibers from the superior cervical ganglion.

Loss of oculomotor nerve [III] function results in complete ptosis or drooping of the superior eyelid, whereas loss of sympathetic innervation to the superior tarsal muscle results in partial ptosis.

Rectus muscles

Four rectus muscles occupy medial, lateral, inferior, and superior positions as they pass from their origins posteriorly to their points of attachment on the anterior half of the eyeball (Fig. 8.89 and Table 8.8). They originate as a group from a common tendinous ring at the apex of the orbit and form a cone of muscles as they pass forward to their attachment on the eyeball.

Superior and inferior rectus muscles

The superior and inferior rectus muscles have complicated actions because the apex of the orbit, where the muscles originate, is medial to the central axis of the eyeball when looking directly forward:

the superior rectus originates from the superior part of the common tendinous ring above the optic canal;

the inferior rectus originates from the inferior part of the common tendinous ring below the optic canal (Fig. 8.90).

Fig. 8.90 Origins of muscles of the eyeball, coronal view.

As these muscles pass forward in the orbit to attach to the anterior half of the eyeball, they are also directed laterally (Fig. 8.89). Because of these orientations:

contraction of the superior rectus elevates, adducts, and internally rotates the eyeball (Fig. 8.91A);

contraction of the inferior rectus depresses, adducts, and externally rotates the eyeball (Fig. 8.91A).

Fig. 8.91 Actions of muscles of the eyeball. A. Action of individual muscles (anatomical action). B. Movement of eye when testing specific muscle (clinical testing).

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The superior branch of the oculomotor nerve [III] innervates the superior rectus, and the inferior branch of the oculomotor nerve [III] innervates the inferior rectus.

To isolate the function of and to test the superior and inferior rectus muscles, a patient is asked to track a physician’s finger laterally and then either upward or downward (Fig. 8.91B). The first movement brings the axis of the eyeball into alignment with the long axis of the superior and inferior rectus muscles. Moving the finger upward tests the superior rectus muscle and moving it downward tests the inferior rectus muscle (Fig. 8.91B).

Medial and lateral rectus muscles

The orientation and actions of the medial and lateral rectus muscles are more straightforward than those of the superior and inferior rectus muscles.

The medial rectus originates from the medial part of the common tendinous ring medial to and below the optic canal, whereas the lateral rectus originates from the lateral part of the common tendinous ring as the common tendinous ring bridges the superior orbital fissure (Fig. 8.90).

The medial and lateral rectus muscles pass forward and attach to the anterior half of the eyeball (Fig. 8.89). Contraction of each medial rectus adducts the eyeball, whereas contraction of each lateral rectus abducts the eyeball (Fig. 8.91A).

The inferior branch of the oculomotor nerve [III] innervates the medial rectus, and the abducent nerve [VI] innervates the lateral rectus.

To isolate the function of and test the medial and lateral rectus muscles, a patient is asked to track a physician’s finger medially and laterally, respectively, in the horizontal plane (Fig. 8.91B).

Extrinsic muscles and eyeball movements

Six of the seven extrinsic muscles of the orbit are directly involved in movements of the eyeball.

For each of the rectus muscles, the medial, lateral, inferior, and superior, and the superior and inferior obliques, a specific action or group of actions can be described (Table 8.8). However, these muscles do not act in isolation. They work as teams of muscles in the coordinated movement of the eyeball to position the pupil as needed.

For example, although the lateral rectus is the muscle primarily responsible for moving the eyeball laterally, it is assisted in this action by the superior and inferior oblique muscles.

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In the clinic

Examination of the eye

Examination of the eye includes assessment of the visual capabilities, the extrinsic musculature and its function, and disease processes that may affect the eye in isolation or as part of the systemic process.

Examination of the eye includes tests for visual acuity, astigmatism, visual fields, and color interpretation (to exclude color blindness) in a variety of circumstances. The physician also assesses the retina, the optic nerve and its coverings, the lens, and the cornea.

The extrinsic muscles are supplied by the abducent nerve [VI], the trochlear nerve [IV], and the oculomotor nerve [III].

The extrinsic muscles work synergistically to provide appropriate and conjugate eye movement:

lateral rectus—abducent nerve [VI];

superior oblique—trochlear nerve [IV];

remainder—oculomotor nerve [III].

The eye may be affected in systemic diseases. Diabetes mellitus typically affects the eye and may cause cataracts, macular disease, and retinal hemorrhage, all impairing vision.

Occasionally unilateral paralysis of the extra-ocular muscles occurs, and is due to brainstem injury or direct nerve injury, which may be associated with tumor compression or trauma. The paralysis of a muscle is easily demonstrated when the patient attempts to move the eye in the direction associated with normal action of that muscle. Typically the patient complains of double vision (diplopia).

Loss of innervation of the muscles around the eye

Loss of innervation of the orbicularis oculi by the facial nerve [VII] causes an inability to close the eyelids tightly, allowing the lower eyelid to droop away causing spillage of tears. This loss of tears allows drying of the conjunctiva, which may ulcerate, so allowing secondary infection.

Loss of innervation of the levator palpebrae superioris by oculomotor nerve [III] damage causes an inability of the superior eyelid to elevate, producing a ptosis. Usually, oculomotor nerve [III] damage is caused by severe head injury.

Loss of innervation of the superior tarsal muscle by sympathetic fibers causes a constant partial ptosis. Any lesion along the sympathetic trunk can induce this. An apical pulmonary malignancy should always be suspected because the ptosis may be part of Horner’s syndrome (see p. 863).

Arteries

The arterial supply to the structures in the orbit, including the eyeball, is by the ophthalmic artery (Fig. 8.90). This vessel is a branch of the internal carotid artery, given off immediately after the internal carotid artery leaves the cavernous sinus. The ophthalmic artery passes into the orbit through the optic canal with the optic nerve.

In the orbit the ophthalmic artery initially lies inferior and lateral to the optic nerve (Fig. 8.92). As it passes forward in the orbit, it crosses superior to the optic nerve and proceeds anteriorly on the medial side of the orbit.

Fig. 8.92 Arterial supply to the orbit and eyeball.

In the orbit the ophthalmic artery gives off numerous branches as follows:

the lacrimal artery, which arises from the ophthalmic artery on the lateral side of the optic nerve, and passes anteriorly on the lateral side of the orbit, supplying the lacrimal gland, muscles, the anterior ciliary branch to the eyeball, and the lateral sides of the eyelid;

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the central retinal artery, which enters the optic nerve, proceeds down the center of the nerve to the retina, and is clearly seen when viewing the retina with an ophthalmoscope—occlusion of this vessel or of the parent artery leads to blindness;

the long and short posterior ciliary arteries, which are branches that enter the eyeball posteriorly, piercing the sclera, and supplying structures inside the eyeball;

the muscular arteries, which are branches supplying the intrinsic muscles of the eyeball;

the supra-orbital artery, which usually arises from the ophthalmic artery immediately after it has crossed the optic nerve, proceeds anteriorly, and exits the orbit through the supra-orbital foramen with the supra-orbital nerve—it supplies the forehead and scalp as it passes across these areas to the vertex of the skull;

the posterior ethmoidal artery, which exits the orbit through the posterior ethmoidal foramen to supply the ethmoidal cells and nasal cavity;

the anterior ethmoidal artery, which exits the orbit through the anterior ethmoidal foramen, enters the cranial cavity giving off the anterior meningeal branch, and continues into the nasal cavity supplying the septum and lateral wall, and ending as the dorsal nasal artery;

the medial palpebral arteries, which are small branches supplying the medial area of the upper and lower eyelids;

the dorsal nasal artery, which is one of the two terminal branches of the ophthalmic artery, leaves the orbit to supply the upper surface of the nose;

the supratrochlear artery, which is the other terminal branch of the ophthalmic artery and leaves the orbit with the supratrochlear nerve, supplying the forehead as it passes across it in a superior direction.

Veins

There are two venous channels in the orbit, the superior and inferior ophthalmic veins (Fig. 8.93).

Fig. 8.93 Venous drainage of the orbit and eyeball.

The superior ophthalmic vein begins in the anterior area of the orbit as connecting veins from the supra-orbital vein and the angular vein join together. It passes across the superior part of the orbit, receiving tributaries from the companion veins to the branches of the ophthalmic artery and veins draining the posterior part of the eyeball. Posteriorly, it leaves the orbit through the superior orbital fissure and enters the cavernous sinus.

The inferior ophthalmic vein is smaller than the superior ophthalmic vein, begins anteriorly, and passes across the inferior part of the orbit. It receives various tributaries from muscles and the posterior part of the eyeball as it crosses the orbit.

The inferior ophthalmic vein leaves the orbit posteriorly by:

joining with the superior ophthalmic vein;

passing through the superior orbital fissure on its own to join the cavernous sinus; or

passing through the inferior orbital fissure to join with the pterygoid plexus of veins in the infratemporal fossa.

Optic nerve

The optic nerve [II] is not a true cranial nerve, but rather an extension of the brain carrying afferent fibers from the retina of the eyeball to the visual centers of the brain.

The optic nerve leaves the orbit through the optic canal (Fig. 8.94). It is accompanied in the optic canal by the ophthalmic artery.

Fig. 8.94 Innervation of the orbit and eyeball.

Oculomotor nerve

The oculomotor nerve [III] leaves the anterior surface of the brainstem between the midbrain and the pons. It passes forward in the lateral wall of the cavernous sinus.

Just before entering the orbit the oculomotor nerve [III] divides into superior and inferior branches (Fig. 8.95). These branches enter the orbit through the superior orbital fissure, lying within the common tendinous ring (Fig. 8.94).

Fig. 8.95 Oculomotor nerve [III] and its divisions.

Inside the orbit the small superior branch passes upward over the lateral side of the optic nerve to innervate the superior rectus and levator palpebrae superioris muscles (Fig. 8.95).

The large inferior branch divides into three branches:

one passing below the optic nerve as it passes to the medial side of the orbit to innervate the medial rectus muscle;

a second descending to innervate the inferior rectus muscle;

the third descends as it runs forward along the floor of the orbit to innervate the inferior oblique muscle (Fig. 8.95).

As the third branch descends, it gives off the branch to the ciliary ganglion. This is the parasympathetic root to the ciliary ganglion and carries preganglionic parasympathetic fibers that will synapse in the ciliary ganglion with postganglionic parasympathetic fibers. The postganglionic fibers are distributed to the eyeball through short ciliary nerves and innervate the sphincter pupillae and ciliary muscles.

Trochlear nerve

The trochlear nerve [IV] arises from the posterior surface of the midbrain, and passes around the midbrain to enter the edge of the tentorium cerebelli. It continues on an intradural path arriving in and passing through the lateral wall of the cavernous sinus just below the oculomotor nerve [III].

Just before entering the orbit, the trochlear nerve ascends, passing across the oculomotor nerve [III] and enters the orbit through the superior orbital fissure above the common tendinous ring (Fig. 8.94). In the orbit the trochlear nerve [IV] ascends and turns medially, crossing above the levator palpebrae superioris muscle to enter the upper border of the superior oblique muscle (Fig. 8.96).

Fig. 8.96 Trochlear nerve [IV] in the orbit.

Abducent nerve

The abducent nerve [VI] arises from the brainstem between the pons and medulla. It enters the dura covering the clivus and continues in a dural canal until it reaches the cavernous sinus.

The abducent nerve enters the cavernous sinus and runs through the sinus lateral to the internal carotid artery. It passes out of the sinus and enters the orbit through the superior orbital fissure within the common tendinous ring (Fig. 8.94). Once in the orbit it courses laterally to supply the lateral rectus muscle.

Postganglionic sympathetic fibers

Preganglionic sympathetic fibers arise from the upper segments of the thoracic spinal cord, mainly T1. They enter the sympathetic chain through white rami communicantes, and ascend to the superior cervical ganglion where they synapse with postganglionic sympathetic fibers.

The postganglionic fibers are distributed along the internal carotid artery and its branches.

The postganglionic sympathetic fibers destined for the orbit travel with the ophthalmic artery. Once in the orbit the fibers are distributed to the eyeball either by:

passing through the ciliary ganglion, without synapsing, and joining the short ciliary nerves, which pass from the ganglion to the eyeball; or

passing through long ciliary nerves to reach the eyeball.

In the eyeball postganglionic sympathetic fibers innervate the dilator pupillae muscle.

Ophthalmic nerve [V₁]

The ophthalmic nerve [V₁] is the smallest and most superior of the three divisions of the trigeminal nerve. This purely sensory nerve receives input from structures in the orbit and from additional branches on the face and scalp.

Leaving the trigeminal ganglion, the ophthalmic nerve [V₁] passes forward in the lateral wall of the cavernous sinus inferior to the trochlear [IV] and oculomotor [III] nerves. Just before it enters the orbit it divides into three branches—the nasociliary, lacrimal, and frontal nerves (Fig. 8.97). These branches enter the orbit through the superior orbital fissure with the frontal and lacrimal nerves outside the common tendinous ring, and the nasociliary nerve within the common tendinous ring (Fig. 8.94).

Fig. 8.97 Ophthalmic nerve [V₁] and its divisions.

Lacrimal nerve

The lacrimal nerve is the smallest of the three branches of the ophthalmic nerve [V₁]. Once in the orbit it passes forward along the upper border of the lateral rectus muscle (Fig. 8.98). It receives a branch from the zygomaticotemporal nerve, which carries parasympathetic and sympathetic postganglionic fibers for distribution to the lacrimal gland.

Fig. 8.98 Relationship of the ophthalmic nerve [V₁] and its divisions to the muscles of the eyeball.

Reaching the anterolateral aspect of the orbit, the lacrimal nerve supplies the lacrimal gland, conjunctiva, and lateral part of the upper eyelid.

Frontal nerve

The frontal nerve is the largest branch of the ophthalmic nerve [V₁] and receives sensory input from areas outside the orbit. Exiting the superior orbital fissure, this branch passes forward between the levator palpebrae superioris and the periorbita on the roof of the orbit (Fig. 8.94). About midway across the orbit it divides into its two terminal branches—the supra-orbital and supratrochlear nerves (Figs. 8.97 and 8.98):

the supratrochlear nerve continues forward in an anteromedial direction, passing above the trochlea, exits the orbit medial to the supra-orbital foramen, and supplies the conjunctiva and skin of the upper eyelid and the skin on the lower medial part of the forehead;

the supra-orbital nerve is the larger of the two branches, continues forward, passing between the levator palpebrae superioris muscle and the periorbita covering the roof of the orbit (Fig. 8.98), exits the orbit through the supra-orbital notch and ascends across the forehead and scalp, supplying the upper eyelid and conjunctiva, the forehead, and as far posteriorly as the middle of the scalp.

Nasociliary nerve

The nasociliary nerve is intermediate in size between the frontal and lacrimal nerves and is usually the first branch from the ophthalmic nerve (Fig. 8.97). It is most deeply placed in the orbit, entering the area within the common tendinous ring between the superior and inferior branches of the oculomotor nerve [III] (Fig. 8.94).

Once in the orbit, the nasociliary nerve crosses the superior surface of the optic nerve as it passes in a medial direction below the superior rectus muscle (Figs. 8.97 and 8.99). Its first branch, the communicating branch with the ciliary ganglion (sensory root to the ciliary ganglion), is given off early in its path through the orbit.

Fig. 8.99 Course of the nasociliary nerve [V₁] in the orbit.

The nasociliary nerve continues forward along the medial wall of the orbit, between the superior oblique and the medial rectus muscles, giving off several branches. These include:

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the long ciliary nerves, which are sensory to the eyeball but may also contain sympathetic fibers for pupillary dilation;

the posterior ethmoidal nerve, which exits the orbit through the posterior ethmoidal foramen to supply posterior ethmoidal cells and the sphenoidal sinus;

the infratrochlear nerve, which distributes to the medial part of the upper and lower eyelids, the lacrimal sac, and skin of the upper half of the nose; and

the anterior ethmoidal nerve, which exits the orbit through the anterior ethmoidal foramen to supply the anterior cranial fossa, nasal cavity, and skin of the lower half of the nose (Figs. 8.97 and 8.99).

Ciliary ganglion

The ciliary ganglion is a parasympathetic ganglion of the oculomotor nerve [III]. It is associated with the nasociliary branch of the ophthalmic nerve [V₁] and is the site where preganglionic and postganglionic parasympathetic neurons synapse as fibers from this part of the autonomic division of the PNS make their way to the eyeball. The ciliary ganglion is also traversed by postganglionic sympathetic fibers and sensory fibers as they travel to the eyeball.

The ciliary ganglion is a very small ganglion, in the posterior part of the orbit immediately lateral to the optic nerve and between the optic nerve and the lateral rectus muscle (Fig. 8.99). It is usually described as receiving at least two, and possibly three, branches or roots from other nerves in the orbit.

Parasympathetic root

As the inferior branch of the oculomotor nerve [III] passes the area of the ciliary ganglion, it sends a branch to the ganglion (the parasympathetic root). The parasympathetic branch carries preganglionic parasympathetic fibers, which enter the ganglion and synapse with postganglionic parasympathetic fibers within the ganglion (Fig. 8.100).

Fig. 8.100 Ciliary ganglion.

The postganglionic parasympathetic fibers leave the ganglion through short ciliary nerves, which enter the posterior aspect of the eyeball around the optic nerve.

In the eyeball the parasympathetic fibers innervate:

the sphincter pupillae muscle, responsible for pupillary constriction; and

the ciliary muscle, responsible for accommodation of the lens of the eye for near vision.

Muscles

Numerous intrinsic and extrinsic muscles are associated with the auricle:

the intrinsic muscles pass between the cartilaginous parts of the auricle and may change the shape of the auricle;

the extrinsic muscles, the anterior, superior, and posterior auricular muscles, pass from the scalp or skull to the auricle and may also play a role in positioning of the auricle.

Both groups of muscles are innervated by the facial nerve [VII].

Innervation

Sensory innervation of the auricle is from many sources (Fig. 8.106):

the outer more superficial surfaces of the auricle are supplied by the great auricular nerve (anterior and posterior inferior portions) and the lesser occipital nerve (posterior superior portion) from the cervical plexus and the auriculotemporal branch of the mandibular nerve [V₃] (anterior superior portion);

the deeper parts of the auricle are supplied by the vagus nerve [X] (the auricular branch) and the facial nerve [VII] (which sends a branch to the auricular branch of the vagus nerve [X]).

Fig. 8.106 Sensory innervation of the auricle.

Vessels

The arterial supply to the auricle is from numerous sources. The external carotid artery supplies the posterior auricular artery, the superficial temporal artery supplies anterior auricular branches, and the occipital artery supplies a branch.

Venous drainage is through vessels following the arteries.

Lymphatic drainage of the auricle passes anteriorly into parotid nodes and posteriorly into mastoid nodes, and possibly into the upper deep cervical nodes.

Innervation

Sensory innervation of the external acoustic meatus is from several of the cranial nerves. The major sensory input travels through branches of the auriculotemporal nerve, a branch of the mandibular nerve [V₃] (anterior and superior walls), and in the auricular branch of the vagus nerve [X] (posterior and inferior walls). A minor sensory input may also come from a branch of the facial nerve [VII] to the auricular branch of the vagus nerve [X].

Innervation

Innervation of the external and internal surfaces of the tympanic membrane is by several cranial nerves:

sensory innervation of the skin on the outer surface of the tympanic membrane is primarily by the auriculotemporal nerve, a branch of the mandibular nerve [V₃] with additional participation of the auricular branch of the vagus nerve [X], a small contribution by a branch of the facial nerve [VII] to the auricular branch of the vagus nerve [X], and possibly a contribution from the glossopharyngeal nerve [IX];

sensory innervation of the mucous membrane on the inner surface of the tympanic membrane is carried entirely by the glossopharyngeal [IX] nerve.

Tegmental wall

The tegmental wall (roof) of the middle ear consists of a thin layer of bone, which separates the middle ear from the middle cranial fossa. This layer of bone is the tegmen tympani on the anterior surface of the petrous part of the temporal bone.

Jugular wall

The jugular wall (floor) of the middle ear consists of a thin layer of bone that separates it from the internal jugular vein. Occasionally, the floor is thickened by the presence of mastoid air cells.

Near the medial border of the floor is a small aperture, through which the tympanic branch from the glossopharyngeal nerve [IX] enters the middle ear.

Membranous wall

The membranous (lateral) wall of the middle ear consists almost entirely of the tympanic membrane, but because the tympanic membrane does not extend superiorly into the epitympanic recess, the upper part of the membranous wall of the middle ear is the bony lateral wall of the epitympanic recess.

Mastoid wall

The mastoid (posterior) wall of the middle ear is only partially complete. The lower part of this wall consists of a bony partition between the tympanic cavity and mastoid air cells. Superiorly, the epitympanic recess is continuous with the aditus to the mastoid antrum (Figs. 8.111 and 8.112).

Fig. 8.112 Mastoid antrum and surrounding bone. A. Diagram. B. High-resolution CT scan of left ear (petrous temporal bone).

Associated with the mastoid wall are:

the pyramidal eminence, a small elevation through which the tendon of the stapedius muscle enters the middle ear; and

the opening through which the chorda tympani nerve, a branch of the facial nerve [VII], enters the middle ear.

Anterior wall

The anterior wall of the middle ear is only partially complete. The lower part consists of a thin layer of bone that separates the tympanic cavity from the internal carotid artery. Superiorly, the wall is deficient because of the presence of:

a large opening for the entrance of the pharyngotympanic tube into the middle ear; and

a smaller opening for the canal containing the tensor tympani muscle.

The foramen for the exit of the chorda tympani nerve from the middle ear is also associated with this wall (Fig. 8.111).

Labyrinthine wall

The labyrinthine (medial) wall of the middle ear is also the lateral wall of the internal ear. A prominent structure on this wall is a rounded bulge (the promontory) produced by the basal coil of the cochlea, which is an internal ear structure involved with hearing (Fig. 8.111).

Associated with the mucous membrane covering the promontory is a plexus of nerves (the tympanic plexus), which consists primarily of contributions from the tympanic branch of the glossopharyngeal nerve [IX] and branches from the internal carotid plexus. It supplies the mucous membrane of the middle ear, the mastoid area, and the pharyngotympanic tube.

Additionally, a branch of the tympanic plexus (the lesser petrosal nerve) leaves the promontory and the middle ear, travels across the anterior surface of the petrous part of the temporal bone, and leaves the middle cranial fossa through the foramen ovale to enter the otic ganglion. Other structures associated with the labyrinthine wall are two openings, the oval and round windows, and two prominent elevations (Fig. 8.111):

the oval window is posterosuperior to the promontory, is the point of attachment for the base of stapes (footplate), and ends the chain of bones that transfer vibrations initiated by the tympanic membrane to the cochlea of the internal ear;

the round window is posteroinferior to the promontory;

posterior and superior to the oval window on the medial wall is the prominence of facial canal, which is a ridge of bone produced by the facial nerve [VII] in its canal as it passes through the temporal bone;

just above and posterior to the prominence of facial canal is a broader ridge of bone (prominence of lateral semicircular canal) produced by the lateral semicircular canal, which is a structure involved in detecting motion.

Vessels

The arterial supply to the pharyngotympanic tube is from several sources. Branches arise from the ascending pharyngeal artery (a branch of the external carotid artery) and from two branches of the maxillary artery (the middle meningeal artery and the artery of the pterygoid canal).

Venous drainage of the pharyngotympanic tube is to the pterygoid plexus of veins in the infratemporal fossa.

Innervation

Innervation of the mucous membrane lining the pharyngotympanic tube is primarily from the tympanic plexus because it is continuous with the mucous membrane lining the tympanic cavity, the internal surface of the tympanic membrane, and the mastoid antrum and mastoid cells. This plexus receives its major contribution from the tympanic nerve, a branch of the glossopharyngeal nerve [IX].

Malleus

The malleus is the largest of the auditory ossicles and is attached to the tympanic membrane. Identifiable parts include the head of malleus, neck of malleus, anterior and lateral processes, and handle of malleus (Fig. 8.114). The head of malleus is the rounded upper part of the malleus in the epitympanic recess. Its posterior surface articulates with the incus.

Inferior to the head of malleus is the constricted neck of malleus, and below this are the anterior and lateral processes:

the anterior process is attached to the anterior wall of the middle ear by a ligament;

the lateral process is attached to the anterior and posterior malleolar folds of the tympanic membrane.

The downward extension of the malleus, below the anterior and lateral processes, is the handle of malleus, which is attached to the tympanic membrane.

Incus

The second bone in the series of auditory ossicles is the incus. It consists of the body of incus, and long and short limbs (Fig. 8.114):

the enlarged body of incus articulates with the head of malleus and is in the epitympanic recess;

the long limb extends downward from the body, paralleling the handle of the malleus, and ends by bending medially to articulate with the stapes;

the short limb extends posteriorly and is attached by a ligament to the upper posterior wall of the middle ear.

Stapes

The stapes is the most medial bone in the osseous chain and is attached to the oval window. It consists of the head of stapes, anterior and posterior limbs, and the base of stapes (Fig. 8.114):

the head of stapes is directed laterally and articulates with the long process of the incus;

the two limbs separate from each other and attach to the oval base;

the base of stapes fits into the oval window on the labyrinthine wall of the middle ear.

Muscles associated with the ossicles

Two muscles are associated with the bony ossicles of the middle ear—the tensor tympani and stapedius (Fig. 8.115 and Table 8.10).

Fig. 8.115 Muscles associated with the auditory ossicles.

Table 8.10 Muscles of the middle ear

Semicircular canals

Projecting in a posterosuperior direction from the vestibule are the anterior, posterior, and lateral semicircular canals (Fig. 8.120). Each of these canals forms two-thirds of a circle connected at both ends to the vestibule and with one end dilated to form the ampulla. The canals are oriented so that each canal is at right angles to the other two.

Cochlea

Projecting in an anterior direction from the vestibule is the cochlea, which is a bony structure that twists on itself two and one-half to two and three-quarter times around a central column of bone (the modiolus). This arrangement produces a cone-shaped structure with a base of cochlea that faces posteromedially and an apex that faces anterolaterally (Fig. 8.121). This positions the wide base of the modiolus near the internal acoustic meatus, where it is entered by branches of the cochlear part of the vestibulocochlear nerve [VIII].

Fig. 8.121 Cochlea.

Extending laterally throughout the length of the modiolus is a thin lamina of bone (the lamina of modiolus, or spiral lamina). Circling around the modiolus, and held in a central position by its attachment to the lamina of modiolus, is the cochlear duct, which is a component of the membranous labyrinth.

Attached peripherally to the outer wall of the cochlea, the cochlear duct creates two canals (the scala vestibuli and the scala tympani), which extend throughout the cochlea and are continuous with each other at the apex through a narrow slit (the helicotrema):

the scala vestibuli is continuous with the vestibule;

the scala tympani is separated from the middle ear by the secondary tympanic membrane covering the round window (Fig. 8.122).

Fig. 8.122 Membranous labyrinth.

Finally, near the round window is a small channel (the cochlear canaliculus), which passes through the temporal bone and opens on its inferior surface into the posterior cranial fossa. This provides a connection between the perilymph-containing cochlea and the subarachnoid space (Fig. 8.122).

Organs of balance

Five of the six components of the membranous labyrinth are concerned with balance. These are the two sacs (the utricle and the saccule) and three ducts (the anterior, posterior, and lateral semicircular ducts).

Organ of hearing

Cochlear duct

The cochlear duct has a central position in the cochlea of the bony labyrinth dividing it into two canals (the scala vestibuli and the scala tympani). It is maintained in this position by being attached centrally to the lamina of modiolus, which is a thin lamina of bone extending from the modiolus (the central bony core of the cochlea), and peripherally to the outer wall of the cochlea (Fig. 8.123).

Fig. 8.123 Membranous labyrinth, cross-section.

Thus, the triangular-shaped cochlear duct has:

an outer wall against the bony cochlea consisting of thickened, epithelial-lined periosteum (the spiral ligament);

a roof (the vestibular membrane), which separates the endolymph in the cochlear duct from the perilymph in the scala vestibuli and consists of a membrane with a connective tissue core lined on either side with epithelium; and

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a floor, which separates the endolymph in the cochlear duct from the perilymph in the scala tympani and consists of the free edge of the lamina of modiolus, and a membrane (the basilar membrane) extending from this free edge of the lamina of modiolus to an extension of the spiral ligament covering the outer wall of the cochlea.

The spiral organ is the organ of hearing, rests on the basilar membrane, and projects into the enclosed, endolymph-filled cochlear duct (Fig. 8.123).

Facial nerve [VII] in the temporal bone

The facial nerve [VII] is closely associated with the vestibulocochlear nerve [VIII] as it enters the internal acoustic meatus of the temporal bone. Traveling through the temporal bone, its path and several of its branches are directly related to the internal and middle ears.

The facial nerve [VII] enters the internal acoustic meatus in the petrous part of the temporal bone (Fig. 8.124A). The vestibulocochlear nerve and the labyrinthine artery accompany it.

Fig. 8.124 A. Facial nerve in the temporal bone. B. Chorda tympani in the temporal bone.

At the distal end of the internal acoustic meatus, the facial nerve [VII] enters the facial canal and continues laterally between the internal and middle ears. At this point the facial nerve [VII] enlarges and bends posteriorly and laterally. The enlargement is the sensory geniculate ganglion. As the facial canal continues, the facial nerve [VII] turns sharply downward and running in an almost vertical direction, it exits the skull through the stylomastoid foramen (Fig. 8.124A).