Anatomy_Gray

Anatomy_Gray. 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. The arterial supply to the structures in the orbit, including the eyeball, is by the ophthalmic artery (Fig. 8.99). 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.


Anatomy_Gray

Anatomy_Gray. In the orbit the ophthalmic artery initially lies inferior and lateral to the optic nerve (Fig. 8.99). As it passes forward in the orbit, it crosses superior to the optic nerve and proceeds anteriorly on the medial side of the orbit.


Anatomy_Gray

Anatomy_Gray. 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; 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


Anatomy_Gray

Anatomy_Gray. 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


Anatomy_Gray

Anatomy_Gray. 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; and 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.


Anatomy_Gray

Anatomy_Gray. There are two venous channels in the orbit, the superior and inferior ophthalmic veins (Fig. 8.100). 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.


Anatomy_Gray

Anatomy_Gray. 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. Because the ophthalmic veins communicate with the cavernous sinus, they act as a route by which infections can spread from outside to inside the cranial cavity. Numerous nerves pass into the orbit and innervate structures within its bony walls. They include the optic nerve [II], the oculomotor nerve [III], the trochlear nerve [IV], the abducent nerve [VI], and autonomic nerves. Other nerves such as the ophthalmic nerve [V1] innervate orbital structures and then travel out of the orbit to innervate other regions.


Anatomy_Gray

Anatomy_Gray. 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 is surrounded by the cranial meninges, including the subarachnoid space, which extends as far forward as the eyeball. Any increase in intracranial pressure therefore results in increased pressure in the subarachnoid space surrounding the optic nerve. This may impede venous return along the retinal veins, causing edema of the optic disc (papilledema), which can be seen when the retina is examined using an ophthalmoscope. The optic nerve leaves the orbit through the optic canal (Fig. 8.101). It is accompanied in the optic canal by the ophthalmic artery. 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.


Anatomy_Gray

Anatomy_Gray. 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. 8.102). These branches enter the orbit through the superior orbital fissure, lying within the common tendinous ring (Fig. 8.101). 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. 8.102). 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, and the third descending as it runs forward along the floor of the orbit to innervate the inferior oblique muscle (Fig. 8.102).


Anatomy_Gray

Anatomy_Gray. 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. 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].


Anatomy_Gray

Anatomy_Gray. Just before entering the orbit, the trochlear nerve ascends, passing across the oculomotor nerve [III] and entering the orbit through the superior orbital fissure above the common tendinous ring (Fig. 8.101). 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. 8.103). 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. 8.101). Once in the orbit it courses laterally to supply the lateral rectus muscle.


Anatomy_Gray

Anatomy_Gray. 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.


Anatomy_Gray

Anatomy_Gray. In the eyeball postganglionic sympathetic fibers innervate the dilator pupillae muscle. 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. 8.104). 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. 8.101).


Anatomy_Gray

Anatomy_Gray. 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. 8.105). It receives a branch from the zygomaticotemporal nerve, which carries parasympathetic and sympathetic postganglionic fibers for distribution to the lacrimal gland. Reaching the anterolateral aspect of the orbit, the lacrimal nerve supplies the lacrimal gland, conjunctiva, and lateral part of the upper eyelid. 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. 8.101). About midway across the orbit it divides into its two terminal branches—the supra-orbital and supratrochlear nerves (Figs. 8.104 and 8.105):


Anatomy_Gray

Anatomy_Gray. 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. 8.105), 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.


Anatomy_Gray

Anatomy_Gray. The nasociliary nerve is intermediate in size between the frontal and lacrimal nerves and is usually the first branch from the ophthalmic nerve (Fig. 8.104). 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] (see Fig. 8.101). 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. 8.104 and 8.106). 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.


Anatomy_Gray

Anatomy_Gray. 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 (Fig. 8.106). These include: 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 (Fig. 8.106).


Anatomy_Gray

Anatomy_Gray. 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. 8.106). It is usually described as receiving at least two, and possibly three, branches or roots from other nerves in the orbit.


Anatomy_Gray

Anatomy_Gray. 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. 8.107). 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.


Anatomy_Gray

Anatomy_Gray. A second branch (the sensory root), passes from the nasociliary nerve to the ganglion (Fig. 8.107). This branch enters the posterosuperior aspect of the ganglion, and carries sensory fibers, which pass through the ganglion and continue along the short ciliary nerves to the eyeball. These fibers are responsible for sensory innervation to all parts of the eyeball; however, the sympathetic fibers also may take alternative routes to the eyeball.


Anatomy_Gray

Anatomy_Gray. The third branch to the ciliary ganglion is the most variable. This branch, when present, is the sympathetic root and contains postganglionic sympathetic fibers from the superior cervical ganglion (Fig. 8.107). These fibers travel up the internal carotid artery, leave the plexus surrounding the artery in the cavernous sinus, and enter the orbit through the common tendinous ring. In the orbit they enter the posterior aspect of the ciliary ganglion, cross the ganglion, and continue along the short ciliary nerves to the eyeball; however, the sympathetic fibers also may take alternative routes to the eyeball.


Anatomy_Gray

Anatomy_Gray. Sympathetic fibers to the eyeball may not enter the ganglion as a separate sympathetic root. Rather, the postganglionic sympathetic fibers may leave the plexus associated with the internal carotid artery in the cavernous sinus, join the ophthalmic nerve [V1], and course into the ciliary ganglion in the sensory root from the nasociliary nerve. In addition, the sympathetic fibers carried in the nasociliary nerve may not enter the ganglion at all and may course directly into the eyeball in the long ciliary nerves (Fig. 8.107). Whatever their path, postganglionic sympathetic fibers reach the eyeball and innervate the dilator pupillae muscle. The globe-shaped eyeball occupies the anterior part of the orbit. Its rounded shape is disrupted anteriorly, where it bulges outward. This outward projection represents about one-sixth of the total area of the eyeball and is the transparent cornea (Fig. 8.108).


Anatomy_Gray

Anatomy_Gray. Posterior to the cornea and in order from front to back are the anterior chamber, the iris and pupil, the posterior chamber, the lens, the postremal (vitreous) chamber, and the retina. The anterior chamber is the area directly posterior to the cornea and anterior to the colored part of the eye (iris). The central opening in the iris is the pupil. Posterior to the iris and anterior to the lens is the smaller posterior chamber. The anterior and posterior chambers are continuous with each other through the pupillary opening. They are filled with a fluid (aqueous humor), which is secreted into the posterior chamber, flows into the anterior chamber through the pupil, and is absorbed into the scleral venous sinus (the canal of Schlemm), which is a circular venous channel at the junction between the cornea and the iris (Fig. 8.108).


Anatomy_Gray

Anatomy_Gray. The aqueous humor supplies nutrients to the avascular cornea and lens and maintains the intra-ocular pressure. If the normal cycle of its production and absorption is disturbed so that the amount of fluid increases, intra-ocular pressure will increase. This condition (glaucoma) can lead to a variety of visual problems. The lens separates the anterior one-fifth of the eyeball from the posterior four-fifths (Fig. 8.108). It is a transparent, biconvex elastic disc attached circumferentially to muscles associated with the outer wall of the eyeball. This lateral attachment provides the lens with the ability to change its refractive ability to maintain visual acuity. The clinical term for opacity of the lens is a cataract.


Anatomy_Gray

Anatomy_Gray. The posterior four-fifths of the eyeball, from the lens to the retina, is occupied by the postremal (vitreous) chamber (Fig. 8.108). This segment is filled with a transparent, gelatinous substance—the vitreous body (vitreous humor). This substance, unlike aqueous humor, cannot be replaced. Walls of the eyeball Surrounding the internal components of the eyeball are the walls of the eyeball. They consist of three layers: an outer fibrous layer, a middle vascular layer, and an inner retinal layer (Fig. 8.108). The outer fibrous layer consists of the sclera posteriorly and the cornea anteriorly. The middle vascular layer consists of the choroid posteriorly and is continuous with the ciliary body and iris anteriorly. The inner layer consists of the optic part of the retina posteriorly and the nonvisual retina that covers the internal surface of the ciliary body and iris anteriorly. The arterial supply to the eyeball is from several sources:


Anatomy_Gray

Anatomy_Gray. The arterial supply to the eyeball is from several sources: The short posterior ciliary arteries are branches from the ophthalmic artery that pierce the sclera around the optic nerve and enter the choroid layer (Fig. 8.108). The long posterior ciliary arteries, usually two, enter the sclera on the medial and lateral sides of the optic nerve and proceed anteriorly in the choroid layer to anastomose with the anterior ciliary arteries. The anterior ciliary arteries are branches of the arteries supplying the muscles (Fig. 8.108)—as the muscles attach to the sclera, these arteries pierce the sclera to anastomose with the long posterior ciliary arteries in the choroid layer. The central retinal artery that has traversed the optic nerve and enters the area of the retina at the optic disc.


Anatomy_Gray

Anatomy_Gray. The central retinal artery that has traversed the optic nerve and enters the area of the retina at the optic disc. Venous drainage of the eyeball is primarily related to drainage of the choroid layer. Four large veins (the vorticose veins) are involved in this process. They exit through the sclera from each of the posterior quadrants of the eyeball and enter the superior and inferior ophthalmic veins. There is also a central retinal vein accompanying the central retinal artery. Fibrous layer of the eyeball The fibrous layer of the eyeball consists of two components—the sclera covers the posterior and lateral parts of the eyeball, about five-sixths of the surface, and the cornea covers the anterior part (Fig. 8.108).


Anatomy_Gray

Anatomy_Gray. The sclera is an opaque layer of dense connective tissue that can be seen anteriorly through its conjunctival covering as the “white of the eye.” It is pierced by numerous vessels and nerves, including the optic nerve posteriorly and provides attachment for the various muscles involved in eyeball movements. The fascial sheath of the eyeball covers the surface of the sclera externally from the entrance of the optic nerve to the corneoscleral junction while internally the surface of the sclera is loosely attached to the choroid of the vascular layer. Continuous with the sclera anteriorly is the transparent cornea. It covers the anterior one-sixth of the surface of the eyeball and, being transparent, allows light to enter the eyeball. Vascular layer of the eyeball The vascular layer of the eyeball consists of three continuous parts—the choroid, the ciliary body, and the iris from posterior to anterior (Fig. 8.108).


Anatomy_Gray

Anatomy_Gray. Vascular layer of the eyeball The vascular layer of the eyeball consists of three continuous parts—the choroid, the ciliary body, and the iris from posterior to anterior (Fig. 8.108). The choroid is posterior and represents approximately two-thirds of the vascular layer. It is a thin, highly vascular, pigmented layer consisting of smaller vessels adjacent to the retina and larger vessels more peripherally. It is firmly attached to the retina internally and loosely attached to the sclera externally. Extending from the anterior border of the choroid is the ciliary body (Fig. 8.108). This triangular-shaped structure, between the choroid and the iris, forms a complete ring around the eyeball. Its components include the ciliary muscle and the ciliary processes (Fig. 8.110).


Anatomy_Gray

Anatomy_Gray. The ciliary muscle consists of smooth muscle fibers arranged longitudinally, circularly, and radially. Controlled by parasympathetics traveling to the orbit in the oculomotor nerve [III], these muscle fibers, on contraction, decrease the size of the ring formed by the ciliary body. The ciliary processes are longitudinal ridges projecting from the inner surface of the ciliary body (Fig. 8.110). Extending from them are zonular fibers attached to the lens of the eyeball, which suspend the lens in its proper position and collectively form the suspensory ligament of the lens. Contraction of the ciliary muscle decreases the size of the ring formed by the ciliary body. This reduces tension on the suspensory ligament of the lens. The lens therefore becomes more rounded (relaxed) resulting in accommodation of the lens for near vision. Ciliary processes also contribute to the formation of aqueous humor.


Anatomy_Gray

Anatomy_Gray. Ciliary processes also contribute to the formation of aqueous humor. Completing the vascular layer of the eyeball anteriorly is the iris (Fig. 8.108). This circular structure, projecting outward from the ciliary body, is the colored part of the eye with a central opening (the pupil). Controlling the size of the pupil are smooth muscle fibers (sphincter pupillae) and myoepithelial cells (dilator pupillae) within the iris (Fig. 8.110): Fibers arranged in a circular pattern make up the sphincter pupillae muscle (Table 8.9), which is innervated by parasympathetics—contraction of its fibers decreases or constricts the pupillary opening. Contractile fibers arranged in a radial pattern make up the dilator pupillae muscle, which is innervated by sympathetics—contraction of its fibers increases or dilates the pupillary opening. Inner layer of the eyeball


Anatomy_Gray

Anatomy_Gray. Inner layer of the eyeball The inner layer of the eyeball is the retina (Fig. 8.108). It consists of two parts. Posteriorly and laterally is the optic part of the retina, which is sensitive to light, and anteriorly is the nonvisual part, which covers the internal surface of the ciliary body and the iris. The junction between these parts is an irregular line (the ora serrata). Optic part of the retina The optic part of the retina consists of two layers, an outer pigmented layer and an inner neural layer: The pigmented layer is firmly attached to the choroid and continues anteriorly over the internal surface of the ciliary body and iris. The neural layer, which can be further subdivided into its various neural components, is only attached to the pigmented layer around the optic nerve and at the ora serrata. It is the neural layer that separates in the case of a detached retina. Several obvious features are visible on the posterior surface of the optic part of the retina.


Anatomy_Gray

Anatomy_Gray. It is the neural layer that separates in the case of a detached retina. Several obvious features are visible on the posterior surface of the optic part of the retina. The optic disc is where the optic nerve leaves the retina (Fig. 8.109). It is lighter than the surrounding retina and branches of the central retinal artery spread from this point outward to supply the retina. As there are no lightsensitive receptor cells in the optic disc, it is referred to as a blind spot in the retina. Lateral to the optic disc a small area with a hint of yellowish coloration is the macula lutea with its central depression, the fovea centralis (Fig. 8.109). This is the thinnest area of the retina and visual sensitivity here is higher than elsewhere in the retina because it has fewer rods (light-sensitive receptor cells that function in dim light and are insensitive to color) and more cones (light-sensitive receptor cells that respond to bright light and are sensitive to color).


Anatomy_Gray

Anatomy_Gray. The ear is the organ of hearing and balance. It has three parts (Fig. 8.113): The first part is the external ear consisting of the part attached to the lateral aspect of the head and the canal leading inward. The second part is the middle ear—a cavity in the petrous part of the temporal bone bounded laterally, and separated from the external canal, by a membrane and connected internally to the pharynx by a narrow tube. The third part is the internal ear consisting of a series of cavities within the petrous part of the temporal bone between the middle ear laterally and the internal acoustic meatus medially. The internal ear converts the mechanical signals received from the middle ear, which start as sound captured by the external ear, into electrical signals to transfer information to the brain. The internal ear also contains receptors that detect motion and position.


Anatomy_Gray

Anatomy_Gray. The external ear consists of two parts. The part projecting from the side of the head is the auricle (pinna) and the canal leading inward is the external acoustic meatus. The auricle is on the side of the head and assists in capturing sound. It consists of cartilage covered with skin and arranged in a pattern of various elevations and depressions (Fig. 8.114). The large outside rim of the auricle is the helix. It ends inferiorly at the fleshy lobule, the only part of the auricle not supported by cartilage. The hollow center of the auricle is the concha of the auricle. The external acoustic meatus leaves from the depths of this area. Just anterior to the opening of the external acoustic meatus, in front of the concha, is an elevation (the tragus). Opposite the tragus, and above the fleshy lobule, is another elevation (the antitragus). A smaller curved rim, parallel and anterior to the helix, is the antihelix.


Anatomy_Gray

Anatomy_Gray. 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 (see Fig. 8.56). Both groups of muscles are innervated by the facial nerve [VII]. Sensory innervation of the auricle is from many sources (Fig. 8.115): 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 (posterosuperior portion) from the cervical plexus and the auriculotemporal branch of the mandibular nerve [V3] (anterosuperior portion).


Anatomy_Gray

Anatomy_Gray. 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]). 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.


Anatomy_Gray

Anatomy_Gray. Lymphatic drainage of the auricle passes anteriorly into parotid nodes and posteriorly into mastoid nodes, and possibly into the upper deep cervical nodes. The external acoustic meatus extends from the deepest part of the concha to the tympanic membrane (eardrum), a distance of approximately 1 inch (2.5 cm) (Fig. 8.116). Its walls consist of cartilage and bone. The lateral one-third is formed from cartilaginous extensions from some of the auricular cartilages and the medial two-thirds is a bony tunnel in the temporal bone. Throughout its length the external acoustic meatus is covered with skin, some of which contains hair and modified sweat glands producing cerumen (earwax). Its diameter varies, being wider laterally and narrow medially.


Anatomy_Gray

Anatomy_Gray. The external acoustic meatus does not follow a straight course. From the external opening it passes upward in an anterior direction, then turns slightly posteriorly still passing upward, and finally, turns again in an anterior direction with a slight descent. For examination purposes, observation of the external acoustic meatus and tympanic membrane can be improved by pulling the ear superiorly, posteriorly, and slightly laterally. 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].


Anatomy_Gray

Anatomy_Gray. The tympanic membrane separates the external acoustic meatus from the middle ear (Figs. 8.117 and 8.118). It is at an angle, sloping medially from top to bottom and posteriorly to anteriorly. Its lateral surface therefore faces inferiorly and anteriorly. It consists of a connective tissue core lined with skin on the outside and mucous membrane on the inside. Around the periphery of the tympanic membrane a fibrocartilaginous ring attaches it to the tympanic part of the temporal bone. At its center, a concavity is produced by the attachment on its internal surface of the lower end of the handle of the malleus, part of the malleus bone in the middle ear. This point of attachment is the umbo of the tympanic membrane. Anteroinferior to the umbo of the tympanic membrane a bright reflection of light, referred to as the cone of light, is usually visible when examining the tympanic membrane with an otoscope.


Anatomy_Gray

Anatomy_Gray. Anteroinferior to the umbo of the tympanic membrane a bright reflection of light, referred to as the cone of light, is usually visible when examining the tympanic membrane with an otoscope. Superior to the umbo in an anterior direction is the attachment of the rest of the handle of the malleus (Fig. 8.118). At the most superior extent of this line of attachment a small bulge in the membrane marks the position of the lateral process of the malleus as it projects against the internal surface of the tympanic membrane. Extending away from this elevation, on the internal surface of the membrane, are the anterior and posterior malleolar folds. Superior to these folds the tympanic membrane is thin and slack (the pars flaccida), whereas the rest of the membrane is thick and taut (the pars tensa). Innervation of the external and internal surfaces of the tympanic membrane is by several cranial nerves:


Anatomy_Gray

Anatomy_Gray. 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.


Anatomy_Gray

Anatomy_Gray. Sensory innervation of the mucous membrane on the inner surface of the tympanic membrane is carried entirely by the glossopharyngeal [IX] nerve. The middle ear is an air-filled, mucous membrane–lined space in the temporal bone between the tympanic membrane laterally and the lateral wall of the internal ear medially. It is described as consisting of two parts (Fig. 8.119): the tympanic cavity immediately adjacent to the tympanic membrane, and the epitympanic recess superiorly.


Anatomy_Gray

Anatomy_Gray. The middle ear communicates with the mastoid area posteriorly and the nasopharynx (via the pharyngotympanic tube) anteriorly. Its basic function is to transmit vibrations of the tympanic membrane across the cavity of the middle ear to the internal ear. It accomplishes this through three interconnected but movable bones that bridge the space between the tympanic membrane and the internal ear. These bones are the malleus (connected to the tympanic membrane), the incus (connected to the malleus by a synovial joint), and the stapes (connected to the incus by a synovial joint, and attached to the lateral wall of the internal ear at the oval window). The middle ear has a roof and a floor, and anterior, posterior, medial, and lateral walls (Fig. 8.120).


Anatomy_Gray

Anatomy_Gray. The middle ear has a roof and a floor, and anterior, posterior, medial, and lateral walls (Fig. 8.120). 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. 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.


Anatomy_Gray

Anatomy_Gray. 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. 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. 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. 8.120 and 8.121).


Anatomy_Gray

Anatomy_Gray. 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. 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. 8.120).


Anatomy_Gray

Anatomy_Gray. The foramen for the exit of the chorda tympani nerve from the middle ear is also associated with this wall (Fig. 8.120). 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. 8.120). 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.


Anatomy_Gray

Anatomy_Gray. 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. 8.120): The oval window is posterosuperior to the promontory, is the point of attachment for the base of the 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 the facial canal, which is a ridge of bone produced by the facial nerve [VII] in its canal as it passes through the temporal bone.


Anatomy_Gray

Anatomy_Gray. Just above and posterior to the prominence of the facial canal is a broader ridge of bone (prominence of the lateral semicircular canal) produced by the lateral semicircular canal, which is a structure involved in detecting motion. Posterior to the epitympanic recess of the middle ear is the aditus to the mastoid antrum, which is the opening to the mastoid antrum (Fig. 8.121). The mastoid antrum is a cavity continuous with collections of air-filled spaces (the mastoid cells), throughout the mastoid part of the temporal bone, including the mastoid process. The mastoid antrum is separated from the middle cranial fossa above by only the thin tegmen tympani. The mucous membrane lining the mastoid air cells is continuous with the mucous membrane throughout the middle ear. Therefore infections in the middle ear can easily spread into the mastoid area.


Anatomy_Gray

Anatomy_Gray. The mucous membrane lining the mastoid air cells is continuous with the mucous membrane throughout the middle ear. Therefore infections in the middle ear can easily spread into the mastoid area. The pharyngotympanic tube connects the middle ear with the nasopharynx (Fig. 8.122) and equalizes pressure on both sides of the tympanic membrane. Its opening in the middle ear is on the anterior wall, and from here it extends forward, medially, and downward to enter the nasopharynx just posterior to the inferior meatus of the nasal cavity. It consists of: a bony part (the one-third nearest the middle ear); and a cartilaginous part (the remaining two-thirds). The opening of the bony part is clearly visible on the inferior surface of the skull at the junction of the squamous and petrous parts of the temporal bone immediately posterior to the foramen ovale and foramen spinosum.


Anatomy_Gray

Anatomy_Gray. 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 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]. The bones of the middle ear consist of the malleus, incus, and stapes. They form an osseous chain across the middle ear from the tympanic membrane to the oval window of the internal ear (Fig. 8.123).


Anatomy_Gray

Anatomy_Gray. Muscles associated with the auditory ossicles modulate movement during the transmission of vibrations. The malleus is the largest of the auditory ossicles and is attached to the tympanic membrane. Identifiable parts include the head of the malleus, neck of the malleus, anterior and lateral processes, and handle of the malleus (Fig. 8.123). The head of the 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 the malleus is the constricted neck of the 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 the malleus, which is attached to the tympanic membrane.


Anatomy_Gray

Anatomy_Gray. The downward extension of the malleus, below the anterior and lateral processes, is the handle of the malleus, which is attached to the tympanic membrane. The second bone in the series of auditory ossicles is the incus. It consists of the body of the incus and long and short limbs (Fig. 8.123): The enlarged body of the incus articulates with the head of the 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. The stapes is the most medial bone in the osseous chain and is attached to the oval window. It consists of the head of the stapes, anterior and posterior limbs, and the base of the stapes (Fig. 8.123): The head of the stapes is directed laterally and articulates with the long process of the incus.


Anatomy_Gray

Anatomy_Gray. The head of the 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 the 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. 8.124 and Table 8.10). The tensor tympani muscle lies in a bony canal above the pharyngotympanic tube. It originates from the cartilaginous part of the pharyngotympanic tube, the greater wing of the sphenoid, and its own bony canal, and passes through its canal in a posterior direction, ending in a rounded tendon that inserts into the upper part of the handle of the malleus. Innervation of the tensor tympani is by a branch from the mandibular nerve [V3].


Anatomy_Gray

Anatomy_Gray. Innervation of the tensor tympani is by a branch from the mandibular nerve [V3]. Contraction of the tensor tympani pulls the handle of the malleus medially. This tenses the tympanic membrane, reducing the force of vibrations in response to loud noises. The stapedius muscle is a very small muscle that originates from inside the pyramidal eminence, which is a small projection on the mastoid wall of the middle ear (Fig. 8.124). Its tendon emerges from the apex of the pyramidal eminence and passes forward to attach to the posterior surface of the neck of the stapes. The stapedius is innervated by a branch from the facial nerve [VII]. Contraction of the stapedius muscle, usually in response to loud noises, pulls the stapes posteriorly and prevents excessive oscillation.


Anatomy_Gray

Anatomy_Gray. Contraction of the stapedius muscle, usually in response to loud noises, pulls the stapes posteriorly and prevents excessive oscillation. Numerous arteries supply the structures in the middle ear: the two largest branches are the tympanic branch of the maxillary artery and the mastoid branch of the occipital or posterior auricular arteries; smaller branches come from the middle meningeal artery, the ascending pharyngeal artery, the artery of the pterygoid canal, and tympanic branches from the internal carotid artery. Venous drainage of the middle ear returns to the pterygoid plexus of veins and the superior petrosal sinus.


Anatomy_Gray

Anatomy_Gray. Venous drainage of the middle ear returns to the pterygoid plexus of veins and the superior petrosal sinus. The tympanic plexus innervates the mucous membrane lining the walls and contents of the middle ear, which includes the mastoid area and the pharyngotympanic tube. It is formed by the tympanic nerve, a branch of the glossopharyngeal nerve [IX], and from branches of the internal carotid plexus. The tympanic plexus occurs in the mucous membrane covering the promontory, which is the rounded bulge on the labyrinthine wall of the middle ear (Fig. 8.125). As the glossopharyngeal nerve [IX] exits the skull through the jugular foramen, it gives off the tympanic nerve. This branch reenters the skull through a small foramen and passes through the bone to the middle ear.


Anatomy_Gray

Anatomy_Gray. Once in the middle ear, the tympanic nerve forms the tympanic plexus, along with branches from the plexus of nerves surrounding the internal carotid artery (caroticotympanic nerves). Branches from the tympanic plexus supply the mucous membranes of the middle ear, including the pharyngotympanic tube and the mastoid area. The tympanic plexus also gives off a major branch (the lesser petrosal nerve), which supplies preganglionic parasympathetic fibers to the otic ganglion (Fig. 8.125). The lesser petrosal nerve leaves the area of the promontory, exits the middle ear, travels through the petrous part of the temporal bone, and exits onto the anterior surface of the petrous part of the temporal bone through a hiatus just below the hiatus for the greater petrosal nerve (Fig. 8.126). It continues diagonally across the anterior surface of the temporal bone before exiting the middle cranial fossa through the foramen ovale. Once outside the skull it enters the otic ganglion.


Anatomy_Gray

Anatomy_Gray. The internal ear consists of a series of bony cavities (the bony labyrinth) and membranous ducts and sacs (the membranous labyrinth) within these cavities. All these structures are in the petrous part of the temporal bone between the middle ear laterally and the internal acoustic meatus medially (Figs. 8.127 and 8.128). The bony labyrinth consists of the vestibule, three semicircular canals, and the cochlea (Fig. 8.128). These bony cavities are lined with periosteum and contain a clear fluid (the perilymph). Suspended within the perilymph but not filling all spaces of the bony labyrinth is the membranous labyrinth, which consists of the semicircular ducts, the cochlear duct, and two sacs (the utricle and the saccule). These membranous spaces are filled with endolymph. The structures in the internal ear convey information to the brain about balance and hearing: The cochlear duct is the organ of hearing. The semicircular ducts, utricle, and saccule are the organs of balance.


Anatomy_Gray

Anatomy_Gray. The cochlear duct is the organ of hearing. The semicircular ducts, utricle, and saccule are the organs of balance. The nerve responsible for these functions is the vestibulocochlear nerve [VIII], which divides into vestibular (balance) and cochlear (hearing) parts after entering the internal acoustic meatus (Fig. 8.128). The vestibule, which contains the oval window in its lateral wall, is the central part of the bony labyrinth (Fig. 8.129). It communicates anteriorly with the cochlea and posterosuperiorly with the semicircular canals. A narrow canal (the vestibular aqueduct) leaves the vestibule, and passes through the temporal bone to open on the posterior surface of the petrous part of the temporal bone.


Anatomy_Gray

Anatomy_Gray. A narrow canal (the vestibular aqueduct) leaves the vestibule, and passes through the temporal bone to open on the posterior surface of the petrous part of the temporal bone. Projecting in a posterosuperior direction from the vestibule are the anterior, posterior, and lateral semicircular canals (Fig. 8.129). 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.


Anatomy_Gray

Anatomy_Gray. 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 the cochlea that faces posteromedially and an apex that faces anterolaterally (Fig. 8.130). 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]. Extending laterally throughout the length of the modiolus is a thin lamina of bone (the lamina of the modiolus, or spiral lamina). Circling around the modiolus, and held in a central position by its attachment to the lamina of the modiolus, is the cochlear duct, which is a component of the membranous labyrinth.


Anatomy_Gray

Anatomy_Gray. 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. 8.131). 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. 8.131). The membranous labyrinth is a continuous system of ducts and sacs within the bony labyrinth. It is filled with endolymph and separated from the periosteum that covers the walls of the bony labyrinth by perilymph.


Anatomy_Gray

Anatomy_Gray. Consisting of two sacs (the utricle and the saccule) and four ducts (the three semicircular ducts and the cochlear duct), the membranous labyrinth has unique functions related to balance and hearing: The utricle, saccule, and three semicircular ducts are part of the vestibular apparatus (i.e., organs of balance). The cochlear duct is the organ of hearing. The general organization of the parts of the membranous labyrinth (Fig. 8.131) places: the cochlear duct within the cochlea of the bony labyrinth, anteriorly, the three semicircular ducts within the three semicircular canals of the bony labyrinth, posteriorly, and the saccule and utricle within the vestibule of the bony labyrinth, in the middle. 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). Utricle, saccule, and endolymphatic duct


Anatomy_Gray

Anatomy_Gray. Utricle, saccule, and endolymphatic duct The utricle is the larger of the two sacs. It is oval, elongated and irregular in shape and is in the posterosuperior part of the vestibule of the bony labyrinth. The three semicircular ducts empty into the utricle. Each semicircular duct is similar in shape, including a dilated end forming the ampulla, to its complementary bony semicircular canal, only much smaller. The saccule is a smaller, rounded sac lying in the anteroinferior part of the vestibule of the bony labyrinth (Fig. 8.131). The cochlear duct empties into it.


Anatomy_Gray

Anatomy_Gray. The saccule is a smaller, rounded sac lying in the anteroinferior part of the vestibule of the bony labyrinth (Fig. 8.131). The cochlear duct empties into it. The utriculosaccular duct establishes continuity between all components of the membranous labyrinth and connects the utricle and saccule. Branching from this small duct is the endolymphatic duct, which enters the vestibular aqueduct (a channel through the temporal bone) to emerge onto the posterior surface of the petrous part of the temporal bone in the posterior cranial fossa. Here the endolymphatic duct expands into the endolymphatic sac, which is an extradural pouch that functions in resorption of endolymph.


Anatomy_Gray

Anatomy_Gray. Functionally, sensory receptors for balance are organized into unique structures that are located in each of the components of the vestibular apparatus. In the utricle and saccule the sense organ is the macula of the utricle and the macula of the saccule, respectively, and in the ampulla of each of the three semicircular ducts it is the crista. The utricle responds to linear acceleration in the horizontal plane and sideways head tilts, while the saccule responds to linear acceleration in the vertical plane, such as forward-backward and upward-downward movements. In contrast, the receptors in the three semicircular ducts respond to rotational movement in any direction. Organ of hearing


Anatomy_Gray

Anatomy_Gray. Organ of hearing 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 the 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. 8.132).


Anatomy_Gray

Anatomy_Gray. 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 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 the modiolus, and a membrane (the basilar membrane) extending from this free edge of the lamina of the 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. 8.132).


Anatomy_Gray

Anatomy_Gray. The spiral organ is the organ of hearing, rests on the basilar membrane, and projects into the enclosed, endolymph-filled cochlear duct (Fig. 8.132). The arterial supply to the internal ear is divided between vessels supplying the bony labyrinth and the membranous labyrinth. The bony labyrinth is supplied by the same arteries that supply the surrounding temporal bone—these include an anterior tympanic branch from the maxillary artery, a stylomastoid branch from the posterior auricular artery, and a petrosal branch from the middle meningeal artery.


Anatomy_Gray

Anatomy_Gray. The membranous labyrinth is supplied by the labyrinthine artery, which either arises from the anteroinferior cerebellar artery or is a direct branch of the basilar artery—whatever its origin, it enters the internal acoustic meatus with the facial [VII] and vestibulocochlear [VIII] nerves and eventually divides into: a cochlear branch, which passes through the modiolus and supplies the cochlear duct; and one or two vestibular branches, which supply the vestibular apparatus. Venous drainage of the membranous labyrinth is through vestibular veins and cochlear veins, which follow the arteries. These come together to form a labyrinthine vein, which eventually empties into either the inferior petrosal sinus or the sigmoid sinus.


Anatomy_Gray

Anatomy_Gray. The vestibulocochlear nerve [VIII] carries special afferent fibers for hearing (the cochlear component) and balance (the vestibular component). It enters the lateral surface of the brainstem, between the pons and medulla, after exiting the temporal bone through the internal acoustic meatus and crossing the posterior cranial fossa. Inside the temporal bone, at the distal end of the internal acoustic meatus, the vestibulocochlear nerve divides to form: the cochlear nerve, and the vestibular nerve. The vestibular nerve enlarges to form the vestibular ganglion, before dividing into superior and inferior parts, which distribute to the three semicircular ducts and the utricle and saccule (see Fig. 8.128).


Anatomy_Gray

Anatomy_Gray. The cochlear nerve enters the base of the cochlea and passes upward through the modiolus. The ganglion cells of the cochlear nerve are in the spiral ganglion at the base of the lamina of the modiolus as it winds around the modiolus (Fig. 8.130). Branches of the cochlear nerve pass through the lamina of the modiolus to innervate the receptors in the spiral organ. 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. 8.133A). The vestibulocochlear nerve and the labyrinthine artery accompany it.


Anatomy_Gray

Anatomy_Gray. The facial nerve [VII] enters the internal acoustic meatus in the petrous part of the temporal bone (Fig. 8.133A). The vestibulocochlear nerve and the labyrinthine artery accompany it. 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. 8.133A).


Anatomy_Gray

Anatomy_Gray. Greater petrosal nerve. At the geniculate ganglion, the facial nerve [VII] gives off the greater petrosal nerve (Fig. 8.133A). This is the first branch of the facial nerve [VII]. The greater petrosal nerve leaves the geniculate ganglion, travels anteromedially through the temporal bone, and emerges through the hiatus for the greater petrosal nerve on the anterior surface of the petrous part of the temporal bone (see Fig. 8.126). The greater petrosal nerve carries preganglionic parasympathetic fibers to the pterygopalatine ganglion. Continuing beyond the bend, the position of the facial nerve [VII] is indicated on the medial wall of the middle ear by a bulge (see Fig. 8.125). Nerve to stapedius and chorda tympani. Near the beginning of its vertical descent, the facial nerve [VII] gives off a small branch, the nerve to the stapedius (Fig. 8.133), which innervates the stapedius muscle, and just before it exits the skull the facial nerve [VII] gives off the chorda tympani nerve.


Anatomy_Gray

Anatomy_Gray. The chorda tympani does not immediately exit the temporal bone, but ascends to enter the middle ear through its posterior wall, passing near the upper aspect of the tympanic membrane between the malleus and incus (Fig. 8.133B). It then exits the middle ear through a canal leading to the petrotympanic fissure and exits the skull through this fissure to join the lingual nerve in the infratemporal fossa. Transmission of sound


Anatomy_Gray

Anatomy_Gray. Transmission of sound A sound wave enters the external acoustic meatus and strikes the tympanic membrane moving it medially (Fig. 8.134). As the handle of the malleus is attached to this membrane, it also moves medially. This moves the head of the malleus laterally. Because the heads of the malleus and incus articulate with each other, the head of the incus is also moved laterally. This pushes the long process of the incus medially. The long process articulates with the stapes, so its movement causes the stapes to move medially. In turn, because the base of the stapes is attached to the oval window, the oval window is also moved medially. This action completes the transfer of a large-amplitude, low-force, airborne wave that vibrates the tympanic membrane into a small-amplitude, high-force vibration of the oval window, which generates a wave in the fluid-filled scala vestibuli of the cochlea.


Anatomy_Gray

Anatomy_Gray. The wave established in the perilymph of the scala vestibuli moves through the cochlea and causes an outward bulging of the secondary tympanic membrane covering the round window at the lower end of the scala tympani (Fig. 8.134). This causes the basilar membrane to vibrate, which in turn leads to stimulation of receptor cells in the spiral organ. The receptor cells send impulses back to the brain through the cochlear part of the vestibulocochlear nerve [VIII] where they are interpreted as sound. If the sounds are too loud, causing excessive movement of the tympanic membrane, contraction of the tensor tympani muscle (attached to the malleus) and/or the stapedius muscle (attached to the stapes) dampens the vibrations of the ossicles and decreases the force of the vibrations reaching the oval window. The temporal and infratemporal fossae are interconnected spaces on the lateral side of the head (Fig. 8.135). Their boundaries are formed by bone and soft tissues.


Anatomy_Gray

Anatomy_Gray. The temporal and infratemporal fossae are interconnected spaces on the lateral side of the head (Fig. 8.135). Their boundaries are formed by bone and soft tissues. The temporal fossa is superior to the infratemporal fossa, above the zygomatic arch, and communicates with the infratemporal fossa below through the gap between the zygomatic arch and the more medial surface of the skull. The infratemporal fossa is a wedge-shaped space deep to the masseter muscle and the underlying ramus of the mandible. Structures that travel between the cranial cavity, neck, pterygopalatine fossa, floor of the oral cavity, floor of the orbit, temporal fossa, and superficial regions of the head pass through it.


Anatomy_Gray

Anatomy_Gray. Of the four muscles of mastication (masseter, temporalis, medial pterygoid, and lateral pterygoid) that move the lower jaw at the temporomandibular joint, one (masseter) is lateral to the infratemporal fossa, two (medial and lateral pterygoid) are in the infratemporal fossa, and one fills the temporal fossa. Bones that contribute significantly to the boundaries of the temporal and infratemporal fossae include the temporal, zygomatic, and sphenoid bones, and the maxilla and mandible (Figs. 8.136 and 8.137). Parts of the frontal and parietal bones are also involved. The squamous part of the temporal bone forms part of the bony framework of the temporal and infratemporal fossae. The tympanic part of the temporal bone forms the posteromedial corner of the roof of the infratemporal fossa, and also articulates with the head of the mandible to form the temporomandibular joint.


Anatomy_Gray

Anatomy_Gray. The tympanic part of the temporal bone forms the posteromedial corner of the roof of the infratemporal fossa, and also articulates with the head of the mandible to form the temporomandibular joint. The lateral surface of the squamous part of the temporal bone is marked by two surface features on the medial wall of the temporal fossa: a transversely oriented supramastoid crest, which extends posteriorly from the base of the zygomatic process and marks the posteroinferior border of the temporal fossa; and a vertically oriented groove for the middle temporal artery, a branch of the superficial temporal artery.


Anatomy_Gray

Anatomy_Gray. Two features that participate in forming the temporomandibular joint on the inferior aspect of the root of the zygomatic process are the articular tubercle and the mandibular fossa. Both are elongate from medial to lateral. Posterior to the mandibular fossa is the external acoustic meatus. The tympanic part of the temporal bone is a flat concave plate of bone that curves inferiorly from the back of the mandibular fossa and forms part of the wall of the external auditory meatus. When viewed from inferiorly, there is a distinct tympanosquamous fissure between the tympanic and squamous parts of the temporal bone. Medially, a small slip of bone from the petrous part of the temporal bone insinuates itself into the fissure and forms a petrotympanic fissure between it and the tympanic part (Fig. 8.136). The chorda tympani nerve exits the skull and enters the infratemporal fossa through the medial end of the petrotympanic fissure.


Anatomy_Gray

Anatomy_Gray. The chorda tympani nerve exits the skull and enters the infratemporal fossa through the medial end of the petrotympanic fissure. The parts of the sphenoid bone that form part of the bony framework of the infratemporal fossa are the lateral plate of the pterygoid process and the greater wing (Fig. 8.136). The greater wing also forms part of the medial wall of the temporal fossa. The greater wings extend one on each side from the body of the sphenoid. They project laterally from the body and curve superiorly. The inferior and lateral surfaces form the roof of the infratemporal fossa and the medial wall of the temporal fossa, respectively.


Anatomy_Gray

Anatomy_Gray. The sharply angled boundary between the lateral and inferior surfaces of the greater wing is the infratemporal crest (Fig. 8.136). Two apertures (the foramen ovale and the foramen spinosum) pass through the base of the greater wing and allow the mandibular nerve [V3] and the middle meningeal artery, respectively, to pass between the middle cranial fossa and infratemporal fossa. In addition, one or more small sphenoidal emissary foramina penetrate the base of the greater wing anteromedial to the foramen ovale and allow emissary veins to pass between the pterygoid plexus of veins in the infratemporal fossa and the cavernous sinus in the middle cranial fossa. Projecting vertically downward from the greater wing immediately medial to the foramen spinosum is the irregularly shaped spine of the sphenoid, which is the attachment site for the cranial end of the sphenomandibular ligament.


Anatomy_Gray

Anatomy_Gray. The lateral plate of the pterygoid process is a vertically oriented sheet of bone that projects posterolaterally from the pterygoid process (Fig. 8.136). Its lateral and medial surfaces provide attachment for the lateral and medial pterygoid muscles, respectively. The posterior surface of the maxilla contributes to the anterior wall of the infratemporal fossa (Fig. 8.136). This surface is marked by a foramen for the posterosuperior alveolar nerve and vessels. The superior margin forms the inferior border of the inferior orbital fissure. The zygomatic bone is a quadrangular-shaped bone that forms the palpable bony prominence of the cheek: A maxillary process extends anteromedially to articulate with the zygomatic process of the maxilla. A frontal process extends superiorly to articulate with the zygomatic process of the frontal bone. A temporal process extends posteriorly to articulate with the zygomatic process of the temporal bone to complete the zygomatic arch.


Anatomy_Gray

Anatomy_Gray. A temporal process extends posteriorly to articulate with the zygomatic process of the temporal bone to complete the zygomatic arch. A small zygomaticofacial foramen on the lateral surface of the zygomatic bone transmits the zygomaticofacial nerve and vessels onto the cheek. A thin plate of bone extends posteromedially from the frontal process and contributes to the lateral wall of the orbit on one side and the anterior wall of the temporal fossa on the other. A zygomaticotemporal foramen on the temporal fossa surface of the plate where it attaches to the frontal process is for the zygomaticotemporal nerve. Ramus of mandible The ramus of the mandible is quadrangular in shape and has medial and lateral surfaces and condylar and coronoid processes (Fig. 8.137). The lateral surface of the ramus of the mandible is generally smooth except for the presence of a few obliquely oriented ridges. Most of the lateral surface provides attachment for the masseter muscle.


Anatomy_Gray

Anatomy_Gray. The posterior and inferior borders of the ramus intersect to form the angle of the mandible, while the superior border is notched to form the mandibular notch. The anterior border is sharp and is continuous below with the oblique line on the body of the mandible. The coronoid process extends superiorly from the junction of the anterior and superior borders of the ramus. It is a flat, triangular process that provides attachment for the temporalis muscle. The condylar process extends superiorly from the posterior and superior borders of the ramus. It consists of: the head of the mandible, which is expanded medially and participates in forming the temporomandibular joint; and the neck of the mandible, which bears a shallow depression (the pterygoid fovea) on its anterior surface for attachment of the lateral pterygoid muscle.


Anatomy_Gray

Anatomy_Gray. The medial surface of the ramus of the mandible is the lateral wall of the infratemporal fossa (Fig. 8.137B). Its most distinctive feature is the mandibular foramen, which is the superior opening of the mandibular canal. The inferior alveolar nerve and vessels pass through this foramen. Immediately anterosuperior to the mandibular foramen is a triangular elevation (the lingula) for attachment of the mandibular end of the sphenomandibular ligament. An elongate groove (the mylohyoid groove) extends anteroinferiorly from the mandibular foramen. The nerve to the mylohyoid is in this groove. Posteroinferior to the mylohyoid groove and mandibular foramen, the medial surface of the ramus of the mandible is roughened for attachment of the medial pterygoid muscle. The temporomandibular joints, one on each side, allow opening and closing of the mouth and complex chewing or side-to-side movements of the lower jaw.


Anatomy_Gray

Anatomy_Gray. The temporomandibular joints, one on each side, allow opening and closing of the mouth and complex chewing or side-to-side movements of the lower jaw. Each joint is synovial and is formed between the head of the mandible and the articular fossa and articular tubercle of the temporal bone (Fig. 8.138A). Unlike most other synovial joints where the articular surfaces of the bones are covered by a layer of hyaline cartilage, those of the temporomandibular joint are covered by fibrocartilage. In addition, the joint is completely divided by a fibrous articular disc into two parts: The lower part of the joint allows mainly the hinge-like depression and elevation of the mandible. The upper part of the joint allows the head of the mandible to translocate forward (protrusion) onto the articular tubercle and backward (retraction) into the mandibular fossa. Opening the mouth involves both depression and protrusion (Fig. 8.138B).


Anatomy_Gray

Anatomy_Gray. Opening the mouth involves both depression and protrusion (Fig. 8.138B). The forward or protrusive movement allows greater depression of the mandible by preventing backward movement of the angle of the mandible into structures in the neck. The synovial membrane of the joint capsule lines all nonarticular surfaces of the upper and lower compartments of the joint and is attached to the margins of the articular disc. The fibrous membrane of the joint capsule encloses the temporomandibular joint complex and is attached: above along the anterior margin of the articular tubercle, laterally and medially along the margins of the articular fossa, posteriorly to the region of the tympanosquamous suture, and below around the upper part of the neck of the mandible. The articular disc attaches around its periphery to the inner aspect of the fibrous membrane.


Anatomy_Gray

Anatomy_Gray. The articular disc attaches around its periphery to the inner aspect of the fibrous membrane. Three extracapsular ligaments are associated with the temporomandibular joint—the lateral, sphenomandibular, and the stylomandibular ligaments (Fig. 8.139): The lateral ligament is closest to the joint, just lateral to the capsule, and runs diagonally backward from the margin of the articular tubercle to the neck of the mandible. The sphenomandibular ligament is medial to the temporomandibular joint, runs from the spine of the sphenoid bone at the base of the skull to the lingula on the medial side of the ramus of the mandible. The stylomandibular ligament passes from the styloid process of the temporal bone to the posterior margin and angle of the mandible. Movements of the mandible


Anatomy_Gray

Anatomy_Gray. The stylomandibular ligament passes from the styloid process of the temporal bone to the posterior margin and angle of the mandible. Movements of the mandible A chewing or grinding motion occurs when the movements at the temporomandibular joint on one side are coordinated with a reciprocal set of movements at the joint on the other side. Movements of the mandible include depression, elevation, protrusion, and retraction (Fig. 8.140): Depression is generated by the digastric, geniohyoid, and mylohyoid muscles on both sides, is normally assisted by gravity, and, because it involves forward movement of the head of the mandible onto the articular tubercle, the lateral pterygoid muscles are also involved. Elevation is a very powerful movement generated by the temporalis, masseter, and medial pterygoid muscles and also involves movement of the head of the mandible into the mandibular fossa.


Anatomy_Gray

Anatomy_Gray. Elevation is a very powerful movement generated by the temporalis, masseter, and medial pterygoid muscles and also involves movement of the head of the mandible into the mandibular fossa. Protraction is mainly achieved by the lateral pterygoid muscle, with some assistance by the medial pterygoid. Retraction is carried out by the geniohyoid and digastric muscles, and by the posterior and deep fibers of the temporalis and masseter muscles, respectively. Except for the geniohyoid muscle, which is innervated by the C1 spinal nerve, all muscles that move the temporomandibular joints are innervated by the mandibular nerve [V3] by branches that originate in the infratemporal fossa. The masseter muscle is a powerful muscle of mastication that elevates the mandible (Fig. 8.141 and Table 8.11). It overlies the lateral surface of the ramus of the mandible.


Anatomy_Gray

Anatomy_Gray. The masseter muscle is a powerful muscle of mastication that elevates the mandible (Fig. 8.141 and Table 8.11). It overlies the lateral surface of the ramus of the mandible. The masseter muscle is quadrangular in shape and is anchored above to the zygomatic arch and below to most of the lateral surface of the ramus of the mandible. The more superficial part of the masseter originates from the maxillary process of the zygomatic bone and the anterior two-thirds of the zygomatic process of the maxilla. It inserts into the angle of the mandible and related posterior part of the lateral surface of the ramus of the mandible. The deep part of the masseter originates from the medial aspect of the zygomatic arch and the posterior part of its inferior margin and inserts into the central and upper part of the ramus of the mandible as high as the coronoid process.


Anatomy_Gray

Anatomy_Gray. The masseter is innervated by the masseteric nerve from the mandibular nerve [V3] and supplied with blood by the masseteric artery from the maxillary artery. The masseteric nerve and artery originate in the infratemporal fossa and pass laterally over the margin of the mandibular notch to enter the deep surface of the masseter muscle. The temporal fossa is a narrow fan-shaped space that covers the lateral surface of the skull (Fig. 8.142A): Its upper margin is defined by a pair of temporal lines that arch across the skull from the zygomatic process of the frontal bone to the supramastoid crest of the temporal bone. It is limited laterally by the temporal fascia, which is a tough, fan-shaped aponeurosis overlying the temporalis muscle and attached by its outer margin to the superior temporal line and by its inferior margin to the zygomatic arch.


Anatomy_Gray

Anatomy_Gray. Anteriorly, it is limited by the posterior surface of the frontal process of the zygomatic bone and the posterior surface of the zygomatic process of the frontal bone, which separate the temporal fossa behind from the orbit in front. Its inferior margin is marked by the zygomatic arch laterally and by the infratemporal crest of the greater wing of the sphenoid medially (Fig. 8.142B)—between these two features, the floor of the temporal fossa is open medially to the infratemporal fossa and laterally to the region containing the masseter muscle. The major structure in the temporal fossa is the temporalis muscle. Also passing through the fossa is the zygomaticotemporal branch of the maxillary nerve [V2], which enters the region through the zygomaticotemporal foramen on the temporal fossa surface of the zygomatic bone.


Anatomy_Gray

Anatomy_Gray. The temporalis muscle is a large, fan-shaped muscle that fills much of the temporal fossa (Fig. 8.143). It originates from the bony surfaces of the fossa superiorly to the inferior temporal line and is attached laterally to the surface of the temporal fascia. The more anterior fibers are oriented vertically while the more posterior fibers are oriented horizontally. The fibers converge inferiorly to form a tendon, which passes between the zygomatic arch and the infratemporal crest of the greater wing of the sphenoid to insert on the coronoid process of the mandible. The temporalis muscle attaches down the anterior surface of the coronoid process and along the related margin of the ramus of the mandible, almost to the last molar tooth.


Anatomy_Gray

Anatomy_Gray. The temporalis muscle attaches down the anterior surface of the coronoid process and along the related margin of the ramus of the mandible, almost to the last molar tooth. The temporalis is a powerful elevator of the mandible. Because this movement involves posterior translocation of the head of the mandible from the articular tubercle of the temporal bone and back into the mandibular fossa, the temporalis also retracts the mandible or pulls it posteriorly. In addition, the temporalis participates in side-to-side movements of the mandible. The temporalis is innervated by deep temporal nerves that originate from the mandibular nerve [V3] in the infratemporal fossa and then pass into the temporal fossa. Blood supply of the temporalis is by deep temporal arteries, which travel with the nerves, and the middle temporal artery, which penetrates the temporal fascia at the posterior end of the zygomatic arch.


Anatomy_Gray

Anatomy_Gray. The deep temporal nerves, usually two in number, originate from the anterior trunk of the mandibular nerve [V3] in the infratemporal fossa (Fig. 8.144). They pass superiorly and around the infratemporal crest of the greater wing of the sphenoid to enter the temporal fossa deep to the temporalis muscle, and supply the temporalis muscle. The zygomaticotemporal nerve is a branch of the zygomatic nerve (see Fig. 8.87, p. 922). The zygomatic nerve is a branch of the maxillary nerve [V2], which originates in the pterygopalatine fossa and passes into the orbit. The zygomaticotemporal nerve enters the temporal fossa through one or more small foramina on the temporal fossa surface of the zygomatic bone. Branches of the zygomaticotemporal nerve pass superiorly between the bone and the temporalis muscle to penetrate the temporal fascia and supply the skin of the temple (Fig. 8.144).


Anatomy_Gray

Anatomy_Gray. Branches of the zygomaticotemporal nerve pass superiorly between the bone and the temporalis muscle to penetrate the temporal fascia and supply the skin of the temple (Fig. 8.144). Normally two in number, these vessels originate from the maxillary artery in the infratemporal fossa and travel with the deep temporal nerves around the infratemporal crest of the greater wing of the sphenoid to supply the temporalis muscle (Fig. 8.144). They anastomose with branches of the middle temporal artery. The middle temporal artery originates from the superficial temporal artery just superior to the root of the zygomatic arch between this structure and the external ear (Fig. 8.144). It penetrates the temporalis fascia, passes under the margin of the temporalis muscle, and travels superiorly on the deep surface of the temporalis muscle. The middle temporal artery supplies the temporalis and anastomoses with branches of the deep temporal arteries.


Anatomy_Gray

Anatomy_Gray. The middle temporal artery supplies the temporalis and anastomoses with branches of the deep temporal arteries. The wedge-shaped infratemporal fossa is inferior to the temporal fossa and between the ramus of the mandible laterally and the wall of the pharynx medially. It has a roof, a lateral wall, and a medial wall, and is open to the neck posteroinferiorly (Fig. 8.145): The roof is formed by the inferior surfaces of the greater wing of the sphenoid and the temporal bone, contains the foramen spinosum, foramen ovale, and the petrotympanic fissure, and lateral to the infratemporal crest of the greater wing of the sphenoid, is open superiorly to the temporal fossa. The lateral wall is the medial surface of the ramus of the mandible, which contains the opening to the mandibular canal.


Anatomy_Gray

Anatomy_Gray. The lateral wall is the medial surface of the ramus of the mandible, which contains the opening to the mandibular canal. The medial wall is formed anteriorly by the lateral plate of the pterygoid process and more posteriorly by the pharynx and by two muscles of the soft palate (tensor and levator veli palatini muscles), and contains the pterygomaxillary fissure anteriorly, which allows structures to pass between the infratemporal and pterygopalatine fossae. The anterior wall is formed by part of the posterior surface of the maxilla and contains the alveolar foramen, and the upper part opens as the inferior orbital fissure into the orbit. Major contents of the infratemporal fossa include the sphenomandibular ligament, medial and lateral pterygoid muscles (Table 8.11), the maxillary artery, the mandibular nerve [V3], branches of the facial nerve [VII] and the glossopharyngeal nerve [IX], and the pterygoid plexus of veins.


Anatomy_Gray

Anatomy_Gray. The sphenomandibular ligament is an extracapsular ligament of the temporomandibular joint. It is attached superiorly to the spine of the sphenoid bone and expands inferiorly to attach to the lingula of the mandible and the posterior margin of the mandibular foramen (Fig. 8.146). The medial pterygoid muscle is quadrangular in shape and has deep and superficial heads (Fig. 8.146): The deep head is attached above to the medial surface of the lateral plate of the pterygoid process and the associated surface of the pyramidal process of the palatine bone, and descends obliquely downward, medial to the sphenomandibular ligament, to attach to the roughened medial surface of the ramus of the mandible near the angle of the mandible. The superficial head originates from the tuberosity of the maxilla and adjacent pyramidal process of the palatine bone and joins with the deep head to insert on the mandible.


Anatomy_Gray

Anatomy_Gray. The superficial head originates from the tuberosity of the maxilla and adjacent pyramidal process of the palatine bone and joins with the deep head to insert on the mandible. The medial pterygoid mainly elevates the mandible. Because it passes obliquely backward to insert into the mandible, it also assists the lateral pterygoid muscle in protruding the lower jaw. The medial pterygoid is innervated by the nerve to the medial pterygoid from the mandibular nerve [V3]. The lateral pterygoid is a thick triangular muscle and like the medial pterygoid muscle has two heads (Fig. 8.147): The upper head originates from the roof of the infratemporal fossa (inferior surface of the greater wing of the sphenoid and the infratemporal crest) lateral to the foramen ovale and foramen spinosum.


Anatomy_Gray

Anatomy_Gray. The lower head is larger than the upper head and originates from the lateral surface of the lateral plate of the pterygoid process, and the inferior part insinuates itself between the cranial attachments of the two heads of the medial pterygoid. The fibers from both heads of the lateral pterygoid muscle converge to insert into the pterygoid fovea of the neck of the mandible and into the capsule of the temporomandibular joint in the region where the capsule is attached internally to the articular disc. Unlike the medial pterygoid muscle whose fibers tend to be oriented vertically, those of the lateral pterygoid are oriented almost horizontally. As a result, when the lateral pterygoid contracts it pulls the articular disc and head of the mandible forward onto the articular tubercle and is therefore the major protruder of the lower jaw. The lateral pterygoid is innervated by the nerve to the lateral pterygoid from the mandibular nerve [V3].


Anatomy_Gray

Anatomy_Gray. The lateral pterygoid is innervated by the nerve to the lateral pterygoid from the mandibular nerve [V3]. When the lateral and medial pterygoids contract on only one side, the chin moves to the opposite side. When opposite movements at the two temporomandibular joints are coordinated, a chewing movement results. The mandibular nerve [V3] is the largest of the three divisions of the trigeminal nerve [V]. Unlike the ophthalmic [V1] and maxillary [V2] nerves, which are purely sensory, the mandibular nerve [V3] is both motor and sensory.


Anatomy_Gray

Anatomy_Gray. Unlike the ophthalmic [V1] and maxillary [V2] nerves, which are purely sensory, the mandibular nerve [V3] is both motor and sensory. In addition to carrying general sensation from the teeth and gingivae of the mandible, the anterior two-thirds of the tongue, mucosa on the floor of the oral cavity, the lower lip, skin over the temple and lower face, and part of the cranial dura mater, the mandibular nerve [V3] also carries motor innervation to most of the muscles that move the mandible, one of the muscles (tensor tympani) in the middle ear, and one of the muscles of the soft palate (tensor veli palatini). All branches of the mandibular nerve [V3] originate in the infratemporal fossa. Like the ophthalmic [V1] and maxillary [V2] nerves, the sensory part of the mandibular nerve [V3] originates from the trigeminal ganglion in the middle cranial fossa (Fig. 8.148):


Anatomy_Gray

Anatomy_Gray. Like the ophthalmic [V1] and maxillary [V2] nerves, the sensory part of the mandibular nerve [V3] originates from the trigeminal ganglion in the middle cranial fossa (Fig. 8.148): The sensory part of the mandibular nerve [V3] drops vertically through the foramen ovale and enters the infratemporal fossa between the tensor veli palatini muscle and the upper head of the lateral pterygoid muscle. The small motor root of the trigeminal nerve [V] passes medial to the trigeminal ganglion in the cranial cavity, then passes through the foramen ovale and immediately joins the sensory part of the mandibular nerve [V3]. Soon after the sensory and motor roots join, the mandibular nerve [V3] gives rise to a small meningeal branch and to the nerve to the medial pterygoid, and then divides into anterior and posterior trunks (Fig. 8.148):


Anatomy_Gray

Anatomy_Gray. Branches from the anterior trunk are the buccal, masseteric, and deep temporal nerves, and the nerve to the lateral pterygoid, all of which, except the buccal nerve (which is predominantly sensory) are motor nerves. Branches from the posterior trunk are the auriculotemporal, lingual, and inferior alveolar nerves, all of which, except a small nerve (nerve to the mylohyoid) that branches from the inferior alveolar nerve, are sensory nerves. The meningeal branch originates from the medial side of the mandibular nerve [V3] and ascends to leave the infratemporal fossa with the middle meningeal artery and reenter the cranial cavity through the foramen spinosum (Fig. 8.148). It is sensory for the dura mater, mainly of the middle cranial fossa, and also supplies the mastoid cells that communicate with the middle ear. Nerve to medial pterygoid


Anatomy_Gray

Anatomy_Gray. Nerve to medial pterygoid The nerve to the medial pterygoid also originates medially from the mandibular nerve [V3] (Fig. 8.148). It descends to enter and supply the deep surface of the medial pterygoid muscle. Near its origin from the mandibular nerve [V3], it has two small branches: One of these supplies the tensor veli palatini. The other ascends to supply the tensor tympani muscle, which occupies a small bony canal above and parallel to the pharyngotympanic tube in the temporal bone. The buccal nerve is a branch of the anterior trunk of the mandibular nerve [V3] (Fig. 8.148). It is predominantly a sensory nerve, but may also carry the motor innervation to the lateral pterygoid muscle and to part of the temporalis muscle.


Anatomy_Gray

Anatomy_Gray. The buccal nerve passes laterally between the upper and lower heads of the lateral pterygoid and then descends around the anterior margin of the insertion of the temporalis muscle to the anterior margin of the ramus of the mandible, often slipping through the tendon of the temporalis. It continues into the cheek lateral to the buccinator muscle to supply general sensory nerves to the adjacent skin and oral mucosa and the buccal gingivae of the lower molars. The masseteric nerve is a branch of the anterior trunk of the mandibular nerve [V3] (Fig. 8.148; also see Fig. 8.141). It passes laterally over the lateral pterygoid muscle and through the mandibular notch to penetrate and supply the masseter muscle.


Anatomy_Gray

Anatomy_Gray. The deep temporal nerves, usually two in number, originate from the anterior trunk of the mandibular nerve [V3] (Fig. 8.148; also see Fig. 8.144). They pass laterally above the lateral pterygoid muscle and curve around the infratemporal crest to ascend in the temporal fossa and supply the temporalis muscle from its deep surface. Nerve to lateral pterygoid The nerve to the lateral pterygoid may originate directly as a branch from the anterior trunk of the mandibular nerve [V3] or from its buccal branch (Fig. 8.148). From its origin, it passes directly into the deep surface of the lateral pterygoid muscle. The auriculotemporal nerve is the first branch of the posterior trunk of the mandibular nerve [V3] and originates as two roots, which pass posteriorly around the middle meningeal artery ascending from the maxillary artery to the foramen spinosum (Fig. 8.149).


Anatomy_Gray

Anatomy_Gray. The auriculotemporal nerve passes first between the tensor veli palatini muscle and the upper head of the lateral pterygoid muscle, and then between the sphenomandibular ligament and the neck of the mandible. It curves laterally around the neck of the mandible and then ascends deep to the parotid gland between the temporomandibular joint and ear. The terminal branches of the auriculotemporal nerve carry general sensation from skin over a large area of the temple. In addition, the auriculotemporal nerve contributes to sensory innervation of the external ear, the external auditory meatus, tympanic membrane, and temporomandibular joint. It also delivers postganglionic parasympathetic nerves from the glossopharyngeal nerve [IX] to the parotid gland.


Anatomy_Gray

Anatomy_Gray. The lingual nerve is a major sensory branch of the posterior trunk of the mandibular nerve [V3] (Fig. 8.149A,B). It carries general sensation from the anterior two-thirds of the tongue, oral mucosa on the floor of the oral cavity, and lingual gingivae associated with the lower teeth. The lingual nerve is joined high in the infratemporal fossa by the chorda tympani branch of the facial nerve [VII] (Fig. 8.149C), which carries: taste from the anterior two-thirds of the tongue, and parasympathetic fibers to all salivary glands below the level of the oral fissure. The lingual nerve first descends between the tensor veli palatini muscle and the lateral pterygoid muscle, where it is joined by the chorda tympani nerve, and then descends across the lateral surface of the medial pterygoid muscle to enter the oral cavity.


Anatomy_Gray

Anatomy_Gray. The lingual nerve enters the oral cavity between the posterior attachment of the mylohyoid muscle to the mylohyoid line and the attachment of the superior constrictor of the pharynx to the pterygomandibular raphe. As the lingual nerve enters the floor of the oral cavity, it is in a shallow groove on the medial surface of the mandible immediately inferior to the last molar tooth. In this position, it is palpable through the oral mucosa and in danger when one is operating on the molar teeth and gingivae (Fig. 8.149C). The lingual nerve passes into the tongue on the lateral surface of the hyoglossus muscle where it is attached to the submandibular ganglion. This ganglion is where the preganglionic parasympathetic fibers carried from the infratemporal fossa into the floor of the oral cavity on the lingual nerve synapse with postganglionic parasympathetic fibers (see Fig. 8.150).


Anatomy_Gray

Anatomy_Gray. The inferior alveolar nerve, like the lingual nerve, is a major sensory branch of the posterior trunk of the mandibular nerve [V3] (Fig. 8.149A–C). In addition to innervating all lower teeth and much of the associated gingivae, it also supplies the mucosa and skin of the lower lip and skin of the chin. It has one motor branch, which innervates the mylohyoid muscle and the anterior belly of the digastric muscle.


Anatomy_Gray

Anatomy_Gray. The inferior alveolar nerve originates deep to the lateral pterygoid muscle from the posterior trunk of the mandibular nerve [V3] in association with the lingual nerve. It descends on the lateral surface of the medial pterygoid muscle, passes between the sphenomandibular ligament and the ramus of the mandible, and then enters the mandibular canal through the mandibular foramen. Just before entering the mandibular foramen, it gives origin to the nerve to the mylohyoid (Fig. 8.149C), which lies in the mylohyoid groove inferior to the foramen and continues anteriorly below the floor of the oral cavity to innervate the mylohyoid muscle and the anterior belly of the digastric muscle. The inferior alveolar nerve passes anteriorly within the mandibular canal of the lower jaw. The mandibular canal and its contents are inferior to the roots of the molar teeth, and the roots can sometimes curve around the canal making extraction of these teeth difficult.


Anatomy_Gray

Anatomy_Gray. The inferior alveolar nerve supplies branches to the three molar teeth and the second premolar tooth and associated labial gingivae, and then divides into its two terminal branches: the incisive nerve, which continues in the mandibular canal to supply the first premolar, incisor, and canine teeth, and related gingivae; and the mental nerve, which exits the mandible through the mental foramen and supplies the lower lip and chin (Fig. 8.149A,B). The mental nerve is palpable and sometimes visible through the oral mucosa adjacent to the roots of the premolar teeth. Chorda tympani and the lesser petrosal nerve Branches of two cranial nerves join branches of the mandibular nerve [V3] in the infratemporal fossa (Fig. 8.150). These are the chorda tympani branch of the facial nerve [VII] and the lesser petrosal nerve, a branch of the tympanic plexus in the middle ear, which had its origin from a branch of the glossopharyngeal nerve [IX] (see Fig. 8.125, p. 953).