Chapter 45: The orbit

Bony orbit

The orbits (figs. 45-1 and 45-2) are two bony cavities occupied by the eyes and associated muscles, nerves, blood vessels, fat, and much of the lacrimal apparatus. Each orbit is shaped like a pear or a four-sided pyramid, with its apex situated posteriorly and its base anteriorly. The orbit is related (1) on its superior side to the anterior cranial fossa and usually to the frontal sinus, (2) laterally to the temporal fossa in (anterior) and to the middle cranial fossa (posterior), (3) on its inferior side to the maxillary sinus, and (4) medially to the ethmoidal and the anterior extent of the sphenoidal sinuses.

The margin of the orbit, readily palpable, is formed by the frontal, zygomatic, and maxillary bones (fig. 45-1A). It may be considered in four parts: superior, lateral, inferior, and medial.

The superior margin, formed by the frontal bone, presents near its medial end either a supraorbital notch or a supraorbital foramen, which transmits the nerve and vessels of the same name.

The lateral margin is formed by the zygomatic process of the frontal bone and the frontal process of the zygomatic bone.

The inferior margin is formed by the zygomatic and maxillary bones. The infraorbital foramen, for the nerve and artery of the same name, is less than 1 cm inferior to the inferior margin.

The medial margin, formed by the maxilla as well as by the lacrimal and frontal bones, is expanded as the fossa for the lacrimal sac. The fossa passes inferiorly through the floor of the orbit as the nasolacrimal canal, which transmits the nasolacrimal duct from the lacrimal sac to the inferior meatus of the nose (fig. 45-11).

Walls of the orbit.

The orbit possesses four walls (fig. 45-1A and C): a roof, lateral wall, floor, and medial wall.

The roof (frontal and sphenoid bones) presents the fossa for the lacrimal gland anterolaterally and the trochlear pit for the cartilaginous or bony pulley of the superior oblique muscle anteromedially. The optic canal lies in the posterior part of the roof, between the roots of the lesser wing of the sphenoid bone. It transmits the optic nerve and ophthalmic artery from the middle cranial fossa.

The posterior aspect of the lateral wall (zygomatic and sphenoid bones) is demarcated by the superior and inferior orbital fissures. The superior orbital fissure lies between the greater and lesser wings of the sphenoid bone. It communicates with the middle cranial fossa and transmits cranial nerves III, IV, and VI, the three branches of the ophthalmic nerve, and the ophthalmic veins (fig. 45-5). The inferior orbital fissure communicates with the infratemporal and pterygopalatine fossae and transmits the zygomatic nerve. The lateral walls of the two orbits are set at approximately a right angle from one another, whereas the medial walls are nearly parallel to each other (fig. 45-3).

The floor (maxilla, zygomatic, and palatine bones) presents the infraorbital groove and canal for the nerve and artery of the same name. The inferior oblique muscle arises anteromedially, immediately lateral to the nasolacrimal canal.

The medial wall (ethmoid, lacrimal, and frontal bones) is very thin. Its main component (the orbital plate of the ethmoid) is papyraceous (paper-thin). At the junction of the medial wall with the roof, the anterior and posterior ethmoidal foramina transmit the nerves and arteries of the same name.

In summary, the orbit communicates with the middle cranial fossa (via the optic canal and superior orbital fissure), the infratemporal and pterygopalatine fossae ( via the inferior orbital fissure), the inferior meatus of the nose (via the nasolacrimal canal), the nasal cavity (via the anterior ethmoidal foramen), and the face ( via supraorbital and infraorbital foramina).

Ophthalmic nerve

The ophthalmic nerve, the first division of the trigeminal (fifth cranial) nerve, is a wholly afferent nerve that supplies the globe and conjunctiva, lacrimal gland and sac, nasal mucosa and frontal sinus, external nose, upper eyelid, forehead, and scalp, It arises from the trigeminal ganglion which contains the cell bodies of its sensory nerve fibers. It divides near the superior orbital fissure into the lacrimal, frontal, and nasociliary nerves. These pass through the superior orbital fissure and traverse the orbit (figs. 45-5 and 45-6). The lacrimal and frontal nerves lie superior to the muscles of the globe, whereas the nasociliary nerve enters the orbit within the cone formed by the muscles.

The lacrimal nerve proceeds along the superior border of the lateral rectus and supplies the lacrimal gland, conjunctiva, and upper eyelid. A communication with the zygomatic nerve (maxillary division of trigeminal) carries some secretory fibers to the lacrimal gland.

The frontal nerve passes anteriorward on the levator palpebrae superioris and divides into the supraorbital and supratrochlear nerves. The supraorbital nerve leaves the orbit through the supraorbital notch or foramen and supplies the forehead, scalp, upper eyelid, and frontal sinus (the lower eyelid is supplied by the maxillary nerve.) The supratrochlear nerve, more medial and much smaller, supplies a small area of the forehead and upper eyelid (fig. 45-1B).

The nasociliary nerve is within the cone of muscles and is therefore on a lower plane than the lacrimal and frontal nerves. The nasociliary nerve is the sensory nerve to the eyeball and is accompanied by the ophthalmic artery. It courses anteriorward, inferior to the superior rectus, crosses the optic nerve (usually superior to it), and is continued medially as the anterior ethmoidal nerve. The nasociliary nerve gives off a communicating branch to the ciliary ganglion, long ciliary nerves, the infra trochlear, and posterior and anterior ethmoidal nerves. The ethmoidal nerves contribute branches to the nasal cavity and external nose. Sympathetic nerve fibers join the nasocillary nerve after entering the orbit with the ophthalmic artery. They will follow branches of thelong cillary nerves to the dilator pupillae muscle.

The area of skin supplied by the ophthalmic nerve (fig. 47-6) is tested for sensation by cotton wool and a pin. Blowing on or touching lightly the cornea causes "closure of the eyes" (corneal reflex), which consists of bilateral contraction of the orbicularis oculi muscles. The afferent limb includes the nasociliary nerve, and the efferent limb is the facial nerve (fig. 46-8A).

Ophthalmic vessels

The ophthalmic artery.

The ophthalmic artery (fig. 45-6A) arises from the internal carotid artery medial to the anterior clinoid process and passes through the optic canal, inferior to the optic nerve. Accompanied by the nasociliary nerve, it turns medially, usually between the superior rectus muscle and the optic nerve. It then turns anteriorward, just superior to the medial rectus muscle. It gives numerous branches, only some of which are significant.

The important central artery and vein of the retina penetrate the optic nerve posterior to the eyeball (fig. 45-5B) and travel within the nerve to supply the retina (figs. 46-3, 46-9, and 46-10). The terminal branches are end-arteries, which can readily be inspected in vivo by opthalmoscopy (fig. 46-9). Obstruction of the central artery by thrombosis or embolism results in immediate blindness in the affected eye. Long and short posterior ciliary arteries pierce the sclera and supply the uvea (middle tunic of the eye), which is highly vascular. The lacrimal artery, which supplies the lacrimal gland and eyelids, gives rise to a recurrent meningeal branch, which anastomoses with the middle meningeal artery and hence provides an anastomosis between the internal and external carotid arteries. Muscular branches are important in that they continue through the muscles to enter the anterior part of the eyeball as the anterior ciliary arteries, which pierce the sclera and supply the iris and ciliary body (fig. 46-10).

Additional branches include the supraorbital, anterior ethmoidal, and palpebral arteries. The terminal branches of the ophthalmic artery are the supratrochlear and dorsal nasal arteries. The latter anastomoses with branches of the facial artery and provides another example of an anastomosis between the internal and external carotid arteries.

The ophthalmic veins.

The superior and inferior ophthalmic veins drain the orbit and have important communications with the facial vein, pterygoid plexus, and cavernous sinus, in which they end directly or indirectly. The superior ophthalmic vein is formed near the root of the nose and allows the spread of superficial infections of the face to the cavernous sinus (figs. 43-21 and 46-5B). The venae vorticosae (fig. 46-10) are four veins that drain the middle layer of the eyeball (the uvea) and end in the ophthalmic vein.

Oculomotor, trochlear and abducent nerves (III, IV, VI)

The oculomotor nerve.

The oculomotor (third cranial) nerve supplies all the muscles of the eyeball except the superior oblique and the lateral rectus muscles. It emerges from the brain stem, passes lateral to the posterior clinoid process, traverses the lateral wall of the cavernous sinus, and divides into superior and inferior divisions, which pass through the superior orbital fissure within the common tendinous ring (fig. 45-5). The oculomotor nerve supplies the levator palpebrae superioris and superior rectus (by its superior division) and the medial rectus, inferior rectus, and inferior oblique muscle (by its inferior division). The oculomotor nerve "opens" the eye, whereas the facial nerve" closes" it (by the orbicularis oculi). The oculomotor nerve also conveys preganglionic parasympathetic nerve fibers to the orbit. A parasympathetic communication arises from the nerve branch to the inferior oblique muscle. This nerve enters the ciliary ganglion and synapses there. Postganglionic parasympathetic nerve fibers pass from the ganglion to the eyeball through the short cilliary nerves, innervating the sphincter pupillae and ciliary muscle (figs. 45-5B and 45-7). In the act of focusing the eyes on a near object (fig. 46-8C), the oculomotor nerves are involved in adduction (medial recti), accommodation (ciliary muscle), and miosis (sphincter pupillae). Paralysis of the oculomotor nerve results in ptosis (paralysis of the levator), abduction (unopposed lateral rectus), and other signs.

The trochlear nerve.

The trochlear (fourth cranial) nerve supplies only the superior oblique muscle of the eyeball. It emerges from the dorsum of the brain stem, winds around the cerebral peduncle, traverses the lateral wall of the cavernous sinus, and passes through the superior orbital fissure just superior to the tendinous ring (fig. 45-5). It then enters the superior oblique muscle. The trochlear nerve is tested by asking the subject to look downward when the eye is in adduction (fig. 45-9, SO).

The abducent nerve.

The abducent (sixth cranial) nerve supplies only the lateral rectus muscle of the eyeball. It emerges from the brain stem at the junction of the pons and the medulla. It enters the dura matter over the dorsum sellae and follows the slope of this bone supeior and anterior, bending sharply anteriorward across the superior border of the apical portion of the petrous part of the temporal bone. This perhaps accounts for abducent involvement in almost any cerebral lesion that is accompanied by increased intracranial pressure. The nerve next traverses the cavernous sinus, passes through the superior orbital fissure within the common tendinous ring (fig. 45-5), and enters the medial side of the lateral rectus muscle. Paralysis of the lateral rectus results in inability to abduct the eye and horizontal diplopia (double vision) which is worst when attempting to look toward the side of the nerve injury.

Ciliary ganglion

The ciliary ganglion (fig. 45-7) is the peripheral ganglion of the parasympathetic system of the eye. It is situated between the optic nerve and the lateral rectus (fig. 45-5). Communications with the nasociliary nerve convey afferent fibers from the eye. A parasympathetic root from the oculomotor nerve contains the only fibers that synapse in the ganglion. The postganglionic parasympathetic fibers pass to the short ciliary nerves (which are branches of the ganglion) and supply the ciliary muscle and sphincter pupillae.

Postganglionic sympathetic fibers from the internal carotid plexus reach and pass through the ciliary ganglion. By way of the short ciliary nerves, the sympathetic fibers supply the dilator pupillae and blood vessels, as well as smooth muscle in the eyelid (superior tarsal muscle) and in the inferior orbital fissure (orbitalis). A sympathetic lesion (Horner syndrome) results in a small pupil (miosis) and mild ptosis (fig. 45-7).

Muscles of eyeball (extraocular muscles)

The eyeball is moved chiefly by six extrinsic muscles: four recti and two oblique muscles (fig. 45-8). These skeletal muscles arise from the posterior aspect of the orbit (except for the inferior oblique muscle) and are inserted into the sclera.

The four recti arise from a common tendinous ring that surrounds the optic canal and a part of the superior orbital fissure. All the structures that enter the orbit through the optic canal and adjacent part of the fissure lie at first within the cone of the recti (fig. 45-4). The four muscles are inserted into the anterior portion of the sclera, 6-8 mm posterior to the sclerocorneal junction (limbus).

The superior oblique muscle arises from the sphenoid bone superomedial to the optic canal. It passes anteriorward, superior to the medial rectus, and through a cartilaginous pulley (the trochlea) attached to the frontal bone. The tendon is thereby directed posterolaterally, running inferior to the tendon of the superior rectus to insert into the posterior sclera. The inferior oblique muscle arises from the maxilla at the anteromedial floor of the orbit, passes in a posterolateral direction, immediately inferior to the inferior rectus to insert into the posterior sclera.

The superior oblique muscle is supplied by the trochlear nerve, the lateral rectus by the abducent nerve, and the others by the oculomotor nerve. Mnemonic: S04, LR6, remainder 3.

Actions of extraocular muscles (fig. 45-9 and table 45-1).

The eye is poised in the fascia and fat of the orbit, and equilibrium is maintained by all the muscles, none of which ever acts alone. Moreover, the two eyes move together in unison (conjugately). Movements may be considered to be around a vertical axis (abduction and adduction), a lateromedial axis (elevation and depression) and even an anteroposterior axis (extorsion and intorsion).

The recti extend from the posterior aspect of the orbit to the anterior aspect of the sclera. The lateral and medial recti are purely an abductor and an adductor, respectively. The superior and inferior recti elevate and depress, respectively, and because of their lateral course, are the only muscles that can do so when the eye is abducted. The trochlea of the superior oblique muscle serves as its functional origin, and hence the two oblique muscles may be said to extend from the anteromedial orbit to the posterior sclera. The superior and inferior oblique muscles depress and elevate, respectively, and because of their lateral course, are the only muscles that can do so when the eye is adducted (fig. 45-9).

Paralysis of an extrinsic eye muscle is noted by (1) limitation of movement in the field of action of the paralyzed muscle and (2) the presence of two images (diplopia) that are separated maximally when an attempt is made to move the eye in the direction of primary action of the paralyzed muscle.

Optic nereve

The optic (second cranial) nerve is the nerve of sight, and it extends from the eye to the optic chiasm. Developmentally, it may be considered as a tract between the retina (a derivative of the brain) and the brain. The nerve fibers, which arise in the retina, converge on the optic disc, pierce the layers of the eye, and receive myelin sheaths. The optic neerve, itself, is surrounded by meningeal sheaths continuous with those of the brain, and also by the subarachnoid space. The optic nerve lies within the cone of the recti (fig. 45-5B), is crossed by the ophthalmic artery and nasociliary nerve, is pierced by the central vessels of the retina, and passes through the optic canal to enter the middle cranial fossa. Intracranially, the optic nerve is related on its inferior side to the internal carotid and ophthalmic arteries and to the hypophysis. The nerve ends in the optic chiasm (fig. 45-6), where the nerve fibers arising from the nasal part of the retina decussate. The decussating fibers are those that transmit the perception of vision in the temporal side of the visual field. Examination of the optic nerve includes ophthalmoscopy, testing of visual acuity, and plotting of the visual field.


The eyelids (palpebrae) (fig. 45-10) are musculofibrous folds in the anterior part of each orbit. Reflex blinking distributes tears and prevents drying of the cornea. The upper eyelid, more extensive and mobile, meets the lower at the medial and lateral angles (canthi). In some people, chiefly Asian, the medial canthus is covered by a fold of skin (epicanthus). The palpebral fissure, the space between the lids, is the opening to the conjunctival sac. The free margin of each lid possesses hairs termed eyelashes (cilia). Three types of glands (associated with the names, Moll, Zeis, and Meibom, respectively) drain into the margins of the lids: (1) ciliary (sweat) glands, (2) sebaceous glands attached to the follicles of the eyelashes, and (3) tarsal glands (about 35 in the upper lid), situated further posteriorly. Infection of one of these three types of glands may result in a stye (hordiolum). Chronic inflammation or obstruction of a tarsal gland produces a small mass or cyst (chalazion).

Medially, the margin of the lid presents the lacrimal punctum and, between the lids, an area termed the lacrimal lake. The floor of the lake shows a "fleshy" mass, the lacrimal caruncle, which lies on a conjunctival fold, the plica semilunaris (fig. 46-6).

The upper eyelid is composed of skin and subcutaneous tissue, muscle (the palpebral part of the orbicularis oculi and the levator palpebrae superioris), fibrous tissue (including the tarsal plate), and mucous membrane (the palpebral part of the conjunctiva).

The subcutaneous tissue usually contains no fat, and fluid can readily accumulate there. The levator palpebrae superioris arises from the sphenoid bone superior to the optic canal and is inserted into the skin of the upper lid and also into the upper border of the tarsal plate by means of the superior tarsal muscle (fig. 45-10). The levator is supplied by the oculomotor nerve; the innervation of the tarsal muscle is sympathetic. Paralysis of the levator results in drooping (ptosis) of the upper lid. The tarsal plate is a fibrous support related posteriorly to the tarsal glands. The ends of the plates are anchored to the orbital margin by lateral and medial palpebral ligaments. The medial palpebral ligament, identifiable on drawing the lids laterally, is anterior to the lacrimal sac, to which it serves as a guide (fig. 45-1B). The superior tarsal muscle connects the levator with the tarsal plate and consists of smooth muscle that is supplied by sympathetic fibers. A lesion of the sympathetic pathway (e.g., of the cervical trunk) may result in mild ptosis (paralysis of the superior tarsal muscle) of the upper lid and miosis, as well as anhidrosis, redness and increased temperature of the skin (Horner syndrome, fig. 45-7).

Direct injury may cause a superficial hematoma of the eyelids and adjacent tissues (a "black eye"), whereas a more deeply placed orbital hematoma occurs after fractures of the anterior or the middle cranial fossa.


The conjunctiva is a connection (conjunction) between the eyelids, sclera and cornea. It is the mucous membrane that lines the posterior surface of the eyelids (palpebral conjunctiva) and the anterior aspect of the globe (bulbar conjunctiva) (fig. 45-10). The potential space, lined by conjunctiva, between the lids and the globe, is termed the conjunctival sac. The mouth of the sac is the palpebral fissure, which varies in size according to the degree to which the" eye is open". The reflections of the conjunctiva from the lids to the globe are known as fornices. The lacrimal glands open into the superior fornix.

The palpebral conjunctiva contains the openings of the lacrimal canaliculi, thereby allowing tears within the conjunctival sac to drain into the nasal cavity. The palpebral conjunctiva is red and vascular and is examined when anemia is suspected.

The bulbar conjunctiva is translucent, thereby allowing the sclera to show through as the "white of the eye". It is colorless, except when its vessels are dilated as a result of inflammation (conjunctivitis). Centrally, it is continuous at the limbus with the anterior epithelium of the cornea. The plica semilunaris, a conjunctival fold at the medial angle of the eye, helps to intercept foreign bodies.

Innervation and blood supply.

The conjunctiva is supplied by branches of the ophthalmic nerve. The vessels of the bulbar conjunctiva are visible. They arise from (1) a peripheral palpebral arcade and (2) the anterior ciliary arteries (fig. 46-10). In acute conjunctivitis (e.g., from wind exposure or infection) the bulbar conjunctiva becomes brick-red (a "blood-shot eye"). In deeper conditions (e.g., diseases of the iris or ciliary body), in which branches of the anterior ciliary arteries are dilated, a rose-pink band of "ciliary injection" is produced around the margins of the cornea.

Lacrimal apparatus

The lacrimal apparatus comprises (1) the lacrimal gland and its ducts and (2) associated passages for drainage: the lacrimal canaliculi and sac and the nasolacrimal duct (fig. 45-11).

The lacrimal gland, lodged in a fossa anterolaterally at the roof of the orbit, rests on the lateral rectus and the levator muscles. The main portion is the orbital part, but a process called the palpebral part projects into the upper lid. A dozen lacrimal ducts leave the palpebral part to enter the superior conjunctival fornix where small accessory lacrimal glands are also found. Tears are secreted by the gland, and they keep the eye moist and free of foreign bodies. The half of the lacrimal secretions that does not evaporate drains into the lacrimal sac.

The secretory fibers to the lacrimal gland are derived from the greater petrosal nerve (of the facial nerve) by way of the nerve of the pterygoid canal (fig. 48-7). The fibers synapse in the pterygopalatine ganglion and reach the gland both directly and by a connection between the zygomatic and lacrimal nerves.

A lacrimal canaliculis is present in each lid. Each begins as an opening, the lacrimal punctum, situated on an elevation termed the lacrimal papilla. The two canaliculi open into the lacrimal sac, which is continuous with the nasolacrimal duct. The sac, lodged in a fossa at the medial margin of the orbit, is partly covered by the medial palpebral ligament (fig. 45-1B). The nasolacrimal duct extends from the lacrimal sac to the inferior meatus of the nasal cavity, and its lumen is marked by valve-like folds. Blinking, which puts tension on the palpebral ligaments, can help drainage of tears.

Additional reading

Duke-Elder,S., and Wybar, K.C., The Anatomy of the Visual System, vol. 2 of System of Ophthalmology, Ed. by S. Duke-Elder, Mosby, St. Louis, 1961. An excellent work of reference for the orbit and eye.

Remington, L. Clinical Anatomy of the Visual System. Butterworth-Heinemann, Boston, 1998. A recent, wellillustrated textbook including descriptions of pupillary and visual pathways.

Whitnall, S. E., The Anatomy of the Human Orbit and Accessory Organs of Vision, 2nd ed., Oxford University Press, London, 1932. The classic study of the orbit.

Wolff's Anatomy of the Eye and Orbit, 8th ed., revised by A. Bron, R. Tripathi, and B. Tripathi. Chapman & Hall, London, 1997. An attractive, well-illustrated text.


45-1. Force applied to the rim of the orbit may be transmitted toward the side of the nose. Which thin bones are likely to be splintered?

45-1. The lacrimal and ethmoid bones are likely to be splintered in injuries to the rim of the orbit, and the ethmoidal air sinuses are frequently opened. A paper model to show the composition of the bony orbit can be useful.

45-2. Which nerve accompanies the ophthalmic artery?

45-2. The nasociliary nerve accompanies the ophthalmic artery. Rarely, the ophthalmic artery arises from the middle meningeal artery, or the middle meningeal artery may come from the ophthalmic artery. These variations are associated with the anastomosis between the lacrimal and middle meningeal arteries, which is ultimately between the internal and external carotid arteries.

45-3. Which is the most important branch of the ophthalmic artery?

45-3. The central artery of the retina, complete obstruction of which results in blindness, is the most important branch of the ophthalmic artery.

45-4. Which cranial nerve is "the weakling of the cranial contents" because of its likely damage from increased intracranial pressure?

45-4. The abducent nerve (C.N. VI), which bends sharply across the petrous part of the temporal bone, is liable to damage from increased intracranial pressure.

45-5. Where is the peripheral relay station of the parasympathetic fibers to the eye?

45-5. The ciliary ganglion, which lies between the optic nerve and the lateral rectus muscle, is the peripheral relay station of the parasympathetic fibers to the eye (fig. 45-7).

45-6. Which nerves enter the orbit within the common tendinous ring?

45-6. Cranial nerves II, III, and VI and the nasociliary nerve (of cranial nerve V) enter the orbit within the common tendinous ring (fig. 45-5A,B).

45-7. Which chief muscles and nerves are concerned with (a) closing the eyelids and (b) opening them?

45-7. The orbicularis oculi (facial nerve) closes the eyelids, whereas the levator palpebrae superioris (oculomotor nerve) "opens the eye".

45-8. What are the main features of Horner syndrome?

45-8. Horner syndrome, caused by a lesion of the cervical sympathetic nerves, is characterized chiefly by ptosis, seeming enophthalmos, and cutaneous hyperemia. The condition, described by Horner in 1869, had been recognized by Claude Bernard in 1862.

45-9. On looking downward and to the right, a patient's left pupil failed to descend. Which muscle is likely to be paralyzed?

45-9. On looking downward and to the right, if the left pupil fails to descend, the left superior oblique muscle is likely to be paralyzed (fig. 45-9).

45-10. On looking upward and to the left, a patient's right pupil failed to ascend. Which muscle is most likely to be involved?

45-10. On looking upward and to the left, if the right pupil fails to ascend, the right inferior oblique muscle is most likely to be involved (fig. 45-9).

Figure legends

Figure 45-1. Anterior aspect of the right bony orbit. A, showing the main sutures (1, 2, 3) between the bones of the margin of the orbit. The infraorbital suture in the maxilla (above the infraorbital foramen) indicates the line of closure of the infraorbital canal. S, surface anatomy of the eye and orbit. C, the bones that form the roof, floor, and walls of the orbit.

Figure 45-2. Approximately horizontal sections of the head as seen by imaging techniques: magnetic resonance imaging (MRI) and computerized tomography (CT). A, MRI. On each side, posterior to the globe, the orbital fat appears white. On the left side of the print, the slightly curved optic nerve is evident, lying between the lateral and medial recti. The peripheral white band represents mainly bone marrow within the skull. Inside the skull, the temporal and occipital lobes of the cerebral hemispheres are visible. In or near the median plane, the nasal cavity and the ethmoidal and sphenoidal sinuses appear black, followed (in gray) by the hypophysial region and optic chiasm, the midbrain (which shows the aqueduct), and the cerebellum. B, CT. In each globe the lens can be seen, and, on the left side of the print, portions of the lateral rectus and optic nerve. The peripheral white band is the skull, outside of which the auricles are visible. The greater wing of the sphenoid forms the lateral wall of the orbit and, further posterior, the squamous part of the temporal bone can be seen lateral to the temporal lobe. The petrous portion of the temporal bone proceeds anteromedially. Mastoid air cells appear as black areas. In or near the median plane, the nasal cavity and the sphenoidal sinus appear black, followed by the dorsum sellae (in white) and, posteriorly, the internal occipital protuberance.

Figure 45-3. Horizontal scheme of the orbits, showing the angles formed by the walls. The dotted lines indicate the longitudinal axes of the orbits. The visual axes, here shown directed toward a distant object, are nearly parallel. After Whitnall.

Figure 45-4. Branches of the ophthalmic nerve, superior aspect.

Figure 45-5. The chief contents of the orbit. A, superior orbital fissure and optic canal, anterior aspect. The optic canal and adjacent part of the fissure are surrounded by the common tendinous ring, from which the four recti arise. The lacrimal, frontal, and trochlear nerves enter the orbit superior to the ring. The superior and inferior ophthalmic veins leave the orbit usually superior and inferior to the ring, respectively. The optic canal contains the optic nerve and the ophthalmic artery. The optic nerve is surrounded by its dural sheath (and by the subarachnoid space, as shown in figure 46-4). B, a superposable view showing the muscular cone anterior to the common tendinous ring. All the structures that pass through the ring lie at first within the muscular cone. The ophthalmic artery crosses (usually superior to) the optic nerve and is accompanied by the nasociliary nerve. The nerves to the extraocular muscles are shown in green, the sensory nerves in yellow, and the ciliary ganglion in mauve. The short ciliary nerves arise from the ganglion. A is modified from Wolff and from Whitnall; B is based partly on von Lanz and Wachsmuth.

Figure 45-6. The right orbit, from above. In A, the levator and superior rectus have been removed to show the artery and nerves that enter within the muscular cone. B is a superposable view. Three nerves can be seen to enter the orbit superior to the muscular cone.

Figure 45-7. The ciliary ganglion and its connections, lateral aspect. Sympathetic fibers are also distributed to the superior tarsal muscle (Fig. 45-10). Horner syndrome results from interruption of the sympathetic pathway anywhere from the hypothalamus (diencephalon) to the eye, e.g., by a tumor or by intracranial thrombosis. Fibers: parasympathetic (green), sympathetic (red), and afferent (blue); preganglionic (continuous lines) and postganglionic (interrupted and dotted lines).

Figure 45-8. The muscles of the globe. A, superior aspect. B, right lateral aspect. Note that the two oblique muscles pass inferior to the corresponding recti. A is based on Krimsky, B on Cogan.

Figure 45-9. Scheme of the muscles acting on the right globe. Note the actions of the superior (SR) and inferior (IR) recti on the abducted eye, and those of the inferior (IO) and superior (SO) obliqui on the adducted eye. The lowest scheme summarizes test positions for the muscles responsible for vertical movements of the abducted and adducted eyes, anterior aspect. The arrows must be followed in strict sequence, i.e., beginning with the horizontal arrows. LR, lateral rectus; MR, medial rectus; OD, oculus dexter (right eye); OS, oculus sinister (left eye).

Figure 45-10. Sagittal section through the eyelids and anterior eye. The levator palpebrae superioris (supplied by the oculomotor nerve) is assisted by the superior tarsal muscle (supplied by the sympathetic). The conjunctival sac is lined by an epithelium (shown in blue) that is continuous with the anterior epithelium of the cornea. Also shown are the anterior chamber, iris, posterior chamber, ciliary zonule, and lens.

Figure 45-11. The lacrimal apparatus (shown in blue). (1) The lacrimal gland, situated superolaterally, is concerned with the production of most of the tears. (2) The conjunctival sac facilitates their distribution. (3) Medially, drainage begins at the lacrimal puncta and canaliculi (near which the plica semilunaris and the lacrimal caruncle can be seen). The letters "s" and "i" indicate the lines of reflection of the conjunctiva at the superior and inferior fornices of the conjunctival sac. Based largely on von Lanz and Wachsmuth.

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