Chapter 18. Orbit

Big Picture

The primary organ responsible for vision is the eye. The eyeball is located within a bony orbital encasement, which protects it. The lacrimal apparatus keeps the eye moist and free of dust and other irritating particles through the production and drainage of tears. Eyelids protect the eye from external stimuli such as dust, wind, and excessive light.

Bony Orbit

The bony orbit is the region of the skull that surrounds the eye and is composed of the following structures (Figure 18-1A):

• Superior wall. Formed by the frontal bone and the lesser wing of the sphenoid bone.
  • Supraorbital foramen. Transmits the supraorbital nerve [cranial nerve (CN) V-1] and vessels to the scalp.
• Lateral wall. Formed by the zygomatic bone and the greater wing of the sphenoid bone.
• Inferior wall. Formed by the maxillary, zygomatic, and palatine bones.
  • Infraorbital foramen. Transmits the infraorbital nerve (CN V-2) and vessels to the maxillary region of the face.
• Medial wall. Formed by the ethmoid, frontal, lacrimal, and sphenoid bones.
  • Anterior and posterior ethmoidal foramina. Transmits the anterior and posterior ethmoidal nerves and vessels, to the nasal cavity and the sphenoid and ethmoid sinuses.
  • Nasolacrimal canal. Formed by the maxillary, lacrimal, and inferior nasal concha bones. Drains tears from the eye to the inferior meatus in the nasal cavity.
• Optic canal. Transmits the optic nerve (CN II) and the ophthalmic artery.
• Superior orbital fissure. An opening between the greater and lesser wings of the sphenoid bone that transmits the oculomotor, trochlear, ophthalmic, and abducens nerves (CNN III, IV, V-1, and VI, respectively), and the ophthalmic veins.
• Inferior orbital fissure. Communicates with the infratemporal and pterygopalatine fossae. The inferior orbital fissure transmits CN V-2 and the infraorbital artery and vein.

Lacrimal Apparatus

The lacrimal gland lies in the superolateral corner of the orbit (Figure 18-1B).

• Tears. The lacrimal gland secretes tears that spread evenly over the eyeball through blinking and cleanse the eye of dust and foreign particles.
• Drainage. Tears drain from the eyeball, via the nasolacrimal duct, into the inferior nasal meatus of the nasal cavity.
• Innervation. The lacrimal gland is innervated by visceral motor parasympathetic neurons from CN VII (Figure 18-4C).

Eyelids

Eyelids protect the eye from foreign particles and from bright light (Figure 18-1C). The external surface of the eyelids is covered by skin, whereas the conjunctiva covers the internal surface.

Conjunctiva

The conjunctiva is a mucous membrane that lines the internal surface of the eyelids. The conjunctiva is reflected at the superior and inferior fornices onto the anterior surface of the eyeball and forms the conjunctival sac when the eyes are closed.

Eyelid Muscles

Tears produced by the lacrimal gland are secreted continually and are spread over the conjunctiva and cornea by movement of the eyelids (blinking). The following muscles are involved in movement of the eyelids:

• Orbicular oculi muscle. Considered a muscle of facial expression. Its circular fibers attach to the anterior surface of the bony orbit as well as in the eyelid. When the facial nerve (CN VII) activates the orbicular oculi muscle, the eyes close.

• Levator palpebrae superioris muscle. The levator palpebrae superioris muscle elevates the upper eyelid and is innervated by CN III.
• Superior tarsal (Müller's) muscle. Composed of smooth muscle and attaches to the inferior surface of the levator palpebrae superioris muscle and the upper eyelid. These two muscles work synergistically to keep the eyelid elevated. The superior tarsal muscle is innervated by postganglionic sympathetic neurons that originate in the superior cervical ganglion and are carried to the orbit by the internal carotid and ophthalmic arteries.

• Ptosis (drooping upper eyelid). Results from the loss of sympathetic innervation to the superior tarsal muscle.
• Anhidrosis (decreased sweating). Results from the loss of sympathetic innervation to sweat glands.
• Miosis (constricted pupil). Results from the loss of sympathetic innervation to the dilator pupillary muscle.
  

Big Picture

The eyeball consists of the sclera, choroid, and retina.

Sclera

The sclera is the white, fibrous covering of the eye into which muscles insert (Figure 18-2A).

• Cornea. The sclera is continuous anteriorly as the cornea, which forms a bulging, transparent region specialized for refracting light as it enters the eye.

Choroid

The choroid is the vascular, middle layer of the eye where blood vessels course (Figure 18-2A). The choroid layer is also composed of the following structures:

Ciliary Apparatus

The anterior region of the choroid is thickened to form the ciliary muscle, which is composed of smooth muscle and is innervated by parasympathetic neurons from CN III (Figure 18-2B).

• Ciliary muscle. This circular muscle surrounds the periphery of the lens. Contraction causes the diameter of the muscle to decrease, whereas relaxation causes the diameter of the muscle to increase. Understanding this concept is essential to understanding how the lens focuses on images near and far.
• Lens. A transparent biconvex structure enclosed in an elastic covering. The lens is held in position by radially arranged suspensory ligaments, which are attached medially to the lens capsule and laterally to the ciliary muscles.

Accommodation

Process by which the lens charges to maintain a focused image from distant or near objects.

• Distant vision. Light rays from distant objects are nearly parallel and do not need as much refraction to bring them into focus (Figure 18-2C). Therefore, to focus on distant objects, the ciliary muscle relaxes, which stretches the suspensory ligaments and flattens the lens. When the lens is flat (least rounded), it is at optimal focal length for distant viewing.
• Near vision. Light rays from close objects diverge and require more refraction to focus (Figure 18-2D). To focus on near objects, parasympathetic neurons in CN III cause ciliary muscle contraction and thus relaxation of the tension on the suspensory ligaments, allowing the lens to become more rounded. When the lens is round, it is at optimal focal length for near viewing.

Iris

The iris is the visible colored part of the eye (Figure 18-2B). The round central opening of the iris is the pupil, which allows light to enter the eye. The iris consists of smooth muscle under autonomic control, which contract reflexively and vary the size of the pupil.

• Sphincter pupillae muscle. Causes pupil constriction in bright light and close vision via visceral motor (parasympathetic) innervation from CN III.
• Dilator pupillae muscle. Causes pupil dilation in dim light and distant vision to enable more light to enter the eye via sympathetic innervation.

Retina

The retina is the innermost layer of the eyeball (Figure 18-2A).

• Optic disc. The optic disc is located medial to the macula lutea. The optic disc has no photoreceptors and is insensitive to light, hence the nickname “the blind spot.” The optic disc consists of the nerve fibers formed by CN II.
• Macula lutea. The macula lutea is a yellow area of the retina that is lateral to the optic disc and contains a central depression called the fovea centralis. This region contains only cones and functions for detailed vision (maximal visual acuity). The eye is positioned such that light rays from the object in the center of the view fall upon this region.
• Rods and cones. Rods are specialized cells for vision in dim light (black, gray, white vision), whereas cones are associated with visual acuity and color vision.

Chambers of the Eye

The eye is divided into the following segments (Figure 18-2B).

• Posterior segment. Known as the vitreous chamber, it is filled with a clear gel called vitreous humor, which contributes to the intraocular pressure.
• Anterior segment. Subdivided by the iris into the anterior chamber (between the cornea and iris) and the posterior chamber (between the iris and lens). The anterior segment is filled with aqueous humor, which has a composition similar to plasma. Unlike vitreous humor, which is constant and is never replaced, aqueous humor forms and drains continually into the venous system via the canal of Schlemm.

Big Picture

Six strap-like extraocular muscles (four rectus and two oblique) control the movement of the eye (elevation, depression, abduction, adduction, intorsion, and extorsion).

Eye Movement

The eye moves in the following three axes of rotation (Figure 18-3A):

• X-axis. A horizontal axis in the axial plane enabling abduction and adduction.
• Y-axis. A vertical axis in the sagittal plane resulting in elevation and depression.
• Z-axis. A horizontal axis in the sagittal plane resulting in intorsion and extorsion.

The extraocular muscles act upon the eye in these three axes to cause the eye to move in the following dimensions:

• Elevation and depression. An upward and downward gaze.
• Abduction and adduction. A lateral and medial gaze.
• Intorsion (medial rotation). An inward medial rotation of the upper pole of the vertical meridian caused by the superior oblique and superior rectus muscles.
• Extorsion (lateral rotation). An outward lateral rotation of the upper pole of the vertical meridian caused by the inferior rectus and inferior oblique muscles.

Each of the six extraocular muscles exerts rotational forces in all three axes (elevation–depression, abduction–adduction, and intorsion–extorsion) to varying degrees, providing fine motor control to direct gaze.

Extraocular Muscles

The movement of each eyeball is controlled by the six extraocular muscles (four rectus and two oblique) innervated by CNN III, IV, and VI. The extraocular muscles are named according to their position on the eyeball (Figure 18-3B and C).

• Superior rectus muscle (CN III). Elevation of the eye, with adduction and intorsion.
• Inferior rectus muscle (CN III). Depression of the eye, with adduction and extorsion.
• Lateral rectus muscle (CN VI). Only action is abduction of the eye.
• Medial rectus muscle (CN III). Only action is adduction of the eye.
• Superior oblique muscle (CN IV). Originates in common with the rectus muscles, courses along the medial wall of the orbit, and then turns and loops through a fibrous sling, called the trochlea, before inserting on the superolateral region of the eyeball. The superior oblique muscle moves the eyeball down and out (depression and abduction), with intorsion.
• Inferior oblique muscle (CN III). Originates on the medial wall of the bony orbit and courses laterally and obliquely to insert on the inferolateral surface of the eyeball. The inferior oblique moves the eyeball up and out (elevation and abduction), with extorsion.

Clinical Examination of the Extraocular Muscles

When performing a physical examination of the eye, a physician will test each of the extraocular muscles and their associated cranial nerves by drawing an “H” pattern in the air in front of the patient's face (Figure 18-3F). The patient is instructed to follow the physician's finger with her eyes only.

• Horizontal line of the “H.” The horizontal line of the “H” will test the medial and lateral rectus muscles. The medial rectus muscle adducts the eye, whereas the lateral rectus muscle abducts the eye. The medial and lateral rectus muscles are the only muscles that move the eye in the horizontal plane and are therefore easy to test.
• Vertical lines of the “H.” The vertical motion of the eye is a little more complex than the horizontal motion. The superior and inferior rectus muscles and the superior and inferior oblique muscles control the vertical motion of the eyeball. When a patient's gaze is straight up, it occurs as a result of the combined action of the superior rectus and inferior oblique muscles. When the gaze is straight down, it is from the combined action of the inferior rectus and superior oblique muscles. Therefore, it is essential in testing the extraocular muscles to ensure that only one muscle is tested at a time, without influence from another extraocular muscle. To test any of these four muscles, each muscle must be isolated from the others.
• Lateral vertical line tests the rectus muscles. When the right eye is fully abducted, only the superior and inferior rectus muscles elevate and depress the eye. This is purely a mechanical property because of the axis of the eye lining up parallel to the line of contraction of the superior and inferior rectus muscles.
• Medial vertical line tests the oblique muscles. When the right eye is fully adducted, only the superior and inferior oblique muscles elevate and depress the eye. Again, this is due to the axis of the muscles paralleling the axis of the eye.

Big Picture

The cranial nerves associated with the orbit are CN II (vision), CN III (eye movement), CN IV (eye movement), CN V-1 (general sensory to eye and scalp), CN VI (eye movement), and CN VII (crying and closing the eyes).

CN II: Optic Nerve

The optic nerve enters the orbit through the optic canal along with the ophthalmic artery and provides special sensory innervation (vision) from the retina to the brain. CN II from one eye joins CN II from the corresponding eye to form the optic chiasma.

CN III: Oculomotor Nerve

The oculomotor nerve emerges from the midbrain and courses anteriorly between the posterior cerebral and the superior cerebellar arteries, through the lateral wall of the cavernous sinus and superior orbital fissure into the orbit, where the nerve bifurcates into superior and inferior divisions (Figure 18-4B and C).

• Superior division. Provides somatic motor innervation to the levator palpebrae superioris and superior rectus muscles.
• Inferior division. Provides somatic motor innervation to the medial rectus, inferior rectus, and inferior oblique muscles. CN III also provides visceral motor innervation. Preganglionic parasympathetic neurons originate in the Edinger–Westphal nucleus and course in the inferior division of CN III and synapse in the ciliary ganglion (between CN II and the lateral rectus muscle). Postganglionic parasympathetic neurons course in the short ciliary nerves to enter the eyeball and innervate the ciliary muscles (lens accommodation) and pupillary sphincter muscles (pupil constriction).

CN IV: Trochlear Nerve

The trochlear nerve arises in the midbrain and is the only cranial nerve to exit from the posterior surface of the brainstem. After curving around the midbrain, CN IV courses through the lateral wall of the cavernous sinus to enter the orbit via the superior orbital fissure (Figure 18-4A). CN IV provides somatic motor innervation to the superior oblique muscle.

CN V-1: Ophthalmic Branch of the Trigeminal Nerve

The ophthalmic nerve courses through the superior orbital fissure and provides general sensory innervation to the orbit via the following three branches (Figure 18-4A–C):

• Lacrimal nerve. Innervates the lacrimal gland, conjunctiva, and skin of the upper eyelid.
• Frontal nerve. Courses superior to the levator palpebrae superioris muscle and bifurcates into the supraorbital and supratrochlear nerves to innervate the skin of the forehead and scalp.
• Nasociliary nerve. Provides general sensory innervation via the following branches:
  • Long ciliary nerves. Supplies the cornea (sensory limb of the pupillary reflex).
  • Posterior ethmoidal nerve. Supplies the sphenoid and ethmoidal sinuses.
  • Anterior ethmoidal nerve. Supplies the ethmoid sinus, nasal cavity, and skin on the tip of the nose.
  • Infratrochlear nerve. Supplies the lower eyelid, conjunctiva, and skin of the nose.

CN VI: Abducens Nerve

The abducens nerve enters the orbit via the superior orbital fissure and provides somatic motor innervation to the lateral rectus muscle (Figure 18-4B).

CN VII: Facial Nerve

In the orbit, the facial nerve “makes you cry” (visceral motor) and “close your eyes” (branchial motor) (Figure 18-4C).

• Preganglionic parasympathetic neurons from CN VII originate in the superior salivatory nucleus (pons) and exit the cranial cavity through the internal acoustic meatus. At the geniculate ganglion, CN VII gives off the greater petrosal nerve, which joins with the deep petrosal nerve (sympathetics from the carotid plexus), becoming the nerve of the pterygoid canal (Vidian nerve). The nerve of the pterygoid canal enters the pterygopalatine fossa, where preganglionic parasympathetic neurons synapse in the pterygopalatine ganglion. Postganglionic parasympathetic neurons join with the zygomatic nerve (CN V-2) and then with the lacrimal nerve (CN V-1) to innervate the lacrimal gland.
• CN VII provides branchial motor innervation to the orbicularis oculi muscle.

Orbital Sympathetics

Sympathetics to the orbit course in the following pathways (Figure 18-4C):

• Postganglionic sympathetic neurons originating in the superior cervical ganglion ascend along the internal carotid artery up to the ophthalmic artery.
• Sympathetic nerves innervate the superior tarsal muscle (keeping the eyelid open) and the dilator pupillae muscles.
• Sympathetic nerves innervate the superior tarsal muscle (keeping the eyelid elevated) and the dilator pupillae muscles.

Big Picture

The ophthalmic artery provides the principal vascular supply to the orbit. The superior and inferior ophthalmic veins drain anteriorly to the facial vein, posteriorly to the cavernous sinus, and inferiorly to the pterygoid plexus.

Ophthalmic Artery

The ophthalmic artery enters the orbit through the optic canal with CN II (Figure 18-5A). Branches travel to the eye, lacrimal gland, eyelids, scalp, extraocular muscles, and the side of the nose. Major branches of the ophthalmic arteries are as follows:

• Central retinal artery. Travels within CN II and at the optic disc sends numerous branches to the retina. The central artery of the retina is the most important branch of the ophthalmic artery because it supplies all the nerve fibers of CN II. Therefore, if the central artery is occluded, there is complete loss of vision in that eye.
• Posterior ethmoidal artery. Courses through the posterior ethmoidal foramen and provides vascular supply to the ethmoid sinus.
• Anterior ethmoidal artery. Courses through the anterior ethmoidal foramen and provides vascular supply to the ethmoid sinus, frontal sinus, nasal cavity, and the external part of the nose.

Ophthalmic Veins

The principal veins that drain the orbit are the superior and inferior ophthalmic veins (Figure 18-5B).

• Superior ophthalmic vein. Formed by the union of the supraorbital, supratrochlear, and angular (facial) veins and communicates posteriorly with the cavernous sinus. The superior ophthalmic vein may also receive tributaries from the inferior ophthalmic vein. Unlike most other veins, the superior ophthalmic vein lacks valves and, therefore, blood may drain anteriorly through the angular vein or posteriorly in the cavernous sinus.

• Inferior ophthalmic vein. Communicates with the pterygoid plexus of veins through the infraorbital fissure, as well as with the infraorbital vein. The inferior ophthalmic vein terminates in the cavernous sinus.