Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

8. Eye and Orbit

8.1 Eye and Orbital Region

A Superficial and deep neurovascular structures of the orbital region

Right eye, anterior view, a Superficial layer. The orbital septum on the right side has been exposed by removal of the orbicularis oculi. b Deep layer. Anterior orbital structures have been exposed by partial removal of the orbital septum.

The regions supplied by the internal carotid artery (supraorbital artery) and external carotid artery (infraorbital artery, facial artery) meet in this region. The anastomosis between the angular vein (extracranial) and superior ophthalmic veins (intracranial) creates a portal of entry by which microorganisms may reach the cavernous sinus (risk of sinus thrombosis, meningitis); therefore it is sometimes necessary to ligate this anastomosis in the orbital region, as in patients with extensive infections of the external facial region (see p.93).

Note the passage of the supra- and infraorbital nerves (branches of CN V-, and CN V2) through the accordingly named foramina. The sensory function of these two trigeminal nerve divisions can be tested at these nerve exit points.

В Surface anatomy of the eye

Right eye, anterior view. The measurements indicate the width of the normal palpebral fissure. It is important to know these measurements because there are a number of diseases in which they are altered. For example, the palpebral fissure may be widened in peripheral facial paralysis or narrowed in ptosis (= drooping of the eyelid) due to oculomotor palsy.

C Structure of the eyelids and conjunctiva

a Sagittal section through the anterior orbital cavity, b Anatomy of the conjunctiva.

The eyelid consists clinically of an outer and an inner layer with the following components:

 Outer layer: palpebral skin, sweat glands, ciliary glands (= modified sweat glands, Moll glands), sebaceous glands (Zeis glands), and two striated muscles, the orbicularis oculi and levator palpebrae (upper eyelid only), innervated by the facial nerve and the oculomotor nerve, respectively.

 Inner layer: the tarsus (fibrous tissue plate), the superior and inferior tarsal muscles (of Muller; smooth muscle innervated by sympathetic fibers), the tarsal or palpebral conjunctiva, and the tarsal glands (Meibomian glands).

Regular blinking (20-30 times per minute) keeps the eyes from drying out by evenly distributing the lacrimal fluid and glandular secretions (seep. 123). Mechanical irritants (e.g., grains of sand) evoke the blink reflex, which also serves to protect the cornea and conjunctiva. The conjunctiva (tunica conjunctiva) is a vascularized, thin, glistening mucous membrane that is subdivided into the palpebral conjunctiva (see above), fornical conjunctiva, and ocular conjunctiva. The ocular conjunctiva borders directly on the corneal surface and combines with it to form the conjunctival sac, whose functions include:

 facilitating ocular movements,

 enabling painless motion of the palpebral conjunctiva and ocular conjunctiva relative to each other (lubricated by lacrimal fluid), and

 protecting against infectious pathogens (collections of lymphocytes along the fornices).

The superior and inferior fornices are the sites where the conjunctiva is reflected from the upper and lower eyelid, respectively, onto the eyeball. They are convenient sites for the instillation of ophthalmic medications. Inflammation of the conjunctiva is common and causes a dilation of the conjunctival vessels resulting in “pink eye.” Conversely, a deficiency of red blood cells (anemia) may lessen the prominence of vascular markings in the conjunctiva. This is why the conjunctiva should be routinely inspected in every clinical examination.

8.2 Eye: Lacrimal Apparatus

A Lacrimal apparatus

Right eye, anterior view. The orbital septum has been partially removed, and the tendon of insertion of the levator palpebrae superioris has been divided. The hazelnut-sized lacrimal gland is located in the lacrimal fossa of the frontal bone and produces most of the lacrimal fluid. Smaller accessory lacrimal glands (Krause or Wolfring glands) are also present. The tendon of levator palpebrae subdivides the lacrimal gland, which normally is not visible or palpable, into an orbital lobe (2/3 of gland) and a palpebral lobe (1 /3). The sympathetic fibers innervating the lacrimal gland originate from the superior cervical ganglion and travel along arteries to reach the lacrimal gland. The parasympathetic innervation of the lacrimal gland is complex (see p.81). The lacrimal apparatus can be understood by tracing the flow of lacrimal fluid obliquely downward from upper right to lower left. From the superior and inferior puncta, the lacrimal fluid enters the superior and inferior lacrimal canaliculi, which direct the fluid into the lacrimal sac. Finally it drains through the nasolacrimal duct to an outlet below the inferior concha of the nose. “Watery eyes” are a typical cold symptom caused by obstruction of the inferior opening of the nasolacrimal duct.

В Distribution of goblet cells in the conjunctiva (after Calabria and Rolando)

Goblet cells are mucous-secreting cells with an epithelial covering. Their secretions (mucins) are an important constituent of the lacrimal fluid (see C). Besides the goblet cells, mucins are also secreted by the main lacrimal gland.

D Mechanical propulsion of the lacrimal fluid

During closure of the eyelids, contraction of the orbicularis oculi proceeds in a temporal-to-nasal direction. The successive contraction of these muscle fibers propels the lacrimal fluid toward the lacrimal passages.

Note: Facial paralysis prevents closure of the eyelids, causing the eye to dryout.

C Structure of the tear film (after Lang)

The tear film is a complex fluid with several morphologically distinct layers, whose components are produced by individual glands. The outer lipid layer, produced by the Meibomian glands, protects the aqueous middle layer of the tear film from evaporating.

E Obstructions to lacrimal drainage (after Lang)

Sites of obstruction in the lacrimal drainage system can be located by irrigating the system with a special fluid. To make this determination, the examiner must be familiar with the anatomy of the lacrimal apparatus and the normal drainage pathways for lacrimal fluid (see A).

a No obstruction to lacrimal drainage (compare with A), bande Stenosis in the inferior or common lacrimal canaliculus. The stenosis causes a damming back of lacrimal fluid behind the obstructed site. In b the fluid refluxes through the inferior lacrimal canaliculus, and in c it flows through the superior lacrimal canaliculus, d Stenosis below the level of the lacrimal sac (postlacrimal sac stenosis). When the entire lacrimal sac has filled with fluid, the fluid begins to reflux into the superior lacrimal canaliculus. In such cases, the lacrimal fluid often has a purulent, gelatinous appearance.

8.3 Eyeball

A Transverse section through the eyeball

Right eye, superior view. Most of the eyeball is composed of three concentric layers (from outside to inside): the sclera, choroid, and retina. The anterior portion of the eyeball has a different structure, however. The outer coat of the eye in this region is formed by the cornea (anterior portion of the fibrous coat). As the “window of the eye,” it bulges forward while covering the structures behind it. At the corneoscleral limbus, the cornea is continuous with the less convex sclera, which is the posterior portion of the outer coat of the eyeball. It is a firm layer of connective tissue that gives attachment to the tendons of all the extraocular muscles. Anteriorly, the sclera in the angle of the anterior chamber forms the trabecular meshwork (see p. 129), which is connected to the canal of Schlemm. On the posterior side of the eyeball, the axons of the optic nerve pierce the lamina cribrosa of the sclera. Beneath the sclera is the vascular coat of the eye, also called the uveal tract. It consists of three parts in the anterior portion of the eye: the iris, ciliary body, and choroid, the latter being distributed over the entire eyeball.

The iris shields the eye from excessive light (see p.128) and covers the lens. Its root is continuous with the ciliary body, which contains the ciliary muscle for visual accommodation (alters the refractive power of the lens, see p. 127). The epithelium of the ciliary body produces the aqueous humor. The ciliary body is continuous at the ora serrata with the middle layer of the eye, the choroid. The choroid organ is the most highly vascularized region in the body and serves to regulate the temperature of the eye and to supply blood to the outer layers of the retina. The inner layer of the eye is the retina, which includes an inner layer of photosensitive cells (the sensory retina) and an outer layer of retinal pigment epithelium. The latter is continued forward as the pigment epithelium of the ciliary body and the epithelium of the iris. The fovea centralis is a depressed area in the central retina that is approximately 4 mm temporal to the optic disk. Incident light is normally focused onto the fovea centralis, which is the site of greatest visual acuity. The interior of the eyeball is occupied by the vitreous humor (vitreous body, see C).

В Reference lines and points on the eye

The line marking the greatest circumference of the eyeball is the equator. Lines perpendicular to the equator are called meridians.

C Vitreous body (vitreous humor) (after Lang)

Right eye, transverse section viewed from above. Sites where the vitreous body is attached to other ocular structures are shown in red, and adjacent spaces are shown in green. The vitreous body stabilizes the eyeball and protects against retinal detachment. Devoid of nerves and vessels, it consists of 98% water and 2% hyaluronic acid and collagen. The “hyaloid canal" is an embryological remnant of the hyaloid artery. For the treatment of some diseases, the vitreous body may be surgically removed (vitrectomy) and the resulting cavity filled with physiological saline solution.

D Light refraction in a normal (emmetropic) eye and in myopia and hyperopia

Parallel rays from a distant light source are normally refracted by the cornea and lens to a focal point on the retinal surface.

* In myopia (nearsightedness), the rays are focused to a point in front of the retina.

* In hyperopia (farsightedness), the rays are focused behind the retina.

E Optical axis and orbital axis

Superior view of both eyes showing the medial, lateral and superior recti and the superior oblique. The optical axis deviates from the orbital axis by 23°. Because of this disparity, the point of maximum visual acuity, the fovea centralis, is lateral to the “blind spot” of the optic disk (see A).

8.4 Eye: Lens and Cornea

A Overview: Position of the lens and cornea in the eyeball

Histological section through the cornea, lens, and suspensory apparatus of the lens. The normal lens is clear and transparent and is only 4 mm thick. It is suspended in the hyaloid fossa of the vitreous body (seep. 124). The lens is attached by rows of fibrils (zonular fibers) to the ciliary muscle, whose contractions alter the shape and focal length of the lens (the structure of the ciliary body is shown in B). Thus, the lens is a dynamic structure that can change its shape in response to visual requirements (seeCb). The anterior chamber of the eye is situated in front of the lens, and the posterior chamber is located between the iris and the anterior epithelium of the lens (seep. 128). The lens, like the vitreous body, is devoid of nerves and blood vessels and is composed of elongated epithelial cells, the lens fibers.

В The lens and ciliary body

Posterior view. The curvature of the lens is regulated by the muscle fibers of the annular ciliary body (see Cb). The ciliary body lies between the ora serrata and the root of the iris and consists of a relatively flat part (pars plana) and a part that is raised into folds (pars plicata). The latter part is ridged by approximately 70-80 radially-oriented ciliary processes, which surround the lens like a halo when viewed from behind. The ciliary processes contain large capillaries, and their epithelium secretes the aqueous humor (see p. 129). Very fine zonular fibers extend from the basal layer of the ciliary processes to the equator of the lens. These fibers and the spaces between them constitute the suspensory apparatus of the lens, called the zonule. Most of the ciliary body is occupied by the ciliary muscle, a smooth muscle composed of meridional, radial, and circular fibers. It arises mainly from the scleral spur (a reinforcing ring of sclera just below the canal of Schlemm), and it attaches to structures including the Bruch membrane of the choroid and the inner surface of the sclera. When the ciliary muscle contracts, it pulls the choroid forward and relaxes the zonular fibers. As these fibers become lax, the intrinsic resilience of the lens causes it to assume the more convex relaxed shape that is necessary for near vision (see Cb). This is the basic mechanism of visual accommodation.

C Reference lines and dynamics of the lens

a Principal reference lines of the lens: The lens has an anterior and posterior pole, an axis passing between the poles, and an equator. The lens has a biconvex shape with a greater radius of curvature posteriorly (1 б mm) than anteriorly (10 mm). Its function is to transmit light rays and make fine adjustments in refraction. Its refractive power ranges from 10 to 20 diopters, depending on the state of accommodation. The cornea has a considerably higher refractive power of 43 diopters.

b Light refraction and dynamics of the lens:

 Upper half of diagram: fine adjustment of the eye for far vision. Parallel light rays arrive from a distant source, and the lens is flattened.

 Lower half of diagram: For near vision (accommodation to objects less than 5 m from the eye), the lens assumes a more rounded shape (seeB). This is effected by contraction of the ciliary muscle (parasympathetic innervation from the oculomotor nerve), causing the zonular fibers to relax and allowing the lens to assume a more rounded shape because of its intrinsic resilience.

D Growth of the lens and zones of discontinuity (after Lang)

a Anterior view, b lateral view.

The lens continues to grow throughout life, doing so in a manner opposite to that of other epithelial structures, i.e., the youngest cells are at the surface of the lens while the oldest cells are deeper. Due to the constant proliferation of epithelial cells, which are all firmly incorporated in the lens capsule, the tissue of the lens becomes increasingly dense with age. A slit-lamp examination will demonstrate zones of varying cell density (zones of discontinuity). The zone of highest cell density, the embryonic nudeus, is at the center of the lens. With further growth, it becomes surrounded by the fetal nucleus. The infantile nucleus develops after birth, and finally the adult nucleus begins to form during the third decade of life. These zones are the basis for the morphological classification of cataracts, a structural alteration in the lens, causing opacity, that is more or less normal in old age (present in 10% of all 80-year-olds).

E Structure of the cornea

The cornea is covered externally by stratified, nonkeratinized squamous epithelium whose basal lamina borders on the anterior limiting lamina (Bowman membrane). The stroma (substantia propria) makes up approximately 90% of the corneal thickness and is bounded on its deep surface by the posterior limiting lamina (Descemet membrane). Beneath is a single layer of corneal endothelium. The cornea does have a nerve supply (for corneal reflexes) but it is not vascularized and therefore has an immunologically privileged status: normally, a corneal transplant can be performed without fear of a host rejection response.

8.5 Eye: Iris and Ocular Chambers

A Location of the iris and the anterior and posterior chambers

Transverse section through the anterior segment of the eye, superior view. The iris, the choroid, and the ciliary body at the periphery of the iris are part of the uveal tract. In the iris, the pigments are formed that determine eye color (seeD). The iris is an optical diaphragm with a central aperture, the pupil, placed in front of the lens. The pupil is 1-8 mm in diameter; it constricts on contraction of the pupillary sphincter (parasympathetic innervation via the oculomotor nerve and ciliary ganglion) and dilates on contraction of the pupillary dilator (sympathetic innervation from the superior cervical ganglion via the internal carotid plexus). Together, the iris and lens separate the anterior chamber of the eye from the posterior chamber. The posterior chamber behind the iris is bounded posteriorly by the vitreous body, centrally by the lens, and laterally by the ciliary body. The anterior chamber is bounded anteriorly by the cornea and posteriorly by the iris and lens.

В Pupil size

a Normal pupil size, b maximum constriction (miosis), c maximum dilation (mydriasis).

The regulation of pupil size is aided by the two intraocular muscles, the pupillary sphincter and pupillary dilator (seeD). The pupillary sphincter, which receives parasympathetic innervation, narrows the pupil while the pupillary dilator, which receives sympathetic innervation, enlarges the pupil. Pupil size is normally adjusted in response to incident light and serves mainly to optimize visual acuity. Normally, the pupils are circular in shape and equal in size (3-5mm). Various influences (listed in C) may cause the pupil size to vary over a range from 1.5 mm (miosis) to 8 mm (mydriasis). A greater than 1-mm discrepancy of pupil size between the right and left eyes is called anisocoria. Mild anisocoria is physiological in some indivuals. Pupillary reflexes such as convergence and the consensual light response are described on p.362.

C Causes of miosis and mydriasis (after Sachsenweger)

Miosis (Bb)

Mydriasis (Be)

Light

Darkness

Sleep, fatigue

Pain, excitement

Miotic agents (parasympatho- mimetics, sympatholytics)

Mydriatic agents ( pa rasy m patho ly tics such as atropine, sympathomimetics such as epinephrine)

Horner syndrome (including ptosis and a narrow palpebral fissure)

Oculomotor palsy

General anesthesia, morphine

Migraine attack, glaucoma attack

D Structure of the iris

The basic structural framework of the iris is the vascularized stroma, which is bounded on its deep surface by two layers of pigmented iris epithelium. The loose, collagen-containing stroma of the iris contains outer and inner vascular circles (greater and lesser arterial circles), which are interconnected by small anastomotic arteries. The pupillary sphincter is an annular muscle located in the stroma bordering the pupil. The radially disposed pupillary dilator is not located in the stroma; rather it is composed of numerous myofibrils in the iris epithelium (myoepithelium). The stroma of the iris is permeated by pigmented connective- tissue cells (melanocytes). When heavily pigmented, these melanocytes of the anterior borderzone of the stroma renderthe iris brown or “black.” Otherwise, the characteristics of the underlying stroma and epithelium determine eye color, in a manner that is not fully understood.

E Normal drainage of aqueous humor

The aqueous humor (approximately 0.3 ml per eye) is an important determinant of the intraocular pressure (seeF). It is produced by the nonpigmented ciliary epithelium of the ciliary processes in the posterior chamber (approximately 0.15ml/hour) and passes through the pupil into the anterior chamber of the eye. The aqueous humor seeps through the spaces of the trabecular meshwork (Fontana spaces) in the chamber angle and enters the canal of Schlemm (venous sinus of the sclera), through which it drains to the episcleral veins. The draining aqueous humor flows toward the chamber angle along a pressure gradient (intraocular pressure = 15 mm Hg, pressure in the episcleral veins = 9 mm Hg) and must surmount a physiological resistance at two sites:

 the pupillary resistance (between the iris and lens) and

 the trabecular resistance (narrow spaces in the trabecular meshwork).

Approximately 85 % of the aqueous humor flows through the trabecular meshwork into the canal of Schlemm. Only 15% drains through the uveoscleral vascular system into the vortical veins (uveoscleral drainage route).

F Obstruction of aqueous drainage and glaucoma

The normal intraocular pressure in adults (15 mm Hg) is necessary for a functioning optical system, partly because it maintains a smooth curvature of the corneal surface and helps keep the photoreceptor cells in contact with the pigment epithelium. When glaucoma is present (see D,p. 127), the intraocular pressure is elevated and the optic nerve becomes constricted at the lamina cribrosa, where it emerges from the eyeball through the sclera. This constriction of the optic nerve eventually leads to blindness. The elevated pressure is caused by an obstruction that hampers the normal drainage of aqueous humor, which can no longer overcome the pupillary or trabecular resistance (seeE). One of two conditions may develop:

 Acute or angle-closure glaucoma (a), in which the chamber angle is obstructed by iris tissue. The aqueous fluid cannot drain into the anterior chamber and pushes portions of the iris upward, blocking the chamber angle.

• Chronic or open-angle glaucoma (b), in which the chamber angle is open but drainage through the trabecular meshwork is impaired (the red bar marks the location of each type of obstruction).

By far the most common form (approximately 90% of all glaucomas) is primary chronic open-angle glaucoma (b), which becomes more prevalent after 40 years of age. The primary goal of treatment is to improve the drainage of aqueous humor(e.g., with parasympathomimetics that induce sustained contraction of the ciliary muscle and pupillary sphincter) or decrease its production.

8.6 Eye: Retina

A Overview of the retina

The retina is the third, innermost layer of the eyeball. It consists mainly of a photosensitive optic part and a smaller, non-photosensitive forward prolongation called the nonvisual retina. The optic part of the retina, shown here in yellow, varies in thickness at different locations. It overlies the pigment epithelium of the uveal tract and is pressed against it by the intraocular pressure. The optic part of the retina ends at a jagged margin, the ora serrata, which is where the nonvisual retina begins (see also B). The site on the retina where visual acuity is highest is the fovea centralis, a small depression at the center of a yellowish area, the macula lutea. The optic part of the retina is particularly thin at this site; it is thickest at the point where the optic nerve emerges from the eyeball at the lamina cribrosa.

В Parts of the retina

The posterior surface of the iris bears a double layer of pigment epithelium, the iridial part of the retina. Just peripheral to it is the ciliary part of the retina, also formed by a double layer of epithelium (one of which is pigmented) and covering the posterior surface of the ciliary body. The iridial and ciliary parts of the retina together constitute the nonvisuai retina— the portion of the retina that is not sensitive to light (compare with A). The nonvisual retina ends at a jagged line, the ora serrata, where the light-sensitive optic part of the retina begins. Consistent with the development of the retina from the embryonic optic cup, two layers can be distinguished within the optic part:

 An outer layer nearer the sclera: the pigmented layer, consisting of a single layer of pigmented retinal epithelium (see Ca).

 An inner layer nearer the vitreous body: the neural layer, comprising a system of receptor cells, interneurons, and ganglion cells (see Cb).

C Structure of the retina

a Schematic diagram of the first three neurons in the visual pathway and their connections, b The ten anatomical layers of the retina.

Light must pass through all the inner layers of the retina (the layers nearest the vitreous body) before reaching the photosensitive elements of the photoreceptors. The direction of transmission of sensory information, however, is inward, opposite to the direction of the incoming light. The first three neurons of the visual pathway are located within the retina. Starting with the outermost neuron, they are as follows (a):

 First neuron: Photoreceptor cells (rods and cones) are light-sensitive sensory cells that transform light stimuli into electrochemical signals. The two types of photoreceptors are rods and cones, named for the shape of their receptor segment. The retina contains 100—125 million rods, which are responsible for twilight and night vision, but only about 6-7 million cones. Different cones are specialized for the perception of red, green, and blue.

 Second neuron: bipolar cells that receive impulses from the photoreceptors and relay them to the ganglion cells.

 Third neuron: retinal ganglion cells whose axons converge at the optic disk to form the optic nerve and reach the lateral geniculate and superior colliculus.

In addition to these largely “vertical” connections, there are also horizontal cells and amacrine cells that function as interneurons to establish lateral connections. In this way the impulses transmitted by the receptor cells are processed and organized while still within the retina (signal convergence). The retinal Muller cells are glial cells that span the neural layer radially from the inner to outer limiting membranes and create a supporting framework for the neurons. External to these cells is the pigment epithelium, whose basement membrane is attached to the Bruch membrane (contains elastic fibers and collagen fibrils) and mediates the exchange of substances between the adjacent choroid (choriocapillaris) and the photoreceptor cells.

Note: The outer segments of the photoreceptors are in contact with the pigment epithelium but are not attached to it. This explains why the retina may become separated from the pigment epithelium (retinal detachment: untreated, leads to blindness). Traditionally, a histological section of the retina consists of ten layers (b) that are formed by elements of the three neurons (e.g., nuclei or cellular processes) that occupy a consistent level within any given layer.

D Optic disk (“blind spot”) and lamina cribrosa

The unmyelinated axons of the retinal ganglion cells (approximately 1 million axons per eye) pass to a collecting point at the posterior pole of the eye, the optic disk. There they unite to form the optic nerve and leave the retina through numerous perforations in the sclera (lamina cribrosa). In the optic nerve, these axons are myelinated by oligodendrocytes.

Note the central retinal artery entering the eye at this location (see p. 132) and note the coverings of the optic nerve. Because the optic nerve is a forward prolongation of the diencephalon, it has all the coverings of the brain (dura mater, arachnoid, and pia mater). It is surrounded by a subarachnoid space that contains cerebrospinal fluid and communicates with the subarachnoid spaces of the brain and spinal cord.

E Macula lutea and fovea centralis

Temporal to the optic disk is the macula lutea. At its center is a funnelshaped depression approximately 1.5mm in diameter, the fovea centralis, which is the site of maximum visual acuity. At this site the inner retinal layers are heaped toward the margin of the depression, so that the cells of the photoreceptors (just cones, no rods) are directly exposed to the incident light. This arrangement significantly reduces scattering of the light rays.

8.7 Eye: Blood Supply

A Blood supply of the eye

Horizontal section through the right eye at the level of the optic nerve, viewed from above. All of the arteries that supply the eye arise from the ophthalmic artery, a terminal branch of the internal carotid artery (see p. 61 ). Its ocular branches are:

 Central retinal artery to the retina (see B)

 Short posterior ciliary arteries to the choroid

 Long posterior ciliary arteries to the ciliary body and iris, where they supply the greater and lesser arterial circles of the iris (see D,

• Anterior ciliary arteries, which arise from the vessels of the rectus muscles of the eye and anastomose with the posterior ciliary vessels

Blood is drained from the eyeball by 4 to 8 vorticose veins, which pierce the sclera behind the equator and open into the superior or inferior ophthalmic vein.

В Arterial blood supply of the optic nerve and optic nerve head

Lateral view. The central retinal artery, the first branch of the ophthalmic artery, enters the optic nerve from below approximately 1 cm behind the eyeball and courses with it to the retina while giving off multiple small branches. The posterior ciliary artery also gives off several small branches that supply the optic nerve. The optic nerve head receives its arterial blood supply from an arterial ring (circle of Zinn and von Haller) formed by anastomoses among the side branches of the short posterior ciliary arteries and central retinal artery.

C Ophthalmoscopic examination of the optic fundus

a Examination technique (direct ophthalmoscopy),

b Normal appearance of the optic fundus.

In direct ophthalmoscopy, the following structures of the optic fundus can be directly evaluated at approximately 16x magnification:

 The condition of the retina

 The blood vessels (particularly the central retinal artery)

 The optic disk (where the optic nerve emerges from the eyeball)

 The macula lutea and fovea centralis

Because the retina is transparent, the color of the optic fundus is determined chiefly by the pigment epithelium and the blood vessels of the choroid. It is uniformly pale red in light-skinned persons and is considerably browner in dark-skinned persons. Abnormal detachment of the retina is usually associated with a loss of retinal transparency, and the retina assumes a yellowish-white color. The central retinal artery and vein can be distinguished from each other by their color and caliber: arteries have a brighter red color and a smaller caliber than the veins. This provides a means for the early detection of vascular changes (e.g., stenosis, wall thickening, microaneurysms), such as those occurring in diabetes mellitus (diabetic retinopathy) or hypertension. The optic disk normally has sharp margins, a yellow-orange color, and a central depression, the physiological cup. The disk is subject to changes in pathological conditions such as elevated intracranial pressure (papilledema with ill-defined disk margins). On examination of the macula lutea, which is 3-4 mm temporal to the optic disk, it can be seen that numerous branches of the central retinal artery radiate toward the macula but do not reach its center, the fovea centralis (the fovea receives its blood supply from the choroid). A common age-related disease of the macula lutea is macular degeneration, which may gradually lead to blindness.

8.8 Orbit: Extraocular Muscles

A Location of the extraocular muscles (extrinsic eye muscles)

Right eye, superior view (a) and anterior view (b).

The eyeball is moved in the orbit by four rectus muscles (superior, inferior, medial, and lateral) and two oblique muscles (superior and inferior). (Innervation and direction of movements are shown in В and D.) The superior oblique arises from the sphenoid bone and the inferior oblique from the medial orbital margin. The four rectus muscles arise from a tendinous ring around the optic canal (common tendinous ring, common annular tendon). All of the extraocular muscles insert on the sclera. The tendon of insertion of the superior oblique first passes through a tendinous loop (trochlea) attached to the superomedial orbital margin, which redirects it posteriorly at an acute angle to its insertion on the temporal aspect of the superior surface of the eyeball. The functional competence of all six extraocular muscles and their coordinated interaction are essential in directing both eyes toward the visual target. It is the task of the brain to process the two perceived retinal images in a way that provides binocular visual perception. If the coordinated actions of these muscles are impaired, due, for example, to the paralysis of one eye muscle (see E), the visual axis of one eye will deviate from its normal position and the patient will perceive a double image (diplopia).

В Innervation of the extraocular muscles

Right eye, lateral view with the temporal wall of the orbit removed. Except for the superior oblique (trochlear nerve) and lateral rectus (abducent nerve), the ocular muscles (superior, medial and inferior rectus and inferior oblique) are supplied by the oculomotor nerve. After emerging from the brainstem, cranial nerves III, IV, and VI first pass through the cavernous sinus (or its lateral wall, see A, p. 138), where they are in close proximity to the internal carotid artery. From there they traverse the superior orbital fissure (see B, p. 138) to enter the orbit and supply their respective muscles.

C Function and innervation of the extraocular muscles

Right eye, superior view with the orbital roof removed. The lateral and medial rectus muscles have only one primary action and one direction of pull (a.b), while the other muscles have secondary actions and directions of pull M).

Muscle

Primary action

Secondary action

Innervation

a Lateral rectus

Abduction

None

Abducent nerve (CN VI)

b Medial rectus

Adduction

None

Oculomotor nerve (CN III), inferior branch

c Superior rectus

Elevation

Adduction and medial rotation

Oculomotor nerve (CN III), superior branch

d Inferior rectus

Depression

Adduction and lateral rotation

Oculomotor nerve (CN III), inferior branch

e Superior oblique

Depression and abduction

Medial rotation

Trochlear nerve (CN IV)

f Inferior oblique

Elevation and abduction

Lateral rotation

Oculomotor nerve (CN III), inferior branch

D The six cardinal directions of gaze

In the clinical evaluation of ocular motility to diagnose oculomotor palsies, six cardinal directions of gaze are tested (see arrows). The muscles that are activated in each direction and their cranial nerves are shown schematically for both eyes.

Note that different muscles may be activated in both eyes for any particular direction of gaze.

For example, gaze to the right is effected by the combined actions of the lateral rectus of the right eye and the medial rectus of the left eye. These two muscles, moreover, are supplied by different cranial nerves (VI and III, respectively).

If one muscle is weak or paralyzed, deviation of the eye will be noted during certain ocular movements (see E).

E Oculomotor palsies

a Complete oculomotor palsy on the right side,

b Trochlear nerve palsy on the right side,

c Abducent nerve palsy on the right side (shown in each case on attempted straightahead gaze).

Oculomotor palsies may result from a lesion involving the nucleus or course of the associated cranial nerve or the eye muscle itself (see p. 72). Depending on the muscle involved, the effect may be a deviated position of the affected eye or diplopia. The patient attempts to compensate for this by adjusting the position of the head.

a In cases of complete oculomotor palsy, the following muscles are paralyzed (followed in parentheses by the observable deficit). Extraocular muscles: superior, inferior and medial recti and inferior oblique (eyeball deviates toward the lower outer quadrant). Intraocular muscles: pupillary sphincter (pupil dilated = mydriasis) and ciliary muscle (loss of near accommodation). Levator palpebrae superioris (drooping of the eyelid = ptosis). If the ptosis is complete, as shown here, complete oculomotor palsy does not produce diplopia because one eye cannot be opened.

b Trochlear nerve palsy disables the superior oblique, whose action is to depress and abduct. The affected eye slightly deviates medially upward.

c Abducent nerve palsy disables the lateral rectus, causing the affected eye to deviate toward the midline.

8.9 Orbit: Subdivisions and Neurovascular Structures

A Subdivision of the orbit into upper, middle, and lower levels

Sagittal section through the right orbit viewed from the medial side. The orbit is lined by periosteum (periorbita) and contains the following structures, which are embedded within the retro-orbital fat: eyeball, optic nerve, lacrimal gland, extraocular muscles, and the neurovascular structures that supply them. The retro-orbital fat is bounded anteriorly by the orbital septum and toward the eyeball by a mobile sheath of connective tissue (bulbar fascia, Tenon’s capsule). The narrow space between the bulbar fascia and sclera is called the episcleral space.

Topographically, the orbit is divided into three levels with the following boundaries:

 Upper level: between the orbital roof and the levator palpebrae superioris

 Middle level: between the superior rectus and the optic nerve

 Lower level: between the optic nerve and the orbital floor

The contents of the different levels are listed in B.

В The three upper orbital levels and their main contents

(The sites of entry of the neurovascular structures into the orbit are described on p. 14.)

Level

Contents

Source/associated structures

Upper level

• Lacrimal nerve

• Branch of ophthalmic nerve (CN \Л])

• Lacrimal artery

• Branch of ophthalmic artery (from internal carotid artery)

• Lacrimal vein

• Passes to superior ophthalmic vein

• Frontal nerve

• Branch of ophthalmic nerve (CN \Л|)

• Supraorbital nerve and supratrochlear nerve

• Terminal branches of frontal nerve

• Supraorbital artery

• Terminal branch of ophthalmic artery

• Supraorbital vein

• Unites with supratrochlear veins to form angular vein

• Trochlear nerve

• Nucleus of trochlear nerve in mesencephalon

Middle level

• Ophthalmic artery

• Branch of internal carotid artery

• Central retinal artery

• Branch of ophthalmic artery

• Posterior ciliary arteries

• Branches of ophthalmic artery

• Nasociliary nerve

• Branch of ophthalmic nerve (CN Vq)

• Abducent nerve

• Abducent nucleus in pons

• Oculomotor nerve, superior branch

• Oculomotor nucleus in mesencephalon

• Optic nerve

• Retina (retinal ganglion cells)

• Short ciliary nerves

• Postsynaptic autonomic fibers to the eyeball

• Ciliary ganglion

• Parasympathetic ganglion for ciliary muscle and pupillary sphincter

• Parasympathetic root

• Presynaptic autonomic fibers of oculomotor nerve

• Sympathetic root

• Postsynaptic fibers from the superior cervical ganglion

• Nasociliary root

• Sensory fibers from eyeball through ciliary ganglion to nasociliary nerve

• Superior ophthalmic vein

• Passes into cavernous sinus

Lower level

• Oculomotor nerve, inferior branch

• Oculomotor nucleus in mesencephalon

• Inferior ophthalmic vein

• Passes into cavernous sinus

• Infraorbital nerve

• Branch of maxillary nerve (CN V2)

• Infraorbital artery

• Terminal branch of maxillary artery (external carotid artery)

C Branches of ophthalmic artery

Right orbit, superior view after opening of the optic canal and orbital roof. The ophthalmic artery is a branch of the internal carotid artery. It runs below the optic nerve through the optic canal to the orbit and supplies the intraorbital structures including the eyeball.

D Veins of the orbit

Right orbit, lateral view with the lateral orbital wall removed and the maxillary sinus opened. The veins of the orbit communicate with the veins of the superficial and deep facial region and with the cavernous sinus (potential spread of infectious pathogens).

E Innervation of the orbit

Right orbit, lateral view with the temporal bony wall removed. The orbit receives motor, sensory and autonomic innervation from four cranial nerves: the oculomotor nerve (CN III), the trochlear nerve (CN IV), the abducent nerve (CN VI), and the ophthalmic division of the trigeminal nerve (CN V-,). The oculomotor nerve also conveys presynaptic parasympathetic fibers to the ciliary ganglion. The postsynaptic sympathetic fibers pass into the orbit byway of the internal carotid plexus and ophthalmic plexus.

8.10 Orbit: Topographical Anatomy

A Intracavernous course of the cranial nerves that enter to the orbit

Anterior and middle cranial fossae on the right side, superior view. The lateral and superior walls of the cavernous sinus have been opened. The trigeminal ganglion has been retracted slightly laterally, the orbital roof has been removed, and the periorbita has been fenestrated. All three of the cranial nerves that supply the ocular muscles (oculomotor nerve, trochlear nerve, and abducent nerve) enter the cavernous sinus, where they come into close relationship with the first and second divisions of the trigeminal nerve and with the internal carotid artery. While the third and fourth cranial nerves course in the lateral wall of the cavernous sinus with the ophthalmic and maxillary divisions of the trigeminal nerve, the abducent nerve runs directly through the cavernous sinus in close proximity to the internal carotid artery. Because of this relationship, the abducent nerve may be damaged as a result of sinus thrombosis or an intracavernous aneurysm of the internal carotid artery.

В Posterior wall of the orbit: common tendinous ring and sites of passage of neurovascular structures through the optic canal and superior orbital fissure

Right orbit, anterior view with most of the orbital contents removed. The optic nerve exits and the ophthalmic artery enters the orbit through the optic canal. Of the neurovascular structures that enter the orbit through the superior orbital fissure, some enter inside the common tendinous ring and some enter outside of it:

 Inside: abducent nerve, nasociliary nerve, superior and inferior branch of the oculomotor nerve

 Outside: superior and inferior ophthalmic veins, frontal nerve, lacrimal nerve, and trochlear nerve

D Topography of the right orbit: contents of the middle level

Superior view. The levator palpebrae superioris and the superior rectus have been divided and reflected backward, and all fatty tissue has been removed to better expose the optic nerve.

Note: The ciliary ganglion is approximately 2mm in diameter and lies lateral to the optic nerve approximately 2 cm behind the eyeball. The parasympathetic innervation for the intraocular muscles (ciliary muscle and pupillary sphincter) is relayed in the ciliary ganglion. The postsynaptic sympathetic fibers for the pupillary dilator, from the superior cervical ganglion, also pass through this ganglion.