Eye Globe Anatomy

For the purpose of this article on eye globe anatomy, the eye will be divided into 2 sections: extraocular (ie, structures outside of the globe) and the ocular (ie, the globe and intraocular structures).

Throughout the years, in the absence of ancillary or diagnostic tools, many descriptive phrases, clichés, or analogies have been used to describe the eye, such as "the eye is the window to the soul," the "eye works like a camera," and "the eye is the only structure that allows us to physically see blood vessels and nerve fibers." We use our eyes daily and in almost every activity we perform (eg, reading, driving, watching). At the foundation, the eye is a sensory organ that detects specifically photons.

Photons within the visible spectrum (ie, light) enters the eye first by passing through the cornea, a clear, dome-shaped structure at the anterior post portion of the globe. Light passing through the cornea is converged (bent) where it passes through the anterior chamber and the pupil, a circular opening regulating the amount of light entering the eye. Light is further converged by the crystalline lens located posterior to the iris.

Light continues through the vitreous humor and the light converges on the retina, specifically the fovea centralis of the macula. Within the layers of the retina, photons trigger a series of electrical and chemical reactions, ultimately sending electrical signals by way of the optic nerve, along with visual pathway to the occipital cortex. Within the occipital cortices, these electrical signals are processed and interpreted, (ie, "seen") by the brain as a visual image.[1]

The image below depicts a sagittal section of the eye.

Several structures exist within the category of extraocular structures, including the orbit, extraocular muscles, conjunctiva, the lacrimal system, and eyelids. The functions associated with these structures include protection and lubrication.

Orbit

The orbit is a cone-shaped (4-sided pyramidal cavity) structure consisting of a base (orbital margin) that opens into the midline of the face, the apex, the narrow end pointed posteriorly into the head, and 4 walls.

In adults, the orbit is formed by 7 bones: (1) frontal, (2) zygoma, (3) maxilla, (4) ethmoid, (5) sphenoid, (6) lacrimal, and (7) palatine. The frontal, ethmoid, and sphenoid are the only 3 unpaired bones of the orbit. The orbital margin (base) is formed by the maxillary, zygomatic, frontal, and lacrimal bones. The lesser wing of the sphenoid and frontal bones form the roof of the orbit, whereas the maxillary, zygomatic, and palatine bones form the floor of the orbit. The medial wall is formed by the sphenoid, ethmoid, lacrimal, and maxillary bones. The lateral wall is formed by the greater wing of the sphenoid and zygomatic bones.

The orbit has a volume of 30 mL, measures 4 cm horizontally and 3.5 cm vertically, and has a depth (anteroposteriorly) of 4.5 cm. Associated with the orbit are foramina and fissures (see Table 1, below), which are important in transmitting nerves, arteries, and veins. The primary function of the orbit is to protect the eye from physical injuries.

Table 1. Contents of Orbital Foramens and Fissures (Open Table in a new window)

 

Extraocular Muscles

The extraocular muscles control the movements of the eye (see Table 2, below). Six extraocular muscles exist: 4 rectus muscles (superior, inferior, medial, and lateral) and 2 oblique muscles (superior and inferior). The extraocular muscles are innervated by 3 cranial nerves (CNs): oculomotor (CN III), trochlear (CN IV), and abducens nerves (CN VI). The levator palpebrae superioris is the other extraocular muscle that, although not directly involved in eye movement, is primarily responsible for elevating the upper eyelid.

The origin of the 4 rectus muscles is the common ring tendon (annulus of Zinn) at the orbital apex. The rectus muscles insert on the sclera anterior to the equator. The insertions of the rectus muscles are not equidistant from the limbus. The imaginary line connecting the insertions of the 4 rectus muscles is known as the Spiral of Tillaux.

The origin of the superior oblique is the sphenoid bone and has a long (10 + 10 mm) tendinous insertion. It inserts superior and temporal to the posterior pole and is covered by the superior rectus. The origin of the inferior oblique is the maxilla and has a muscular insertion temporal and inferior to the geometric posterior pole and covers the inferior rectus.

Table 2. Extraocular Muscles (Open Table in a new window)

Conjunctiva

The conjunctiva is a thin, transparent mucous membrane overlying the anterior-most portion of the sclera and lining the inner surfaces of the eyelids. The conjunctiva is divided into the limbal, bulbar, forniceal, and palpebral regions. Associated with the conjunctiva are goblet cells, which produce mucus and eccrine glands: the conjunctival glands (of Krause) and the accessory lacrimal glands (of Wolfring). The conjunctival glands (of Krause) are concentrated in the upper fornix, whereas the accessory lacrimal glands (of Wolfring) are associated with the tarsus.

Lacrimal Gland and the Nasolacrimal System

The lacrimal gland is nestled within the fossa of the frontal bone located in the anterior superotemporal quadrant of the orbit. The gland is divided into the orbital lobe and the palpebral lobe by the tendon of the levator palpebrae superioris. Ducts from both lobes traverse through the palpebral lobe and empty into the conjunctival fornix temporally. Lacrimal fluid is collected by 2 lacrimal canaliculi, which drain into the lacrimal sac at the medial canthus of the eye. These tears then drain into the inferior nasal meatus via the nasolacrimal duct. See the following image.

Eyelids

The eyelids are designed to protect, nourish and sustain the cornea and the anterior sclera. Anatomically, the eyelid is divided into 2 lamellae, anterior and posterior, demarcated by the gray line. The anterior lamella is composed of the epithelium and orbicularis oculi, whereas the tarsus and palpebral conjunctiva form the posterior lamella.

Tunica Fibrosa

Sclera

The sclera is a dense, fibrous tissue that forms the outermost layer of the eye (see the image below). It protects the eyes and provides for extraocular muscle insertion. Posteriorly, the sclera is perforated by the optic nerve at the lamina cribrosa.

Scleral thickness is not uniform. Anteriorly, the sclera is 0.6 mm thick; 0.3 mm thick at the insertion of the rectus muscles; 0.5 mm thick at the equator; and 1.0 mm thick at the posterior pole.

Externally, the sclera is covered by the episclera, containing episcleral vessels, and the anterior and posterior plexus.

Cornea

The cornea is a clear and transparent layer anterior on the eye. It is the eye's main refracting surface. This layer is avascular and exhibits the following 5 layers:

1. The epithelium is a stratified squamous nonkeratinized epithelium (5-6 layers of cells); it is highly sensitive due to numerous nerve endings and has excellent regenerative power

2. The Bowman membrane is structureless and acellular

3. The substantia propria (stroma) forms 90% of the cornea's thickness; fibrils of the stroma criss-cross at 90º angles, and these fibrils are of types I, III, V, and VII collagen

4. The Descemet membrane is structureless, homogeneous, and measures 3-12 microns; it is composed of the anterior banded zone and the posterior nonbanded zone; the Descemet membrane is rich in type IV collagen fibers

5. The endothelium is a single layer of simple cuboidal and hexagonal cells that line the inner surface of the cornea; the endothelium is derived from the neural crest and functions to transport fluid from the anterior chamber to the stroma; Because the cornea is avascular, its nutrients are derived mainly from diffusion from the endothelium layer

Tunica vasculosa (uveal tract)

Choroid

The choroid is a spongy, brown membrane with extensive venous plexuses, which has the following 4 layers:

1. The epichoroid layer bridges the space between the sclera and choroid

2. The vessels layer forms the bulk of the choroid layer and contains melanocytes

3. The choriocapillaris is a layer of capillaries lined by fenestrated type II endothelium that supplies nutrition to the outer portion of the retina

4. The Bruch membrane is a shiny, homogeneous membrane that lies between the choriocapillaris and retina

Ciliary body

The ciliary body is the thickest part of the tunica vasculosa. It encircles the eye anterior to the ora serrata. The ciliary processes are radiating folds of smooth muscles.

The bulk of the ciliary body is made up of smooth muscle arranged in meridional, radial, and equatorial bands. They function to contract and relax the zonule fibers, which results in altered tension on the capsule of the lens.

Iris

The iris is the most anterior part of the uvea. It has a central aperture, the pupil. Peripherally, the iris is attached to the ciliary body, and, anteriorly, it rests against the anterior surface of the lens, thus separating the anterior chamber from the posterior chamber. The anterior surface is irregular with crypts and furrows; posteriorly, the surface shows shallow furrows and is uniformly black due to the 2 layers of pigmented epithelium.

The iris has both a sphincter and a dilator pupillary muscle. The sphincter pupillae muscle lies as a ring of smooth at the pupillary margin and is supplied by the parasympathetic fibers of CN III. The dilator pupillae muscle is thin and radially oriented; it is supplied by the sympathetic fibers.

Lens

The lens is a crystalline structure, biconvex, and covered by a lens capsule. Attached to it are the zonular fibers that pass to the ciliary body as the suspensory ligament. The lens is avascular and derives it nutrients from the aqueous humor. It is elastic and transparent and held in place by suspensory ligament or zonule.

Chambers of the Eye

The anterior chamber is a space bounded anteriorly by the posterior surface (endothelium) of the cornea, and posteriorly by the lens, iris, and anterior surface of the ciliary body. Circumferentially, the lateral border of the anterior chamber is occupied by the trabecular meshwork, through which aqueous humor is drained into the scleral venous sinus (canal of Schlemm).

The posterior chamber is bounded anteriorly by the iris, posteriorly by the lens and zonule fibers, and peripherally by the ciliary processes.

Aqueous Humor

The aqueous humor is watery fluid that fills both the anterior and posterior chambers of the eye. It is secreted partially by the ciliary epithelium and partially by diffusion from capillaries in the ciliary processes. The aqueous humor contains diffusable materials of blood plasma but has a low protein content.

Scleral Venous Sinus (Canal of Schlemm)

The scleral venous sinus, or canal of Schlemm, is an annular vessel encircling the eye. The canal is lined by endothelium and its function is to drain the aqueous humor.

Trabecular Meshwork

The trabecular meshwork is a spongelike tissue that is interposed between the anterior chamber and the scleral venous sinus. The trabeculae are made up of a core of collagenous fibers that are covered by endothelium.

Vitreous Body

The vitreous body is a clear, transparent gel that fills the space between the retina and the lens that adheres to the retina. Its function is to maintain the shape and turgidity of the eye and to permit passage of light rays to the retina.

Refractive Media

Refractive media include all transparent structures through which light rays must pass to reach the retina, such as the cornea, anterior chamber, lens, and vitreous body.

Retina

The retina is the innermost layer of the eyeball, which is composed of photoreceptor cells. In the posterior pole, a shallow depression is termed the fovea centralis (see the following image). This area is the point of greatest visual acuity. This area is composed of only cones. Around the fovea is an area containing yellow pigment termed the macula lutea.

The layers of the retina are as follows:

• Pigment epithelium (layer closest to the choroid layer)

• Layer of rods and cones

• External limiting membrane

• Outer nuclear layer

• Outer plexiform layer

• Inner nuclear layer

• Inner plexiform layer

• Ganglion cell layer

• Optic nerve fiber layer

• Internal limiting membrane (layer closest to the vitreous body)

Pigment epithelium

The pigment epithelium is a single layer of polygonal cells regular in shape. The nuclei are spherical and lie toward the cell base. Numerous melanin granules are seen. Functionally, the pigment epithelium absorbs light and prevents reflection, and it is also involved in the nutrition of photoreceptors. The pigment epithelium is essential for the formation of rhodopsin and its movement by storing and releasing vitamin A, a rhodopsin precursor.

Layer of rods and cones

Rods are slender, cylindrical cells and number about 130 x 106. Cones have a flask shape and number about 67 x 106. Nuclei in the cones are larger than those of the rods, and they are also less dense. They do not contain rhodopsin but have pigment that is sensitive to blue, green, and red light.

External and internal limiting membrane

The external limiting membrane supports the photoreceptor cells. The internal limiting membrane is the basal lamina of the Muller cells, separating the retina from the vitreous body.

Outer and inner nuclear layers

The outer nuclear layer is composed of the nucleated portion of the rod and cone cells. The inner nuclear layer contains the cell bodies and nuclei of the bipolar neurons as well as supporting cells named Muller cells.

Outer and inner plexiform layers

The outer plexiform layer contains synapses made between the rod and cone cells and the bipolar cells. The inner plexiform layer contains the synapses between the bipolar neurons and the ganglionic cells.

Ganglion cell layer

The ganglion cell layer contains cell bodies and nuclei of the ganglion cells. Neuroglia are also present.

Optic nerve fibers

The optic nerve fibers contain axons of the ganglion cells that pass radially to form the optic nerve.