Anatomy of the Eyeball

Eyeball Anatomy

The adult eyeball is about 2.5 cm (1 inch) wide, but only the front one-sixth is exposed, while the rest is protected inside the orbit (eye socket).

Eyeball Anatomy: Three Layers

The wall of the eyeball consists of three layers: fibrous layer, vascular layer, and inner layer (retina)

Fibrous Layer

The superficial layer of the eyeball and consists of the anterior cornea and posterior sclera

Fibrous Layer: Cornea

The outermost, clear layer of the eyeball, made of the anterior cornea and posterior sclera, which also helps focus light onto the retina due to its curvature.

Fibrous Layer: Cornea: Layers

The cornea has an outer layer of nonkeratinized stratified squamous epithelium, a middle layer of collagen fibers and fibroblasts, and an inner layer of simple squamous epithelium.

Fibrous Layer: Sclera

The white, tough outer layer of the eyeball, made of dense connective tissue with mostly collagen fibers and fibroblasts, provides strength and protection and serves as an attachment site for the extraocular muscles.

Fibrous Layer: Scleral Venous Sinus

The circular channel at the junction where the cornea meets the sclera drains aqueous humor into the bloodstream.

Vascular Layer

The middle layer of the eyeball and consists of the choroid, ciliary body, and iris

Vascular Layer: Choroid

A dark, vascular layer that supplies nutrients to the retina, while its melanin absorbs stray light, preventing reflection and scattering so the image on the retina stays sharp and clear.

Vascular Layer: Choroid: Albinism

Because albinos lack melanin, they often need sunglasses to protect themselves from light and glare from even moderately bright light.

Vascular Layer: Ciliary Body

A dark, ring-shaped tissue inside the eye, located just behind the iris, that extends from the ora serrata to the junction of the sclera and cornea.

Vascular Layer: Ciliary Body: Ora Serrata

The jagged, serrated boundary line inside the eye, just posterior to the junction of the sclera and cornea

Vascular Layer: Ciliary Body: Melanocytes

Like the choroid, the ciliary body appears dark brown in color because it contains melanin-producing melanocytes

Vascular Layer: Ciliary Body: Components

The ciliary body consists of ciliary processes and ciliary muscle.

Vascular Layer: Ciliary Body: Ciliary Processes

Small, finger-like protrusions on the surface of the ciliary body that contain blood capillaries and secrete aqueous humor to support the lens via extensions of zonular fibers (elastic connective tissue fibers).

Vascular Layer: Ciliary Body: Ciliary Muscle

A circular band of smooth muscle that contracts or relaxes the zonular fibers, altering the shape of the lens to adapt for near or far vision.

Vascular Layer: Iris

The colored, ring-shaped muscle at the front of the eye, which controls how much light enters through the pupil, consists of melanocytes and circular and radial smooth muscle fibers.

Vascular Layer: Iris: Color

The amount of melanin in the iris determines the eye color; dark-colored eyes has its iris contain large amounts of melanin, whereas light-colored has its iris contain fewer amounts of melanin.

Vascular Layer: Iris: Pupil

The dark, circular opening in the center of the iris that allows light to enter the eye

Vascular Layer: Iris: Pupil: Red Eye

If bright light is directed into the pupil, the person’s eyes appear red because of the reflected light from the blood vessels on the surface of the retina

Vascular Layer: Iris: Pupil: Sphincter Pupillae

When bright light stimulates the eye, parasympathetic fibers of the oculomotor (III) nerve cause the sphincter pupillae to contract, decreasing the pupil’s size (constriction).

Vascular Layer: Iris: Pupil: Dilator Pupillae

When dim light stimulates the eye, sympathetic neurons cause the dilator pupillae to contract, increasing the pupil’s size (dilation).

Inner Layer (Retina)

The inner layer of the eyeball is the retina, which is a light-sensitive layer of nerve tissue that lines the back three-quarters of the eyeball

Inner Layer (Retina): Ophthalmoscope

An instrument used to examine the interior of the eye, specifically the retina, blood vessels, and the optic (II) nerve, by flashing a light into the eye.

Inner Layer (Retina): Optic Disc

The blind spot on the retina where nerve fibers leave the eye to form the optic nerve

Inner Layer (Retina): Central Retinal Artery

A small blood vessel branching from the ophthalmic artery that supplies oxygen and nutrients to the inner layers of the retina

Inner Layer (Retina): Central Retinal Vein

The main blood vessel that drains oxygen-poor blood away from the retina through the optic disc

Inner Layer (Retina): Components

The retina consists of a pigmented layer and a neural layer.

Inner Layer (Retina): Pigmented Layer

A sheet of melanin-containing epithelial cells located between the choroid and the neural part of the retina, which also help absorb stray light rays, similar to the choroid

Inner Layer (Retina): Neural Layer

A multilayered outgrowth of the brain that processes visual data before sending nerve impulses into axons that form the optic nerve.

Inner Layer (Retina): Neural Layer: Components

The retina has three layers of retinal neurons—photoreceptors, bipolar cells, and ganglion cells—separated by inner and outer synaptic layers where the neurons communicate with each other.

Inner Layer (Retina): Two Cells

The bipolar cell layer contains horizontal and amacrine cells, which form lateral circuits in the retina that modify and refine signals as they travel from photoreceptors to bipolar cells to ganglion cells.

Inner Layer (Retina): Photoreceptors

The photoreceptor cell layer has specialized light-detecting cells cells that convert light rays into nerve impulses for the brain

Inner Layer (Retina): Photoreceptors: Types

There are two types of photoreceptors: rods and cones; each retina has about 6 million cones and 120 million rods.

Inner Layer (Retina): Photoreceptors: Rods

Specialized photoreceptor cells in the retina that make low-light and night vision possible, but they not provide color vision, as they see the world in black and white in dim light.

Inner Layer (Retina): Photoreceptors: Cones

Specialized, light-sensitive photoreceptor cells in the retina that produce color vision.

Inner Layer (Retina): Photoreceptors: Cones: Types

The retina has three types of cones—blue, green, and red—and color vision comes from different combinations of these cones being stimulated.

Inner Layer (Retina): Photoreceptors: Cones: Legal Blindness

The cone system is responsible for most of our visual experiences, and losing cone function can result in legal blindness.

Inner Layer (Retina): Signal Pathway

Information flows from photoreceptors to bipolar cells to ganglion cells, and the ganglion cell axons leave the eye at the optic disc to form the optic (II) nerve, which carries visual information to the brain.

Inner Layer (Retina): Optic Disc

The point where ganglion cell axons leave the eyeball to form the optic nerve— also known as the blind spot because it contains no rods or cones.

Inner Layer (Retina): Macula

A yellow, specialized central area of the retina in the eye that is responsible for sharp, detailed, and color vision

Inner Layer (Retina): Fovea Centralis

The small depression in the center of the macula that only contains cones, providing the sharpest and clearest vision, or visual acuity

Lens

A clear, biconvex structure located directly behind the pupil and lens that focuses light onto the retina

Lens: Crystallins

An abundant, water-soluble structural protein found primarily in the lens and cornea of the eye that provides transparency and refractive power for clearer vision

Interior of the Eyeball

The lens divides the interior of the eyeball into two segments; the anterior segment and the posterior segment.

Interior of the Eyeball: Anterior Segment

The space between the cornea and lens is divided into two chambers: the anterior chamber between the cornea and iris, and the posterior chamber between the iris and lens.

Interior of the Eyeball: Anterior Segment: Aqueous Humor

Both chambers of the anterior segment are filled with aqueous humor, a transparent watery fluid that nourishes the lens and cornea

Interior of the Eyeball: Aqueous Humor: Pathway

Aqueous humor continuously filters from the ciliary processes, flows from the posterior chamber → pupil → anterior chamber, then drains into the scleral venous sinus and then into the blood, and is replaced about every 90 minutes.

Interior of the Eyeball: Posterior Segment

The larger posterior portion of the eyeball, occupying about four-fifths, is located between the lens and the retina.

Interior of the Eyeball: Posterior Segment: Vitreous Humor

A clear, jelly-like fluid behind the lens that holds the retina against the choroid, helping provide a smooth surface for clear images.

Interior of the Eyeball: Posterior Segment: Vitreous Humor: Composition

The vitreous humor forms during embryonic development and is mostly water, with collagen fibers, hyaluronic acid, and phagocytic cells that remove debris and help keep the eye clear.

Vitreous Humor: Replacement

Unlike aqueous humor, vitreous humor is not continuously replaced.

Interior of the Eyeball: Posterior Segment: Vitreal Floaters

Harmless specks that appear to dart through the visual field when debris in the vitreous humor casts shadows on the retina, mainly found in older individuals

Interior of the Eyeball: Posterior Segment: Hyaloid Canal

A narrow channel running through the posterior segment from the optic disc to the back of the lens that contains the hyaloid artery in the fetus.

Intraocular Pressure

The pressure inside the eyeball is produced mainly by aqueous humor and partly by vitreous humor, which maintains the eyeball's shape and prevents it from collapsing.

Intraocular Pressure: Normal Value

Normal intraocular pressure is about 16 mmHg (millimeters of mercury).

Eye Injury: Puncture Wound

A puncture wound to the eyeball can cause loss of aqueous and vitreous humor, decreasing intraocular pressure, which may lead to retinal detachment or blindness