HOW THE EYE WORKS

How The Eye Works

In a number of ways, the human eye works much like a digital camera:

1. Light is focused primarily by the cornea — the clear front surface of the eye, which acts like a camera lens.
2. The iris of the eye functions like the diaphragm of a camera, controlling the amount of light reaching the back of the eye by automatically adjusting the size of the pupil (aperture).
3. The eye's crystalline lens is located directly behind the pupil and further focuses light. Through a process called accommodation, this lens helps the eye automatically focus on near and approaching objects, like an autofocus camera lens.
4. Light focused by the cornea and crystalline lens (and limited by the iris and pupil) then reaches the retina — the light-sensitive inner lining of the back of the eye. The retina acts like an electronic image sensor of a digital camera, converting optical images into electronic signals. The optic nerve then transmits these signals to the visual cortex — the part of the brain that controls our sense of sight.

Conjunctiva Function

The primary functions of the conjunctiva are:

• Keep the front surface of the eye moist and lubricated.
• Keep the inner surface of the eyelids moist and lubricated so they open and close easily without friction or causing eye irritation.
• Protect the eye from dust, debris and infection-causing microorganisms.

The conjunctiva has many small blood vessels that provide nutrients to the eye and lids. It also contains special cells that secrete a component of the tear film to help prevent dry eye syndrome.

Sclera Definition

The sclera is the dense connective tissue of the eyeball that forms the "white" of the eye. It is continuous with the stroma layer of the cornea. The junction between the white sclera and the clear cornea is called the limbus.

Some of the nourishment of the sclera comes from the blood vessels in the episclera, which is a thin, loose connective tissue layer that lies on top of the sclera and under the transparent conjunctiva that covers the sclera and episclera. Larger episcleral blood vessels are visible through the conjunctiva.

Other nourishment of the sclera comes from the underlying choroid, which is the vascular layer of the eyeball that is sandwiched between the sclera and the retina.

Sclera Function

The sclera, along with the intraocular pressure (IOP) of the eye, maintains the shape of the eyeball.

The tough, fibrous nature of the sclera also protects the eye from serious damage — such as laceration or rupture — from external trauma.

The sclera also provides a sturdy attachment for the extraocular muscles that control the movement of the eyes.

Cornea Definition

The cornea is the clear front surface of the eye. It lies directly in front of the iris and pupil, and it allows light to enter the eye.

The cornea has five layers. From front to back, these layers are:

1. *The corneal epithelium. This outer layer of the cornea is five to seven cells thick and measures about 50 microns — making it slightly less than 10 percent of the thickness of the entire cornea. Epithelial cells are constantly being produced and sloughed off in the tear layer of the surface of the eye. The turnover time for the entire corneal epithelium is about one week.
2. *Bowman's layer. This is a very thin (8 to 14 microns) and dense fibrous sheet of connective tissue that forms the transition between the corneal epithelium and the underlying stroma.
3. *The corneal stroma. This middle layer of the cornea is approximately 500 microns thick, or about 90 percent of the thickness of the overall cornea. It is composed of strands of connective tissue called collagen fibrils. These fibrils are uniform in size and are arranged parallel to the cornea surface in 200 to 300 flat bundles called lamellae that extend across the entire cornea. The regular arrangement and uniform spacing of these lamellae is what enables the cornea to be perfectly clear.
4. *Descemet's membrane. This very thin layer separates the stroma from the underlying endothelial layer of the cornea. Descemet's (pronounced "DESS-eh-mays") membrane gradually thickens throughout life.
5. *The corneal endothelium. This is the innermost layer of the cornea. The back of the endothelium is bathed in the clear aqueous humor that fills the space between the cornea and the iris and pupil. The corneal endothelium is only a single layer of cells thick and measures about 5 microns. Most of the endothelial cells are hexagonal (six-sided), but some may have five or seven sides. The regular arrangement of these cells is sometimes called the endothelial mosaic.

Cornea Function

As already mentioned, the clear cornea allows light to enter the eye for vision. But it has another very important function as well — the cornea provides approximately 65 to 75 percent of the focusing power of the eye.

Most refractive errors — nearsightedness, farsightedness and astigmatism — are due to a less-than-optimal curvature or symmetry of the cornea. Presbyopia, on the other hand, is due to an aging change in the crystalline lens.

In addition to allowing light to enter the eye and providing most of the focusing power of the eye, individual parts of the cornea have specialized functions:

*Corneal epithelium. The corneal epithelium provides an optimal surface for the tear film to spread across the surface of the eye to keep it moist and healthy and to maintain clear, stable vision.

*Bowman's layer. The dense nature of Bowman's layer helps prevent corneal scratches from penetrating into the corneal stroma. Corneal abrasions that are limited to the outer epithelial layer generally heal without scarring; but scratches that penetrate Bowman's layer and the corneal stroma typically leave permanent scars that can affect vision.

*Corneal endothelium. The single layer of cells that forms the endothelium maintains the fluid content within the cornea. Damage to the corneal endothelium can cause swelling (edema) that can affect vision and corneal health.

Uvea Definition

The uvea is the pigmented middle layer of the eyeball. It has three segments: the iris, the ciliary body and the choroid.

The uvea is the pigmented middle layer of the eye, consisting of the iris, ciliary body and choroid.

*Iris. The iris of the eye is the thin, circular structure made of connective tissue and muscle that surrounds the pupil. The color of our eyes is determined by the amount of pigment in the iris.

*Ciliary body. The second part of the uvea is the ciliary body. It surrounds the iris and cannot be seen because it's located behind the opaque sclera (white of the eye).

*Choroid. The posterior portion of the uvea is the choroid, which is sandwiched between the tough outer sclera of the eyeball and the retina in the back of the eye.

Uvea Function

Each component of the uvea has a specific function:

*Iris. In addition to giving the eye its color, the iris acts like the diaphragm of a camera and controls the size of the pupil. One muscle within the iris constricts the pupil in bright light (full sunlight, for example), and another iris muscle dilates (enlarges) the pupil in dim lighting and in the dark.

*Ciliary body. The ciliary body holds the lens of the eye in place. It is connected to the lens with a network of many tiny ligaments (called ciliary zonules or zonules of Zinn) that suspend the lens in place behind the pupil. The ciliary body also secretes the clear aqueous fluid that fills the space in the anterior segment of the eye between the cornea and the iris and lens, and it contains the muscle that controls accommodationof the eye.

*Choroid. The posterior portion of the uvea — the choroid — contains many tiny blood vessels and has the vital role of nourishing the retina.

Pupil Definition

The pupil is the opening in the center of the iris (the structure that gives our eyes their color). The function of the pupil is to allow light to enter the eye so it can be focused on the retina to begin the process of sight.

Typically, the pupils appear perfectly round, equal in size and black in color. The black color is because light that passes through the pupil is absorbed by the retina and is not reflected back (in normal lighting).

The pupil is the circular opening (aperture) of the eye. It is surrounded by the iris, which gives the eye its color.

If the pupil has a cloudy or pale color, typically this is because the lens of the eye (which is located directly behind the pupil) has become opaque due to the formation of a cataract.

There's another common situation when the pupil of the eye changes color — when someone takes your photo using the camera's flash function. Depending on your direction of gaze when the photo is taken, your pupils might appear bright red. This is due to the intense light from the flash being reflected by the red color of the retina.

Pupil Function

Together, the iris and pupil control how much light enters the eye. Using the analogy of a camera, the pupil is the aperture of the eye and the iris is the diaphragm that controls the size of the aperture.

The size of the pupil is controlled by muscles within the iris — one muscle constricts the pupil opening (makes it smaller), and another iris muscle dilates the pupil (makes it larger). This dynamic process of muscle action within the iris controls how much light enters the eye through the pupil.

In low-light conditions, the pupil dilates so more light can reach the retina to improve night vision. In bright conditions, the pupil constricts to limit how much light enters the eye (too much light can cause glare and discomfort, and it may even damage the lens and retina).

Retina Definition

The retina is the sensory membrane that lines the inner surface of the back of the eyeball. It's composed of several layers, including one that contains specialized cells called photoreceptors.

Light rays are focused by the cornea and lens onto the retina, where vision begins. The macula is a tiny, highly sensitive area of the retina that controls central vision and color vision.

There are two types of photoreceptor cells in the human eye — rods and cones.

Rod photoreceptors detect motion, provide black-and-white vision and function well in low light. Cones are responsible for central vision and color vision and perform best in medium and bright light.

Rods are located throughout the retina; cones are concentrated in a small central area of the retina called the macula. At the center of the macula is a small depression called the fovea. The fovea contains only cone photoreceptors and is the point in the retina responsible for maximum visual acuity and color vision.

Retina Function

Photoreceptor cells take light focused by the cornea and lens and convert it into chemical and nervous signals which are transported to visual centers in the brain by way of the optic nerve.

In the visual cortex of the brain these signals are converted into images and visual perceptions.