Human Eye is the most sensitive part of body. When surveyed about the five senses — sight, hearing, taste, smell and touch — people consistently report that their eyesight is the mode of perception they value (and fear losing) most.
Despite this, many people don’t have a good understanding of the anatomy of the eye, how vision works, and health problems that can affect the eye.
Structure and Function of Human Eye
To understand how the eye sees, it helps to know the eye structures and functions:
Cornea: Light enters through the cornea, the transparent outer covering of the eye. The eyeball is rounded, so the cornea acts as a lens. It bends or refracts light.
Aqueous Humor: The fluid beneath the cornea has a composition similar to that of blood plasma. The aqueous humor helps to shape the cornea and provides nourishment to the eye.
Iris and Pupil: Light passes through the cornea and aqueous humor through an opening called the pupil. The size of the pupil is determined by the iris, the contractile ring that is associated with eye color. As the pupil dilates (gets bigger), more light enters the eye.
Lens: While most of the focusing of light is done by the cornea, the lens allows the eye to focus on either near or distant objects. Ciliary muscles surround the lens, relaxing to flatten it to image distant objects and contracting to thicken the lens to image close-up objects.
Vitreous Humor: A certain distance is required to focus light. The vitreous humor is a transparent watery gel that supports the eye and allows for this distance.
Light rays enter the eye through the cornea, the clear front “window” of the eye. The cornea’s refractive power bends the light rays in such a way that they pass freely through the pupil the opening in the center of the iris through which light enters the eye.
The iris works like a shutter in a camera. It has the ability to enlarge and shrink, depending on how much light is entering the eye.
After passing through the iris, the light rays pass thru the eye’s natural crystalline lens. This clear, flexible structure works like the lens in a camera, shortening and lengthening its width in order to focus light rays properly.
Light rays pass through a dense, transparent gel-like substance, called the vitreous that fills the globe of the eyeball and helps the eye hold its spherical shape.
In a normal eye, the light rays come to a sharp focusing point on the retina. The retina functions much like the film in a camera. It is responsible for capturing all of the light rays, processing them into light impulses through millions of tiny nerve endings, then sending these light impulses through over a million nerve fibers to the optic nerve.
Because the keratoconus cornea is irregular and cone shaped, light rays enter the eye at different angles, and do not focus on one point the retina, but on many different points causing a blurred, distorted image.
In summary, the cornea is the clear, transparent front covering which admits light and begins the refractive process. It also keeps foreign particles from entering the eye.
The pupil is an adjustable opening that controls the intensity of light permitted to strike the lens. The lens focuses light through the vitreous humor, a clear gel-like substance that fills the back of the eye and supports the retina.
The retina receives the image that the cornea focuses through the eye’s internal lens and transforms this image into electrical impulses that are carried by the optic nerve to the brain. We can tolerate very large scars on our bodies with no concern except for our vanity. This is not so in the cornea. Even a minor scar or irregularity in the shape can impair vision. No matter how well the rest of the eye is functioning, if the cornea is scarred, clouded or distorted, vision will be affected.
In keratoconus, the irregular shape of the cornea does not allow it to do its job correctly, leading to distortion of the image it passed to the retina and transmitted to the brain.
Rods are used for monochrome vision in poor light, while cones are used for color and for the detection of fine detail. Cones are packed into a part of the retina directly behind the retina called the fovea, which is responsible for sharp central vision.
When light strikes either the rods or the cones of the retina, it’s converted into an electric signal that is relayed to the brain via the optic nerve. The brain then translates the electrical signals into the images a person sees.
Although appearing to be one clear membrane, the cornea is composed of five distinct layers of tissue, each with its own function.
Epithelium is the thin outermost layer of fast-growing and easily-regenerated cells.
Bowman’s layer consists of irregularly-arranged collagen fibers and protects the corneal stroma. It is 8 to 14 microns thick.
Stroma, the transparent middle and thickest layer of the cornea is made up of regularly-arranged collagen fibers and keratocytes (specialized cells that secrete the collagen and proteoglycans needed to maintain the clarity and curvature of the cornea)
Descemet’s membrane is a thin layer that serves as the modified basement membrane of the corneal endothelium.
Endothelium is a single layer of cells responsible for maintaining proper fluid balance between the aqueous and corneal stromal compartments keeping the cornea transparent.