Structure of Human Eye

Although the eyes are not considered traditional vital organs, they are the main organs that facilitate sight. Sight is a major factor in how we navigate the world. Without it, we would have a hard time doing even the simplest tasks. Eyesight is an intricate process that requires many of the structures inside the eye to work together.

The Outer Structure

• The entire eyeball is encased in a structure called the sclera. It is a tough material that protects the eyeball from damage. The cornea is the part of the sclera that covers your iris, or the colored part of the eye. While the sclera is white, the cornea is clear. This allows light to pass through it, on to the other structures in the eye.

The Pupil and Iris

• The process of eyesight begins with light. Light enters through the hole in the middle of the eye. This hole is called the pupil, and it is in charge of receiving light. It is essential that the right amount of light be allowed into the pupil. This is where the iris, the colored part of the eye, comes in. The iris is full of muscles that allow it to adjust the size of the pupil's opening. When there is not enough light, the iris adjusts the pupil's opening to receive the maximum amount. When there is too much light or the light is too bright, the iris contracts or dilates the pupil's opening so that it only takes in enough light to function.

The Lens

• The next stop for the light that enters the eye is the lens. The lens is a clear structure that is very similar to a contact lens. The purpose of the lens is to focus the light toward the back of the eyeball so that it is more likely to reach its destination. The lens is mobile, too. It is held in place by a network of fibers attached to the ciliary muscle. The ciliary muscle's function is to adjust the lens's thickness depending on how you need to use it. When you are looking at things that are up close, the ciliary muscle works to make the lens thicker. Images that are far away require that the lens be much thinner.

The Vitreous Humor and the Retina

• After the light passes through the lens, it must pass through the vitreous humor, a gel-like substance made up of water and collagen. The vitreous humor has several functions. It gives the eye its shape and makes up most of its volume, too. The vitreous humor also keeps the retina in place and prevents it from flopping around. The eye needs all its structures to function, but the retina is probably the most important one. The retina is a light-sensitive tissue that lines the back of the eye. As light hits the retina, it sets off a chain of chemical and electrical impulses that travel along the optical nerve.

Retinal Photoreceptors

• The optical nerve is a network of nerve cells attached to the light-sensitive cells, or photoreceptors, in the retina. The impulses that these retina cells create travel along the optical nerve to the brain. Inside the brain, these impulses are interpreted into a picture. There are two types of photoreceptors. Cones are responsible for detailed central vision and are located in the center of the retina. Rods are located around the edges of the retina. They are responsible for night and side vision but do not recognize color. There are far fewer rods than cones. The combination of the two types of photoreceptors gives the brain a complete picture of what your eyes are seeing.