The eye is a very interesting and complex organ within the human body. Its main function is to provide sight.
The outermost layer of the eye is called the sclera and it maintains the shape of the eye. The front clear part of this is called the cornea. Extraocular muscles are the muscles that move the eye and these are attached to the sclera. All light must pass through the cornea first and then moves onto the choroid which is the second layer of the eye. This part contains blood vessels that supply blood to the different parts of the eye and contains two structures called the iris and ciliary body. Within the iris there are two muscles the sphincter and dilator. The dilator muscle makes the iris smaller and pupil larger which allows more light into the eye. The sphincter muscle makes the iris larger and pupil smaller which allows less light into the eye. Pupil size can change from 2 millimeters to 8 millimeters on average and when the size of the pupil changes, it changes the amount of light that enters the eye 30 times. The innermost layer of the eye is called the retina and this is the light-sensing portion of the eye. Contained in this layer are rod cells, which are responsible for vision in low light, and cone cells that are responsible for color vision and detail. The macula is located in the back part of the eye in the center of the retina. In the center of the macula is the fovea centralis and this part contains only cones as it is used for seeing fine details. The retina also contains a chemical called rhodopsin, known as "visual purple." This chemical converts light into electrical impulses that the brain then measures as vision. Retinal nerve fibers are connected at the back of the eye, forming the optic nerve, which sends the electrical impulses to the brain. The optic disk is where the optic nerve and blood vessels exit the retina. This is a blind spot on the retina because there are no rods or cones. Yet we are unaware of this blind spot because each eye covers the blind spot of the other eye. This is some of the basic anatomy of the human eye. (Bianco, M.D. 2)
There are many infections, diseases, etc. that cause vision problems or even blindness. Glaucoma is when drainage of an aqueous humor which is produced in the ciliary body and then drained through the canal of schlemm is blocked. The conjunctiva, which keeps the eye moist, can have an infection called conjunctivitis also known as pink eye. The eye is protected from injuries by a bony cavity called the orbital cavity and eyelids do this by blinking. Eyelashes and eyebrows also protect the eye from particles that may harm it. Nearsightedness, also known as myopia, is when a person is able to see near objects well but has difficulty seeing objects that are farther away. Farsightedness, also known as hyperopia, is when a person is able to see distant objects well but has difficulty seeing objects that are closer. Astigmatism is an uneven curvature of the cornea that causes distortion in vision. The older we get the more the lens becomes less elastic and loses its ability to change shape. This is called presbyopia. It is more noticeable when we try to see things up close, because the ciliary body must contract to make the lens thicker and the loss of elasticity prevents the lens from becoming thicker. This causes us to lose our ability to focus on certain objects. A cataract is cloudiness in the lens that blocks light from reaching the retina and can be corrected with surgery. Trachoma is an infection caused by an organism called Chlamydia trachomatis and is disease is the most common cause of blindness is the world, however is rare in the
The eye relies on refraction in order to focus on various objects. Refraction takes place by the aqueous humor, which is a liquid on top of the lens. Light is refracted as it comes into the eye by the liquid, is then refracted more by the lens, and then more by the vitreous humor, which is a jelly-like substance located in the space between the lens and the retina. The lens is critical in creating a sharp image so it can adjust quickly when focusing objects at different distances and this process is known as accommodation. (Polarization 1)
The eye is a very interesting and vital organ in the human body. It works to help us focus in on images at various distances. By this we are able to observe the world around us and enjoy its beauty.
Works Cited
Bianco, M.D., Carl. ""How Vision Works"." How Stuff Works. 1998. 2 Mar 2008 http://health.howstuffworks.com/eye4.htm
unknown, unknown. "Polarization; and the Human Eye." University. 2 Mar 2008 http://physics.bu.edu/~duffy/PY106/Eye.html
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