Human Eye

Larissa Loss
Winter Break Blog
Mr. Wirth
March 2, 2008


Anatomy:
The eye is a very complex organ in the human body. It is made out of various parts to function properly. In order for the eye to keep its round shape there is a tough, outer layer called the sclera. The front sixth of this layer is called the cornea. The cornea is where all light must first pass through before it enters the eye. The extraocular muscles control the movement of the eye. The muscular area that is attached the lens is called the ciliary body. It contracts and relaxes to control the size of the lens. The part of the eye which is colored is called the iris. The iris has two muscles, the dilator and the sphincter. The dilator makes the iris smaller, and therefore allowing the pupils to become larger and allowing more light into the eye. The sphincter makes the iris larger and the pupil smaller, therefore allowing less light into the eye.
The retina is the inmost part of the eye and controls the light-sensing portion of the eye. In the retina it has rod and cone cells. The rods are used for vision in dimmer light, while the cone cells control the color vision and detail. In the center of the macula which is located in the center of the retina, is the fovea centralis. The fovea centralis only has cones and is accountable for seeing detail very clearly. The rhodopsin which is located in the retina is responsible for changing light into electrical impulses that send messages to the brain to interpret the vision. The optic nerve which is located in the back of the eye consists of nerve fibers and which conducts the electrical impulses to the brain. The blind spot in the eye is called the optic disk, because there are no cone or rod cells located in that area of the brain.

There are six muscles attached to the sclera to control the movement of the eye. The six muscles consist of the medial rectus, lateral rectus, superior rectus, inferior rectus, superior oblique, and inferior oblique. The medial rectus controls the eyes moving towards the nose. The lateral rectus controls the eye moving away from the nose. The superior rectus raises the eye, the inferior rectus lowers the eye, and the superior and inferior oblique rotates the eye.

Vision: light and color:
The color vision in the eye consists of retinal and photopsins. There are three different kinds of color-sensitive pigments, red-sensitive pigment, green-sensitive pigment, and blue-sensitive pigment. The human eye can sense most colors when red green and blue are mixed together.
As light enters the eye, it eventually reaches the retina. In the retina contains rods and cones. Rods are responsible for vision in dim light, and cones are responsible for the color and detail. When light hits both cells a complex chemical reaction occurs. The chemical that is formed creates electrical impulses to the optic nerve. On the outer layers of the rods there is a chemical called rhodopsin. On the cones the chemicals are called color pigments. The retina contains millions of rods and cones and is pigmented black called melanin which lessens the amount of reflection and is responsible for sharp and detailed vision. The macula is the central part of the retina and has the highest concentration of only cones which provide the most clarity to our vision. Rhodopsin is a mixture of proteins and is unstable when exposed to light. This physical change causes rhodopsin to break down into several immediate compounds forming metarhodopsin II (activated rhodopsin). “This chemical causes electrical impulses that are transmitted to the brain and interpreted as light.”(Bianco). This is the primary visual cortex where visual interpretation occurs in the occipital lobe of the brain.

Normal Vision and Vision Problems:

Normal vision in the human eye is 20/20 vision. This is determined by reading a Snellen eye chart standing 20 feet away. However, not all people have perfect vision. Nearsightedness is when you can see things up close, but not far away. People who can see far away objects, but not close objects are farsighted. People with imperfect vision can be helped by wearing contacts or glasses.
The eye is an important and complex part of our body. We have natural defenses to protect it from harm. Our eyebrows prevent sweat from running into them and our eyelashes collect dust and dirt from floating in, as does our eyelid protect from foreign bodies. Tears are important for cleaning and constantly bathing our eye with moisture. We take our eyes for granted and forget how imperative they are for daily function.


Works Cited
Bianco Md., Dr. Carl. "How Vision Works." How Stuff Works. 1998. 2 Mar. 2008 .
"Your Sense of Sight." Sight. Think Quest Junior. 2 Mar. 2008 .