THE HUMAN EYE
Author’s Name
Mr. Steeves 1?2 Biology
Date
Introduction
The eye is considered one of the most important of the five senses that humans possess. The eye allows one to see and to distinguish between different objects, colors and different intensities of light. Like a camera, a healthy eye records images, but to a higher quality compared to that of a camera (Adams et al, 1987). The eye collects data and information, and sends that out to the brain to make sense of what is being observed. The eye can see things as far away as a star and quickly adjusts its focus to view a grain of sand only inches away (Goldberg, 2012). Unlike a camera, the eye has the ability to focus instantly from a distant object to a near object. Considering the size of the eye no camera can match the eye’s proficiency concerning color range, depth of field, wide ranges of light levels and focus speed, (North, 2010). The eye is wonderfully designed to undertake all of its tasks with considerable ease.
Parts and Functions
The conjunctiva lines the insides of the eyelids and is the outermost covering of the frontal part of the eye (Adams et al, 1987). The conjunctiva is a transparent membrane that covers the front part of the eye except for the cornea. The conjunctiva is nourished by tiny blood vessels and secretes oils and
mucous to moisten and lubricate the eye (St. Lukes Cataract & Laser Institute, 2010). The cornea is the transparent, front part of the eye that covers the iris and pupil, and allows light to enter the eye (Figure 1). It provides two thirds of the eye’s focusing power and is very sensitive. More than anywhere else in the body, the cornea has the most nerve endings. The cornea is about ½ millimeter thick and comprises of five layers: the epithelium, Bowman's membrane, stroma, Descemet's membrane and the endothelium.
Underneath the cornea and in front of the iris is a watery fluid called the aqueous humor and provides the necessary nutrients for the cornea and the lens (Adams et la, 1987). It is secreted by the ciliary epithelium which supports the lens (Zeuthen & Stein, 2002). The iris is the colored part of the eye which gets its color from a pigment called melanin that protects against damage from the sun (MedicineNet, 2012). The iris regulates the light levels in the eye (St.Lukes Cataract & Laser Institute, 2010).
In the center of the iris is the pupil. The iris has tiny muscles that widen and contract the pupil when exposed to different intensities of light. Lying around the edge of the pupil are the sphincter muscles, which decrease the size of the pupil when the eye is exposed to bright light. By shrinking the pupil, it prevents too much light from entering the eye (Goldberg, 2012). The pupil also shrinks in order to focus on a nearby object. The dilator muscles that run across the iris widen the eye in dim light which allows more light inside the eye (Miller & Levine).
Under the iris is a transparent, biconvex structure called the lens (Ananth, 2010). Its main function is to refract light on to the retina. The lens is flexible, and can change its form instantly. When focusing on a near object, the lens thickens. The lens becomes less flexible at about the age of forty, and therefore makes viewing near objects more difficult (St. Lukes Cataract & Laser Institute, 2010). There is a thin, transparent membrane that covers the lens called the lens capsule, and helps to change the shape of the lens when it is focusing on an object.
Also, behind the iris and beside the lens is the ciliary body (Figure 1). Fibers called zonules are attached to the ciliary body, and they act as guy wires as they suspend the lens inside the eye (Lukes Cataract & Laser Institute, 2010). The ciliary body composes of the ciliary muscles and the ciliary epithelium. The ciliary muscles are smooth muscle fibers that control and modify the lens’s shape.
Eighty percent of the eye is made of a jelly-like substance called the vitreous humor. It fills the eye cavity, and helps the eye keep its shape (Montgomery, 1998). Ninety nine percent of the vitreous humor is made up of water. The rest is made of collagen fibers, proteins, salts and sugars (Smith, 2012). If objects get into the vitreous humor it will not drift out on its own. One way to remove impurities is through a vitrectomy.
The retina is the innermost layer of the eye (Miller & Levine, 2008). It lines the back of the eye and is composed of multiple layers (St.Lukes Cataract & Laser Institute, 2010). It is a sensory tissue composed of millions of photoreceptors. These photoreceptors seize light rays and convert them into electrical impulses that are then sent along the optic nerve to the brain. The brain then takes these impulses and turns them into images. There are two different kinds of photoreceptors; cones and rods (Figure 2).
Another part of the retina is the macula, which is positioned approximately in the center of the retina (St. Lukes Cataract & Laser Institute, 2010). The macula is responsible for the eye’s central vision and color vision. Central vision is used for viewing details and for things like reading. “Cones” are a type of photoreceptor that are most densely found in the fovea; the most center part of the macula. They work best in bright light, and give the ability to see color. Photoreceptors called “rods” are found spread throughout the peripheral retina. The rods function best in dim light and are responsible for peripheral and night vision.
The optic nerve is located in the back of the eye, and is responsible for transmitting the electrical impulses from the photoreceptors to the brain (Albert & Gamm, 2007). The eye has a blind spot, also known as the optic disk; it is located in front of the optic nerve (Adams et al, 1987). It is called the blind spot, because there are no cones or rods there. One common theory about how people don’t notice this blind spot suggests that the brain fills in the blind spot with clues from the surrounding environment (Cherry, 2012).
The choroid is the middle layer of the eyeball’s wall and it rests just underneath the retina (Guinness, E.A. 1987). It is a dark, pigmented layer that contains many blood vessels that supply nutrients to the inner part of the eye. The choroid’s function is to absorb light and prevents it from reflecting within the eyeball. Finally, the sclera is the outermost layer of the eyeball and it maintains the eye’s shape (Miller & Levine, 2008). It also serves as the attachment point for the muscles that move the eye.
Diseases and Disorders
Glaucoma
Glaucoma is the leading cause of irreversible blindness (MedicineNet, 2012). About six million people worldwide are blind because of it. Glaucoma is usually caused by elevated fluid pressure in the eye that damages the delicate fibers of the optic nerve. Glaucoma mostly starts with the loss of peripheral vision, and if left undiagnosed can lead to the loss of central vision. The symptoms are hard to detect and until they are, glaucoma is already in the advanced stages.
Astigmatism
There are two types of astigmatism; corneal astigmatism and lenticular astigmatism (Nordqvist, 2009). Corneal astigmatism is a condition in which the cornea is oblong. This causes light to be bent improperly onto the retina, and it creates a distorted image. Lenticular astigmatism is similar to corneal astigmatism; however, it is the lens that is irregularly shaped. High blood sugar can cause the lens to change shape, meaning that diabetic people commonly develop lenticular astigmatism.
Macular Degeneration
About 1.75 million people in the United States alone are affected by macular degeneration (Haddrill, 2012), and it is the leading cause of vision loss in Canada (Zafar, 2012). There are two types of macular degeneration; wet and dry. Dry macular degeneration is when yellowish deposits called drusen form on the retina underneath the macula (Mogk, 2012). This causes the macula to deteriorate over time. Dry macular degeneration affects about ninety nine percent of individuals with macular degeneration. Wet macular degeneration is when blood vessels from the choroid begin to grow underneath the macula. When these unusual blood vessels break or leak, it damages the macula and causes it to lift and pull away. However, both kinds of macular degeneration do not cause any pain which makes it hard to detect, but there are some symptoms such as straight lines appearing to be wavy are early signs of macular degeneration (Connolly, 2012).
Cataracts
Cataracts are a painless condition where the lens begins to cloud over causing vision quality to decrease and even blindness (Canada.com, 2010). Some symptoms include; blurred vision, poor night vision, faded color, glares and halos. Cataracts are very common occurring in people usually over the age of fifty. In most cases, cataracts occurs in both eyes.
Treatments
There are medications for glaucoma that reduce eye pressure and can prevent further eye damage (CHealth, 1996). However, laser surgery is the best way to get rid of glaucoma. Laser trabeculoplasty is the most common treatment for glaucoma. It’s a painless, fifteen minute, outpatient procedure.
Eye glasses or contact lenses can be worn to correct astigmatism. People with minor cases of astigmatism may even go without corrective lens (Keenan, 2009). Bladeless Lasik surgery can also correct astigmatism by changing the shape of the cornea. However, lenticular astigmatism is presently impossible to correct without taking out the lens and replacing it with a plastic or silicon lens (Hagele, 2012).
Treatment for dry macular degeneration is very little (Canada.com, 2010). Doctors recommend good nutrition to help slow down the disease. Wet macular degeneration can be treated with laser surgery. Laser surgery destroys the tiny blood vessels that leak onto macula. Light activated medications can also be injected into a vein, and this causes the abnormal blood vessels to close.
Cataracts can only be removed with surgery (Simon, 2009). Patients with cataracts can improve their vision by ninety five percent with surgery. In most surgeries, the clouded lens is removed and replaced by either an acrylic or silicone lens.
Conclusion
The human eye is a fascinating organ that should not be taken for granted. In the normal 90-90 degree angle position, the human eye can see about 576 megapixels (Howmanyarethere.net, 2012) compared to the world’s highest resolution Nikon camera which has 36.3 megapixels (Pachal, 2012). Sight is a wonderful gift, and much should be done to ensure the proper care and protection of our eyes. Technology has come a long way and many eye disorders can be treated. With further research, more conditions can be reversed. There are still many diseases that have no cures or treatments. With a healthy diet and lifestyle, people can prevent / improve eye health throughout their lifetime.
Literature Cited
Goldberg, M. (2012) Eye. In World Book Online Advanced. Retrieved November 21, 2012 from: http://www.worldbookonline.com/advanced/article?=id=ar189340&st=human+eye
North, T. (2010, Sept. 2nd). The camera: Not as good as the human eye. Retrieved Nov. 2012, from Shutter Photo:
http://www.shutterphoto.net/article/the-cameras-not-as-good-as-the-human-eye/
Hien & Jason. (2000). A perception of vision. Retrieved Nov. 20, 2012, from Oracle Thinkquest: http://library.thinkquest.org/C005949/anatomy/vitreous.htm
St. Lukes Cataract & Laser Institute. (2010). Conjunctiva. Retrieved Nov. 21, 2012, from
http://www.stlukeseye.com/anatomy/conjunctiva.html
St. Lukes Cataract & Laser Institute. (2010). Cornea. Retrieved Nov. 21, 2012, from
http://www.stlukeseye.com/anatomy/cornea.html
St. Lukes Cataract & Laser Institute. (2010). Iris. Retrieved Nov. 21, 2012, from
http://www.stlukeseye.com/anatomy/iris.html
St. Lukes Cataract & Laser Institute. (2010). Ciliary. Retrieved Nov. 21, 2012, from
http://www.stlukeseye.com/anatomy/ciliary.html
St. Lukes Cataract & Laser Institute. (2010). Accommodation. Retrieved Nov. 21, 2012, from http://www.stlukeseye.com/eyeq/Accomodation.html
Finnegan, J. (2003). What Is descemet's membrane? Retrieved Nov. 21 2012, from Wise Geek: http://www.wisegeek.com/what-is-descemets-membrane.htm
MedicineNet. Definition of melanin. Retrieved Nov. 21, 2012, from
http://www.medterms.com/script/main/art.asp?articlekey=4340
Parwani, N. & Rogers, K. (2007) Lens. In Encyclopedia Britannica. Retrieved Nov. 21, 2012, from http://www.britannica.com/EBchecked/topic/336040/lens
Zeuthen, T. & Stein, W. (2002). International Review of Cytology: A survey of Cell Biology. San Diego, California: Academic Press.
http://books.google.ca/books?id=rYho3d9dWfUC&pg=PA422&dq=eye+ciliary+epithelium&hl=en&sa=X&ei=ZgCsUN66N7K00AH4zICgBA&ved=0CDMQ6AEwAQ#v=onepage&q=eye%20ciliary%20epithelium&f=false
St. Lukes Cataract & Laser Institute. (2010). Retina. Retrieved Nov. 24, 2012 from
http://www.stlukeseye.com/anatomy/retina.html
Albert, D. & Gamm, D. (2007) Optic nerve. In Encyclopedia Britannica. Retrieved Nov. 24, 2012, from http://www.britannica.com/EBchecked/topic/430325/optic-nerve
Cherry, K. (2012). What is a blind spot. Retrieved Nov. 26, 2012 from About.com:
http://psychology.about.com/od/bindex/g/blind-spot.htm
MedicineNet. Glaucoma. Retrieved Nov.26, 2012 from
http://www.medicinenet.com/glaucoma/article.htm#what_is_glaucoma
Guinness, A. E. et al. (1988). ABC’s of the body: The workings of the eye. United States of America: Readers Digest Association, Inc.
Nordqvist, C. (2009). What is astigmatism? What causes astigmatism? Retrieved Nov. 29, 2012 from Medical News Today: http://www.medicalnewstoday.com/articles/158810.php
Haddrill, M. (2012) Age related macular degeneration. Retrieved Dec. 8, 2012, from All About Vision: http://www.allaboutvision.com/conditions/amd.htm
Zafar, A. (2012). Macular degeneration study fuels drug therapy debate. Retrieved Nov. 26, 2012, from CBCnews | Health
http://www.cbc.ca/news/health/story/2012/07/04/macular-degeneration-lucentis- avastin-study.html
Miller, K & Levine, J. (2008). Prentice Hall Biology: The senses. Boston, Massachusetts: Pearson Education, Inc.
Mogk, G. L. (2012). The difference between wet and dry macular degeneration. Retrieved Nov. 5, 2012, from Vision Aware: http://www.visionaware.org/section.aspx?FolderID=6&SectionID=134&DocumentID=5972
Canada.com (2010). Cataracts. Retrieved Dec. 5, 2012, from http://bodyandhealth.canada.com/channel_condition_info_details.asp?channel_id=2022&disease_id=28&relation_id=16665