Eyes
( Structure )

 

Introduction
Overview of Structure
Cornea and sclera
Iris
Lens
Retina
Visual purple
 

Introduction

The eye is shaped like a ball, with a slight bulge at the front. It is this bulge that a person sees when looking at the eyes of someone else. When the eyelids are closed, the bulge is covered. The rest of the eye is protected by the bones of the skull. Each part of the human eye has a special function.                       

                                       Click Photo to see an Enlarged Image


Overview of Structure

1.  CORNEA
2.  EPITHELIUM
3.  IRIS
4.  PUPIL
5.  RETINA
6.  SCLERA
7.  LENS


CORNEA
A curved band of strong, clear tissue on the surface of the eye, the cornea focuses light onto the retina.
   EPITHELIUM    
 The epithelium provides a thin protective layer for the cornea and heals very quickly when disturbed.
IRIS
The coloured part of the eye is actually a muscle that controls the size of the pupil.
PUPIL
The black circular area in the middle of the eye controls the amount of light reaching the retina.
RETINA
A membrane on the inner wall of the eye, similar to the film in a camera, the retina changes light into images that are transferred to the brain via the optic nerve.
SCLERA
The outer white coat of the eye, the sclera also provides protection.
LENS
A natural lens behind the pupil changes shape to allow the eye to focus. As a result of aging, the natural lens hardens, resulting in presbyopia, the loss of reading vision.

 

Cornea and sclera

The eye is made of three coats, or tunics. The outermost coat consists of the cornea and the sclera; the middle coat contains the main blood supply to the eye and consists of the choroid, the ciliary body, and the iris. The innermost layer is the retina. The sclera, or the white of the eye, is composed of tough fibrous tissue. On the exposed area of the eye the scleral surface is covered with a mucous membrane called the conjunctiva. This protects the eye from becoming dry. The cornea, a part of the sclera, is the transparent window of the eye through which light passes. The focusing of light begins in the cornea. Behind the cornea is a watery fluid called the aqueous humor. This fluid fills a curved, crescent-shaped space, thick in the center and thinner toward the edges. The cornea and the aqueous humor together make an outer lens that refracts, or bends, light and directs it toward the center of the eye.

Iris

Behind the aqueous humor is a colored ring called the iris. The color of the iris is inherited and does not affect vision. The iris is like a muscular curtain that opens and closes. It controls the amount of light entering the eye through the pupil, an opening in the iris. The pupil looks like a black spot. Light from everything a person sees must go through the pupil. When more or less light is needed to see better, the pupil becomes larger or smaller through the movement of the muscle in the iris. The aqueous humor flows through the pupil into a small space between the iris and the lens. A simple way to see how the pupils respond to light is to stand in front of a mirror with the eyes closed, covered by the hands for about ten seconds. When the hands are removed and the eyes opened, the pupils begin to get smaller, or contract, in response to the light. When light is reduced, pupils expand; when it is increased, they contract. The choroid is a layer of blood vessels and connective tissue squeezed between the sclera and the retina. It supplies nutrients to the eye. The ciliary body is a muscular structure that changes the shape of the lens.

Lens

Behind the pupil and iris are the crystalline lens and the ciliary muscle. The muscle holds the lens in place and changes its shape. The lens is a colorless, nearly transparent double convex structure, similar to an ordinary magnifying glass. Its only function is to focus light rays onto the retina. The lens is made of elongated cells that have no blood supply. These cells obtain nutrients from the surrounding fluids the aqueous humor in front and the vitreous body, a clear jelly, behind. The shape of the lens essentially that of a flattened globe can be changed by the movement of the ciliary muscles surrounding it. Hence, the eye can focus clearly on objects at widely varying distances. The ability of the lens to adjust from a distant to a near focus is called accommodation. By contracting, the ciliary muscle pushes the lens to make it thicker in the middle. By relaxing, the muscle pulls the lens and flattens it. To see objects clearly when they are close to the eyes the lens is squeezed together and thickened. To see distant objects clearly it is flattened. For people with normal vision, the relaxed ciliary muscle flattens the lens enough to bring objects into sharp focus if they are 20 feet (6 meters) or more from the eye. To see closer objects clearly, the ciliary muscle must contract in order to thicken the lens. Young children can see objects clearly at distances as close as 2 inches (6.4 centimeters). After about age 45 most people must have objects farther and farther away in order to see them clearly. The lens becomes less elastic as a person grows older.

Retina

The retina is a soft, transparent layer of nervous tissue made up of millions of light receptors. The retina is connected to the brain by the optic nerve. All of the structures needed to focus light onto the retina and to nourish it are housed in the eye, which is primarily a supporting shell for the retina. When light enters the eye it passes through the lens and focuses an image onto the retina. The retina has several layers, one of which contains special cells named for their shapes rods and cones. Light-sensitive chemicals in the rods and cones react to specific wavelengths of light and trigger nerve impulses. These impulses are carried through the optic nerve to the visual center in the brain. Here they are interpreted, and sight occurs. Light must pass through the covering layers of the retina to reach the layer of rods and cones. There are about 75 to 150 million rods and about 7 million cones in the human retina. Rods do not detect lines, points, or color. They perceive only light and dark tones in an image. The sensitive rods can distinguish outlines or silhouettes of objects in almost complete darkness. They make it possible for people to see in darkness or at night. Cones are the keenest of the retina's receptor cells. They detect the fine lines and points of an image. The cones, for example, make it possible to read these words. There are three types of cones that receive color sensations. One type absorbs light best in wavelengths of blue-violet and another in wavelengths of green; a third is sensitive to wavelengths of yellow and red.



Light Rays [1] falling on the eye pass through optic nerve fibres [2], ganglia [3] and bipolar nerve cells [4] before reaching the rods [5] and cones [6], the eye's photo-receptor cells found in front of the pigment cell layer [7]. Cones [orange], concerned with colour vision, are concentrated in the middle, forming the fovea [8]. Rods [green], effective in black and white, are mostly on the periphery. The atio of rods to cones is 18:1.


Visual purple

Rods detect images in the dark because the cells contain a rose-red pigment called visual purple, or rhodopsin. When exposed to bright light, visual purple undergoes a chemical change in which it loses its color. This causes the rods to lose their sensitivity to light, thus enabling the eye to endure glaring light. Before the eye can see in the dark, visual purple must be re-formed in the retina. As more visual purple is produced, the eye's sensitivity to light increases. Thus when a person enters a darkened motion-picture theater his eyes do not contain much visual purple. As the visual purple is re-formed, the person can see better. In a short time his eyes' sensitivity to light is multiplied about 2,000 times. Visual purple can be produced only if the body has a sufficient quantity of vitamin A (see Vitamins). Lack of vitamin A in the diet may lead to night blindness, or nyctalopia.

Vitamin A, also called retinol, is a fat-soluble vitamin that is readily destroyed upon exposure to heat, light, or air. The vitamin has a direct role in vision and is a component of a pigment present in the retina of the eye. It is essential for the proper functioning of most body organs and also affects the functioning of the immune system. Vitamin A deficiency results in various disorders that most commonly involve the eye and the epithelial tissues the skin and the mucous membranes lining the internal body surfaces. An early symptom of vitamin A deficiency is the development of night blindness, and continued deficiency eventually results in loss of sight. If deficiency is prolonged, the skin may become dry and rough. Vitamin A deficiency may also result in defective bone and teeth formation. Excessive intake of vitamin A causes a toxic condition. The symptoms may include nausea, coarsening and loss of hair, drying and scaling of the skin, bone pain, fatigue, and drowsiness. There may also be blurred vision and headache in adults, and growth failure, enlargement of the liver, and nervous irritability in children.


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Compiled by D. M Brewer - November 1997