Vision helps us detect desired targets, threats and changes in our physical environment and to adapt accordingly. So, how does the visual system accomplish this? Read this article to find more about vision disorders and vision therapy, eye & vision care!
Structure of the Eye:
Our eye contains eyelids, eyelashes, eyebrows and lachrymal glands. A thin layer covers the front portion of the eye. This thin layer is called as Conjunctiva. The eye ball is located in the eye socket and only 1/6 portion of the eye is visible to us.
Eye includes 3 main layers which are sclerotic layer or sclera, retina, and choroid layer. The outer most tough, fibrous, non-elastic, thick, and white colored layer is sclera. This sclera bulges out and form cornea and the end of sclera connect to the optic nerve. The second layer is the choroid layer. This layer is black in color and contains a lot of vessels. It encloses the eye except the part pupil. The part formed by the choroid layer around the pupil is iris. Radial and circular muscles are present in iris. Biconvex Lens is present immediately behind the pupil is attached to the ciliary muscles and suspensory ligaments.
The lens divides the inner eyeball as aqueous chamber and vitreous chamber. Vitreous chamber is filled with jelly-like fluid whereas the aqueous chamber is filled with water like fluid. The retina contains the cells called rods and cones. The area of no vision called blind spot and the area of best vision, called yellow spot are present in the retina. The yellow spot is called Macula or Fovea.
Functioning of the Eye: The Visual Sensation
You might think of the eye as a sort of “video camera” that the brain uses to make motion pictures of the world. Like a camera, the eye gathers light through a convex lens, focuses it, and forms an image in the retina at the back of the eye. The Lens turns the image left to right and upside down. This visual reversal may have influenced the very structure of the brain, which tends to maintain this reversal in its sensory processing regions.
Thus, most information from the sense organ crosses over to the opposite side of the brain. Similarly, “maps” of the body are typically reversed and inverted in the brain’s sensory areas. But while a digital camera simply forms an electronic image, the eye forms an image that gets the extensive further processing in the brain.
The unique characteristics of the eye that makes it different from other sense organs, lies in its ability to make the information from light waves then transform the characteristics of light into neural signals that the brain can process. This happens in the retina, the light-sensitive layer of cells at the back of the eye that acts much like the light-sensitive chip in a digital camera.
As with a camera, things can go wrong. For example, the lenses of those who are “nearsighted” focus images short of the retina; in those who are “farsighted,” the focal point extends behind the retina. However, images are not sharp without corrective lenses.
The real work in the retina is performed by light-sensitive cells known as photoreceptors. These photoreceptors consist of 2 different types of specialized cells the rods and cones that absorb light energy and respond by creating nerve impulses.
But why there are 2 sorts of photoreceptors? Our eyes function sometimes in bright light and sometimes in near darkness. These 2 types of processors involving distinct receptor cell types named for their shapes have evolved for this purpose.
Nearly 125 million tiny rods containing the pigment rhodopsin “see in the dark” that is, they detect low intensities of light at night, though they cannot make the fine distinctions that give rise to our sensations of color.
Cells and tissues in the Eye
Making the fine distinctions necessary for color vision is the job of the nearly seven million cones containing the pigment iodopsin that come into play in brighter light. Each cone is specialized to detect the light waves we sense either red, blue or yellow and the array of colors formed by their combinations. Thus the yellow field, the bright red morning sun, the blue sky and all other colors in nature are sensed.
The cones concentrate most in the very center of the retina, in a small region called the fovea, which gives us our sharpest vision. With movements of our eyeballs, we use the fovea to scan whatever interests us visually, the features of the face or perhaps, a flower.
There are other types of cells in the retina that do not directly respond to light. These handle the job of collecting impulses from many photoreceptors (rods and cones) and shuttling them on to the nerve cells. Presence of some other receptor cells sensitive to edges and boundaries of objects and those to light and shadow and motion in the retina also have been reported recently.
Bundled together, the nerve cells make up the optic nerve, which transports visual information from the eye to the brain. However, it is important to understand that the optic nerve carries no light. Only patterns of nerve impulses conveying information derived from the incoming light are carried. Each of the eyes collects slightly different view of an object. The brain puts the two views together and a three-dimensional picture is formed.
There is a small area of the retina in each eye which has no photoreceptors and where everyone is blind. This blind spot is located at the point where the optic nerve exists each eye, and the result is a gap in the visual field. You do not experience blindness there because what one eye misses is registered by another eye, and the brain “fills in” the spot with information that matches the background.
Eye Protection:
Each eye is protected by eyelids, lachrymal or tear glands, eyelashes, and eyebrows. A thin membrane called conjunctiva covers the front part of the eye. The conjunctiva is made up of the transparent epithelium. It is also a protective cover to the eye.
Whenever an unwanted substance comes in contact with this thin layer the lachrymal glands are stimulated to wash with substance out of the eye. The fluids that are filled in the eyeball (vitreous and aqueous chambers) protect the lens and other parts of an eye from the mechanical shocks. The cornea is the clean window in the sclera in front of the iris. It protects the eye from the direct exposure to light.
Do you know?
While issuing identity cards like AADHAR, they take photographs of the eyes. Do you know why did they take the photo of your eye? Iris patterns are specific to every individual and can be used for identification just as our fingerprints.
Eye: Some Structures that bring about adjustments
The iris is a muscular structure which adjusts the size of the pupil which is nothing but a gap between the Iris in front of the lens. Adjustments are made depending on the light intensity. Ciliary muscles and suspensor ligaments are capable of adjusting the focal length of the eye lens.
The lenses in our eyes are very special. They are biconvex and crystalline in nature. Their shape is adjustable to some extent that is their focal length can be changed with the help of cilliary muscles and suspensory ligaments. They can change the shape of the lens from a moderate to more convex form.
Diseases and defects of the eye
When your mind deceives you by interpreting a stimulus pattern incorrectly, you are experiencing an illusion. Such illusions can help us understand some fundamental properties of sensation particularly the discrepancy between what we see and external reality. The main diseases and defects of the eye are
- Night blindness
- Xerophthalmia
- Myopia (nearsightedness)
- Hypermetropia (farsightedness)
- Glaucoma
- Cataract
- Colour blindness
Tips to take of our eyes
- Wash eyes with fresh water at least thrice and four times per day
- Keep the distance between the book and eyes about 25cm while reading
- Don’t strain and stress your eyes continuously
- Include food materials like green leafy vegetables, carrots etc rich in vitamin A in your diet
- Work under good lighting
- Don’t rub your eyes if anything falls in them, just wash the eyes immediately
- Avoid using tongue, ring, blowing air etc to remove dust in the eye
- Consult the eye specialist immediately for any vision-related problems
- Avoid seeing lightning, gas welding sparks and eclipse etc
