"Nature and books belong to the eyes that see them," – Ralph Waldo Emerson
Helen Keller (born deaf & blind) had said: ‘Of all the senses, sight must be the most delightful’. Indeed, it would become impossible to perceive our surrounding and take pleasure in them without eyes. Our daily activities would be in jeopardy without vision. Can you imagine working, driving, eating and any of our daily chores without eyesight? Our eyes let us enjoy special moments of our life; your first day at high school, sweet memories with friends, the amorous look in the eyes of your love, the day when you first saw your child taking his first step. Every single moment is viewed by our eyes and stored in our memories for a lifetime. But how much do we know about this treasured gift? Let us get acquainted with our eyes.
Structure of our eyes
You will be stunned to know about the complex anatomy of our eyes in spite of its size being that of a ping pong ball. Each eye is a fluid-filled sphere enclosed by three layers of tissue. Only the innermost layer of the eye, the retina, contains neurons that are sensitive to light and are capable of transmitting visual signals to central targets. The immediately adjacent layer of tissue includes three distinct but continuous structures collectively referred to as the uveal tract.
The largest component of the uveal tract is the choroid and extending from it, near the front of the eye is the ciliary body, a ring of tissue that encircles the lens. It consists of a muscular component that is important for adjusting the refractive power of the lens, and a vascular component, known as ciliary process, that produces the fluid that fills the front of the eye. The most anterior component of the uveal tract is the iris, the coloured portion of the eye with an opening at the centre called pupil. The sclera forms the outermost fibrous tissue layer of the eye, which is transformed in front of the eye into the cornea, a specialized transparent tissue that permits light rays to enter the eye.
Beyond the cornea, light rays pass through two distinct fluid environments before striking the retina. In the anterior chamber, just behind the cornea and in front of the lens, lies aqueous humor, a clear, watery liquid that supplies nutrients to both of these structures. The space between the back of the lens and the surface of the retina is filled with a thick, gelatinous substance called the vitreous humor, which accounts for about 80% of the volume of the eye.
How the eye works
If you think that only a glance is enough to perceive the information about our world, you are wrong! A complex process is involved in visual perception. Our eyes are just like a camera. The rays of light from an object fall on the cornea, which curve them through the pupil into the iris. The lens, similar to that of a camera, focuses the light on the ‘camera film’ of our eye, i.e. retina. This light is then transformed into impulses by the optic nerve and send to brain for further comprehension. For a clear view, our eyes have to undergo several adjustments. Failure of which would lead to serious visual defects.
The eye moves itself until the image, whether static or moving, is accurately focused on the retina. It also carries out similar movement to adapt itself to changing light intensities, from the faint light of stars at night to bright sunlight. Six muscles, namely, the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique muscles are responsible for such eye movements. Sometimes we are fascinated by the capability of our eyes to perceive objects with equal precision, no matter how far or near they are present from us. The credit goes to the lens and suspensory ligaments. The latter contract or relax to control the curvature of the lens. The latter attains a more convex shape to accommodate rays of light coming from closer objects and relaxes while focusing light from distant ones.
Without eyes, the colourful world would be turned worthless. It is our ability to distinguish between colours which make our surrounding all the more appealing to us! The different wavelengths of light emitting from various colours are detected by photoreceptor cells of retina, namely, rod and cone cells. Rod cells become active in dim light while the cone cells are mainly responsible for our colour vision and send the stimuli to the brain for ultimate perception.
Disorders of vision
Optical discrepancies among the various components of the eye cause a majority of the human population to have some form of refractive error, called ametropia. People who are unable to bring distant objects into clear focus are said to be nearsighted, or myopic. Myopia can be caused by the corneal surface being too curved, or by the eyeball being too long. In either case, with the lens as flat as it can be, the image of distant objects focuses in front of, rather than on, the retina.
People who are unable to focus on near objects are said to be farsighted, or hyperopic. Hyperopia can be caused by the eyeball being too short or the refracting system too weak. Even with the lens in its most rounded-up state, the image is out of focus on the retinal surface. Both myopia and hyperopia are correctable by appropriate lenses—concave (minus) and convex (plus), respectively—or by the increasingly popular technique of corneal surgery. Sometimes, the distorted shape of cornea or lens makes the focused light smeared on the retina, hence giving rise to another refractive disorder called astigmatism.
As one approaches the age of 40, the lens loses its flexibility and hence leads to anomalous focusing of light on the retina, a condition called presbyopia. Apart from these refractive disorders, our eyes have to go through several maladies frequently. One of the most menacing diseases is the inflammation of the covering membrane of eyeball or conjunctiva, called conjunctivitis, due to viral and bacterial infections. The lens might become cloudy and opaque leading to condition called cataracts which result in blurry vision and even blindness. Age-related macular degeneration (AMD) causes a progressive loss of central vision with age and severely limits the ability to perform visual tasks.
Our eyes are precious to us yet often we fail to pay attention to it, without even giving a single thought about severity of the outcome – blindness. Relieve your eyes from stress during long hours of work. Splash them with water or weak tea solution to ease them. You can also place a pad of cotton, soaked in rose water on the eye lids for about 15 minutes to get rid of any eye irritations. If none of them is handy, just put a slice of cucumber on each eye and relax. You can replace the cucumber with raw potato to alleviate puffy eyes.
Our eyes are the windows of our mind and heart; so take care of them and keep them glowing forever. Liya Das (The Caribbean Current)