Of the five senses, sight is by far the most important. Our eyes tell us much more about the world around us than the rest of the senses, and the part of the brain that ”deals” with the sight is far greater than the parts for the other senses.
Eyes are a complex, intricate and very sensitive creation. Each eyeball is a spherical body of about 25 mm in diameter. The outer part of the occiput is composed of three concentric layers of tissue.
The solid most complete layer is the sclera (the white of the eye). Its exposed surface in the front part of the eye is covered with a transparent conjunctiva, which also covers the inner surface of the eyelids. In the front part of the eye – in the sclera and in the ocular joint – is a cornea, a transparent creation which is sometimes called the window of the eye.
Under the sclera is a choroid, a layer rich in blood vessels that supply the outer half of the retina with oxygen and nutrients. In the front part of the eye the choroid is thicker and forms the ciliary body. In front of the ciliary body is a rounded fabric made of muscular fibers, called iris, whose color varies from individual to individual. In the center of the iris there is an opening, a pupil, which looks like a black circle. Light enters the eye through the pupil. The amount of light depends on the contraction or dilation of the pupil, and this process is governed by iris muscles. Immediately behind the iris and the pupil there is a transparent, elastic lens that is associated with the ciliary body. Those ciliary muscles shape the lens and thereby sharpen the image of a subject at different distances. The space between the cornea and the lens is filled with a transparent watery substance – aqueous humour. Behind the lenses there is a glassy, transparent gelatinous substance that makes up most of the eye.
The third and deepest layer, the retina, covers the back three-quarters of the eyeball. The retina also covers the layer of light-sensitive nerve cells; those are so-called rods and cones. Light passes through the pupil and lens to the retina, forming a reversed image of the object we are watching. Rods are unusually sensitive to the intensity of light and allow us to see even in the low light; cons perceive color and tiny details. Rods and const (each eye has about 125 million rods and 7 million cons) transform the received sensations of color, shape and intensity of light into the nervous impulses. These impulses are transferred via nerve fibers of the retina to the visual nerve, a nerve bundle that connects the back of the eyeball with the brains. A place where the visual nerve emerges from the retina is called the visual nerve disk (or “blind spot”). The brain then interprets the impulses received from the eye.
Eye disorders are divided into four groups. The first group includes disturbances in the refraction (breaking) of the ray of light, e.g. short-sightedness and long-sightedness. The other group deals with disturbances of visible parts of the eye – mostly eyelids, eyelashes, sclera, iris and lenses. The third group refers to two forms of glaucoma, a disease that is caused by a defective flow of aqueous humour. The fourth and last group includes disorders affecting the back of the eye – primarily the retina and its blood supply system, as well as the muscles and other tissues surrounding the eyeball in its bone cavity called the occiput.
Medications and eyes
Medications used in the treatment of eye disorders can cause side effects that are harmful to the eye. Although the majority of these side effects are usually just unpleasant and rarely dangerous, some may be serious and, in some cases, cause cataracts, glaucoma, or even (although rarely) blindness. Therefore, such medicines may only be given by an expert.
Some drugs against general disorders or illness can also negatively affect the eye. Thus, for example, corticosteroid drugs can cause cataract, while the soothing substances (taken by the elderly) can enhance the occurrence of acute glaucoma. Such side effects do not occur in all cases, but you should take care of them if you are taking medicines for some other illness and if you start feeling unwell.
All the colors we see are a combination of three colors – red, green and blue – in the rays of light entering our eyes. Special cells in the retina, cons, contain a color-sensitive substance that reacts to one of those colors. However, some people experience a color vision disorder due to the partial or complete lack of color-sensitive substances in the cons.
Color blindness is a common, though defective term for a very common condition where some people can not differentiate between colors. ”True” color blindness, i.e. seeing all pictures in the shades of gray, is extremely rare.
The most common disorder is the inability to distinguish between red and green colors in dim light. In normal light the colors are seen normally; many people, colorblind, believe that all people see those colors in the same way at dim light, and are not aware of the defect unless they are tested.
The next most common disorder is the inability to distinguish between the same colors during the normal light. Anyone with this defect will be undoubtedly aware of the problem and will know how to deal with it in everyday life. Thus, for example, a person can distinguish the red and green lights on the semaphore by their position.
All these color blind problems are found in 8% of men and only 0.5% of women. They are almost always hereditary, and therefore present since birth (they are rarely a result of some illness) and are only transmitted by women. There is no cure for color blindness; however, this condition does not seriously affect everyday life.