THE HUMAN EYE
The Human Eye
The Human Eye is a specialized sense organ of sight. It is a window to the outside world and it makes us "SEE".
Eye is a very unique and delicate organ. It is made up of very specialized and delicate tissues. We have two eyes which produce an image of the objects on the Retina. Each eye produces a separate image. These two images are so similar that they are fused together in the brain and this produces the sensation of a single image which is a three dimensional image. This is the function of "Binocular Vision".
A full term child's eyes at 3 months are very well developed as compared to other organs, but the tissues are immature and the size of the eyes is small. In terms of Refraction, a child's eyes are small so that they require Plus Glasses to the tune of +30.00 Dioptre. This is normal. The eyes develop and their development is completed upto the age of 5 years. If any obstruction to its development occurs during this time, such as a Congenital Cataract, or a Squint, then there is a suppression of vision and the eyes search for vision by expressing fine side-to-side movements of the eyes. This is called as "Nystagmus" and it means subnormal vision of the eyes.
Refractive Error - means a person requires a spectacular correction for seeing clearly. Here the rays of light are not brought to a focus on the Retina. They come to a focus either in front of or behind the retina, or there may be multiple foci of the image. Such Errors of Refraction require correction by glasses which a person has to wear infront of the eyes. The Refractive Errors may be Myopia where the person needs Minus glasses or Hypermetropia where the person requires Plus glasses, or Astigmatism, where the person requires Cylindrical glasses to correct the Refractive Error.
A person with refractive error can wear spectacles or contact lenses. Contact Lenses are worn in front of and on the cornea. We also get coloured cosmetic contact lenses which are used by some persons to change the colour of the eyes to blue, green or brown.
High degrees of Myopia i.e. between -3.0 Dioptre and -8.0 Dioptre can be corrected by Refractive surgery if the spects number is stable and not changing for the last 2 years and the age of the person is above 20 years.
After the age of 40 years, a person requires Plus glasses to see near objects clearly. This happens because the Power of Accomodation of the Human Crystalline Lens is gradually reduced so that the person is not able to increase the converging power of the eye and the near objects are blurred. Such a condition is called as Presbyopia. A presbyopic person can wear Plus glasses to bring the near objects in sharp focus on the retina. Nowadays, Bifocal and multi-focal Contact Lenses are also available for Presbyopic persons.
The Human Eyes are unique in the sense that we have two eyes which are situated in front of the head. These two eyes work simultaneously so that human beings have Binocular Vision which is a three dimensional vision so that there is a depth perception also. This is not so in most animals where the two eyes are situated on the sides of the head so that they work independently and not together. They do not have a Binocular vision and they lack depth perception. They however have a large field of vision.
Some common eye problems :
Redness of Eyes may be due to infection, inflammation or allergy of ocular tissues. In urban areas where there is dust, smoke and particulate contaminants suspended in the atmosphere, allergic reactions of the eyes occur commonly. In rural areas there is agricultural produce so that infections of the eyes with bacteria and fungi are commonly seen. Treatment of the redness of the eyes is based upon the type of contaminant and disease.
Watering of the Eyes is also called as "epiphora". A person may have ocular surface disturbance in which case there occurs a tear film dysfunction. Similar condition occurs in persons who work on computers for extended times. While working on computers, one forgets to blink the eyes frequently and a break up time ( BUT ) of tears is accelerated. Such cases have a burning and gritty sensation in the eyes. Use of tear substitutes helps in a long way in such cases.
Blurring of vision and loss of vision can occur due to various eye conditions.
Refractive Errors are by far the commonest cause of blurring of vision. They are corrected by prescribing spectacular glasses and by wearing contact lenses. Refractive surgery in cases of high myopia between -3 D and -8 D are done to get rid of glasses.
Commonest cause of blurring of vision in elderly population is due to formation of Cataract. Here the transparent crystalline lens of the eye turns opaque. This requires surgical removal of the opaque lens and it is replaced by an Intra - Ocular - Lens - Implant. The patient does not need to wear thick plus glasses after a cataract+iol surgery for cataract.
Another cause of blurring of vision is Glaucoma wherein the Intra - Ocular - Pressure of the eyes rises above normal limits. This results in a gradual loss of vision due to Optic Atrophy. Here the Optic nerve of the eyes turns white and there occurs a permanent loss of vision in the eye. Glaucoma is treatable by medicines and surgery if detected early.
Other causes of blurring of vision or loss of vision are diseases which cause interference of the transparency of the ocular media due to corneal opacity, cataract and vitreous pathology, and also diseases of the retina and optic nerve.
Systemic diseases like Diabetes and Hypertension cause blindness in the long run if these diseases are not well controlled by treatment.
Diseases of the Retina, Optic Nerve, Visual pathway and the Visual Cortex of the brain can also lead to blindness. These are Neurological causes of blindness and mostly they cause irreversible blindness.