Vision

  
Sight Defects
Braille

 
CNIB
Canadian organization for the blind. Extensive Vision Information Centre.

Humans are born with two eyes. Each eye is a spherical organ of 2.5 cm (1 inch) in diameter. The eyes are located in the head's eye-sockets. The front of the eye is open to the external environment to capture the light waves required to see. This light is admitted to the interior of the eye through the cornea, a tough membrane that bulges in the front-centre of the eye. Behind the cornea is the aqueous humor, a chamber of transparent fluid separating the cornea from the lens. The light enters the front of the lens through the pupil. A muscle around the edge of the pupil automatically contracts and relaxes to allow as much or as little light into the lens as necessary. The lens is a flattened sphere made of layers of transparent fibres. Ligaments connect this to the ringlike ciliary muscle surrounding the lens. This muscle flattens and expands to 'focus' the lens. Behind this is a thin sac, the hyaloid membrane, which contains a transparent gel-like substance called the vitreous humor, which maintains the eyeball's shape. After this is the retina. Light-sensitive nerve cells packed closely together on the outer surface of the retina. Directly behind the pupil is a small yellowish spot called the macula lutea, which contains the fovea centralis, the area of the eye with the most visual acuity. The optic nerve connects to the brain from below the fovea. A small area of the retina has no light-sensitive cells, forming a 'blind-spot'.

How it works
The eye works like a simple camera. It focuses by flattening and rounding the lens, called accommodation. When an object is far away, the lens remains flat. But when the object is closer, the ciliary muscle must contract, making the lens rounder, to focus properly. The lens hardens with age, making it difficult for older people to see clearly at a close range without the need for eyeglasses. Because of the complexity of this organ, many common conditions exist for many people, requiring corrective lenses. When a person is unable to use his or her sense of vision, he or she is referred to as blind.

The eye is extremely fragile, so many protective mechanisms are required. First, the surface of the eye is covered with the sclera, a clear protective coating. More obvious are the folds of skin surrounding the eye-sockets, called eyelids, which close to protect the eye. On the ends of these folds are eyelashes, small hairs that keep dust and other tiny foreign objects from falling into the eye. Inside the eyelids a thin membrane called the conjunctiva covers the visible sclera. To keep the exterior of the eye moist, tear glands, or lacrimal organs, are situated at the outside corner of the each eye. They secret a salty lubricant which washes away any foreign matter. Meibomian glands on the edges of the eyelids also produce a lubricant. Humans blink (rapidly close and reopen their eyelids) unconsciously about every six seconds, which allows these lubricants to coat the eye. If the body senses dust on the eye, you may blink more, or more tears will be produced. Eyebrows soak up moisture from perspiration and rain before it can drip into the eye, and the body edges of the eye-socket, also referred to as the orbit, protect the eye from injury due to collisions.

Animals’ Eyes
In the animal world, there are two types of eyes: single and compound. Human eyes are of the single-eye type, and are of average complexity. The least complex are those of jellyfish, while nocturnal birds, cats, and bats have a larger number of light-sensitive cells that humans, allowing them to see well in darker conditions. Compound-eyes, found in anthropoids, use many individual facets to create separate images that combine into an image similar to a mosaic.