Eye

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The **eye** is a round organ for **sensing** light so organisms can see. It is the first part of the visual system. About 97 percent of animal species have eyes.  Image-resolving eyes are present in cnidaria, molluscs, vertebrates, annelids and arthropods.

In mammals, two kinds of cells, *rods* and *cones*, allow sight by sending signals through the optic nerve to the brain.

Some animals can see light that humans cannot see. They can see ultraviolet or infrared light.

The lens on the front part of the acts like a camera lens. It can be pulled flatter by muscles inside the eye, or allowed to become rounder. As some people get older, they may not be able to do this perfectly. Many people are born with other small problems or get them later in life. Eyeglasses (or contact lenses) may fix the problem.

Like different cameras, different eyes have different abilities. They may have higher or lower resolution, the ability to detect small details. They may have different performance in low light; nocturnal animals can see better at night than daytime animals. They may have different ability to distinguish colours.

## Parts of the eye 
The human eye is composed of several different parts. These parts may or may not be the same in other animals. They are:
* Cornea: the outermost, transparent layer that protects the iris and pupil. 
* Pupil: the black circle in the middle of the eye, through which light passes. 
* Iris: the colorful circle of the eye around the pupil. It can be brown, blue, green, etc. Its main function is to regulate the amount of light coming into the eye.
* Sclera: the large, white field around the iris that keeps the eyeball in shape.
* Lens: behind the cornea there is a transparent bioconvex lens of very short focal length.This lens is held in the centre of eye ball with the help of ciliary muscles.
* Retina: has the cells which turn light into nerve impulses.
* Optic nerve: the nerve that connects the eye to the brain. Optical information is taken to the rear of the brain for processing: see cerebral cortex.

## Types of eye 
Today, ten different types of eyes are known. Most ways of capturing an image have evolved at least once.

One way to categorize eyes is to look at the number of "chambers". *Simple eyes* are made of only one concave chamber, perhaps with a lens. Compound eyes have many such chambers with their lenses on a convex surface. and some annelids possess both.

### Simple eyes 
#### Pit eyes 
Pit eyes are set in a depression in the skin. This reduces the angles at which light can enter. It allows the organism to say where the light is coming from.

This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above). 

Compound eyes are common in arthropods, annelids, and some bivalved molluscs.

## Evolution of the eye
The evolution of eyes started with simplest light-sensitive patches in unicellular organisms. These eye-spots do nothing but detect if the surroundings are light or dark. Most animals have a biochemical 'clock' inside. These simple eye-spots are used to adjust this daily clock, which is called circadian rhythm. Some snails, for example, see no image (picture) at all, but they sense light, which helps them stay out of bright sunlight.

More complex eyes have not lost this function. A special type of cells in the eye senses light for a different purpose than seeing. These cells are called *ganglion* cells. They are located in the retina. They send their information about light to the brain along a different path (the *retinohypothalamic tract*). This information adjusts (synchronizes) the animal's circadian rhythm to nature's light/dark cycle of 24 hours. The system also works for some blind people who cannot see light at all.

Eyes that are a little bit better are shaped like cups, which lets the animal know where the light is coming from.

More complex eyes give the full sense of vision, including color, motion, and texture. These eyes have a round shape that makes light rays focus on the back part of the eye, called the *retina*.

### Other 
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Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialized zones of ommatidia organized into a fovea area which gives sharp vision. In this zone the eyes are flattened and the facets  are larger. The flattening allows more ommatidia to receive light from a spot. This gives a higher resolution.

The body of *Ophiocoma wendtii*, a type of brittle star, is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many chitons.

## Sign and symptoms of disease or illness
* *pupil:* Argyll Robertson pupils (picture)
* Adie pupil (picture)
* Marcus Gunn pupil

**cornea:* Fleischer ring
* Kayser–Fleischer ring
* Hudson–Stahli line

* *iris:* Brushfield spots
* Lisch nodule

* *conjunctiva:* Bitot's spots
* Arlt's line

* *retina:* Hollenhorst plaque
* Roth's spot
* Fuchs spot

* *others:* Alexander's law
* Hirschberg test
* Siegrist streaks

* *eyelid:* Abadie's sign of exophthalmic goiter
* Boston's sign
* Dalrymple's sign
* Stellwag's sign
* *lacrimal:* Schirmer's test

## References