The complete eye, which includes the eyeball, muscles nerves, blood vessels and the structures that produce and drain tears are all housed in a bony cavity called the orbit. Each orbit is made up of several bones and is shaped like a pear.

Eyeball

The eyeball consists mainly of the following:

The sclera is a relatively tough white outer layer of the eye. The sclera is covered near the front of the eye by the conjunctiva. It is the thickest layer made from fibrous connective tissues.

The conjunctiva is a thin mucous membrane, which lines the inside of the eyelids but which also stretches to the edge of the cornea.

The cornea is a transparent, clear dome-like structure on the front surface of the eye over the black circle called the pupil. The cornea serves as a protective covering for the eye and also functions to focus light on the retina located at the back of the eye. The cornea has numerous nerve endings and is therefore very sensitive to touch.

The iris is a circular colored ring on the front portion of the eye. It is a thin layer of fine muscles. After the light passes through the cornea it goes through the pupil which is actually a hole in the iris. The main function of the iris is to control the amount of light entering the eye. In doing so, the iris makes the pupil dilate when less light enters and shrink when the amount of light is excessive, as in a camera.

The lens is located behind the iris. It is a small clear disc and is relatively dry containing only 65% water. It is made of a transparent elastic protein. Its main function is to focus light on the retina. It does this by using small muscles called the ciliary muscles to increase its thickness when focusing on near objects and thinning itself when focusing on far objects. The lens is attached to the ciliary muscles by fine thread like structures.

The retina is a kind of screen at the back of the eye on which an inverted image of the object being viewed is formed. The retina is a thin transparent membrane and is the part of the eye that actually sees. It represents the film in a camera. The retina contains millions of light-sensitive sensory cells or photoreceptors and blood vessels to nourish them. These sensory cells absorb light.

The most sensitive part of the retina is called the macula where the density of photoreceptors is the highest. The macula helps in producing detailed images, as in high resolution films where the grain density is high. The macula is responsible for the critical function of focusing. It is the part of the retina most used for normal vision.

Each photoreceptor is connected to a nerve fiber which when bunched together form the optic nerve. The photoreceptors convert the light into electrical impulses and the optic nerves carry these electrical impulses to the brain.

There are two types of photoreceptors, cones and rods. Cones contribute in producing sharp, detailed, central and color vision and are mainly present in the macula. The rods are responsible for night and peripheral vision. In short, cones function best in daylight and rods at night or in dark environments. The rods are mainly present in the peripheral regions of the retina. They are more numerous than cones and are more sensitive to light but do not register color vision. Rods can only differentiate between shades of grey. That is why we do not see colors in the dark.

Eyeball interior

The eyeball is divided into two chambers each of which is filled with fluid. The front chamber stretches from the inside of the cornea to the front part of the lens and is filled with a fluid called the aqueous humor which nourishes the internal structure. The rear chamber extends from the back of the lens to the retina. It is filled with a fluid called the vitreous humor. These fluids create a pressure which fills out the eyeball and helps maintain its shape.

The front chamber is also divided into two sections. The front section extends from the cornea to the iris and the rear section extends from the iris to the lens. Usually the aqueous humor is produced in the rear section and flows slowly through the pupil into the front section. The fluid then drains out of the eye through channels present where the iris meets the cornea.