The eye is located in a cavity in the skull known as the optic cavity. The eye itself
is a hollow sphere of fat. The walls that encase the eye, known as tunics, are filled with
liquids known as humors. These humors help maintain the eyes shape. It is held in orbit by
layers of fat. There are muscles all around the eye, which are responsible for its
movement. To the posterior of the eye there is the optic nerve. The optic nerve reaches
behind the eye, to the brain. The area of the eye that is not encased in fat or is
connected to a nerve has a separate type of protection. To the top and bottom part of the
eye, eyelids are located. At the ends of the eyelids are the eyelashes. These two sections
of the eye lock out foreign substances from 
entering the eye. The anterior part of the
eye, which is not protected, has a layer of oily secretion that lubricates, and protects
the eye. This oil is manufactured in the meibomian glands.
The tunics of the eye are mostly responsible for vision. The three
tunics are the sclera, the choroid, and the retina. The sclera is a white, thick,
connective tissue. The sclera is the white region of the eye. The central region of this
tissue is a transparent area, which houses the cornea. The cornea is responsible for the
allotment of light into the eye. The cornea is mainly made up of nerve fibers; thus it can
clearly recognize pain. When a large amount of light enters the eye, the nerves in the
cornea are responsible for the pain felt.
The second coat of the eye, the choroid, lies behind the sclera. The
main purpose of the choroid is to regulate the light passing through the eye. It is a
blood rich area that houses pigment. This pigment contains certain chemicals that prevent
light from being transferred throughout the body. The Choriod is connected to two muscles,
the iris and the cilliary body. The iris has an opening, the pupil, through which light
passes through. The pupil acts like a camera lens, regulating the amount of light that
travels throughout the body by opening.
The innermost area of the three eye tunics is the retina. The retina is
the primary tunic that regulates sight. The retina consists of millions of receptor cells
known as rods and cones. Rods and cones are also known as photoreceptors, due to the fact
that the primarily react to light. Electrical signals travel throughout the two
photoreceptors via a two-neuron chain. From the neuron chain the signal travels to bipolar
cells then to the ganglion. Before the signal leaves the retina, nerve impulses are sent
to the optic cortex, the result of all is sight.
