Sight

     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.