The eyeballs are extensions of the brain and are made up of lymph, brain, nerve and muscle tissue. Whereas most of the body is nourished by the blood, the eyes are fed through the lymph system, which is why movement is so important. The eyeballs rest in the eye orbits, protective cavities formed by the connection of seven bones. The eyelids consist of skin, muscles and conjuctiva, a membrane that lines the inner lids and part of the eyeball. Blinking is very important in lubricating the eyes and keeping their surfaces clean and refreshed. Surrounding each eyeball are six ocular muscles. There are four Recti muscles, which attach on the top, bottom and sides of the eye and two Oblique muscles, which wrap around the eyes. These extrinsic muscles are responsible for both voluntarily pulling the eyes in different directions and for involuntarily changing the shape of the eye for accommodation, or focusing.
The eyeball itself consists of three layers.
1) The first layer is the external, fibrous layer composed of the sclera (thick white part of the eye) and cornea (transparent area that allows light to enter the eye).
2) The second layer is called the uveal tract and is composed of the choroid (provides oxygen and nourishment to retina), ciliary body (responsible for changing the shape of the lens for accommodation or focusing) and iris (colored area that regulates the amount of light that enters the eye).
3) The third layer is the nervous layer and is composed of the retina (where light focuses on the back of the inside of the eye containing light-sensitive cells called rods and cones).
The inside of the eye is divided by the crystalline lens (contracts and flattens to focus at different distances) into the aqueous chamber and vitreous chamber (transparent, jelly-like substance that fills the inside of the eyes). The electrical signals recorded on the retina get sent via the optic nerve to the visual cortex in the back of the brain for processing and interpretation.