Although small, the eye is a very complex organ. It is approximately 1 inch wide, 1 inch deep and 0.9 inches tall.
The tough, outermost layer of the eye is the sclera. It maintains the eye's shape. The front sixth of this layer is clear. It's called the cornea. All light must pass through the cornea when it first enters the eye. Attached to the sclera are muscles that move the eye, called extraocular muscles.
The second layer of the eye is called the choroid (or uveal tract). It contains blood vessels to supply blood to structures of the eye.
The front part of the choroid has two structures:
- The ciliary body, a muscular area attached to the lens. It contracts and relaxes controlling the size of the lens for focusing.
- The iris, the colored part of the eye. The iris's color is determined by the color of the connective tissue as well as pigment cells. Fewer pigment cells make the eyes blue; more pigment makes the eyes brown. The iris contains two muscles. The dilator makes the iris smaller (and the pupil larger). This allows more light into the eye. And the sphincter, which makes the iris larger (and the pupil smaller). This allows less light into the eye.
- The innermost layer of the eye is the retina, the light-sensing portion. It contains rod cells, responsible for vision in low light, and cone cells, responsible for color vision and detail. All the way in the back of the eye (at the center of the retina) is the macula. At the center of the macula is the fovea centralis area. This area contains only cones and helps the eye to see fine detail clearly.
- The optic nerve is formed by retinal nerve fibers that collect at the back of the eye. The optic nerve conducts electrical impulses to the brain.
- The lens - used to fine tune vision - is a clear, bi-convex structure about 0.4 inches in diameter. The lens can change shape because it's attached to muscles in the ciliary body.
Lacrimal glands, located above the outer segment of each eye, produce tears. Tears drain into the inner corner of the eye. The tears then move into the lacrimal sac, through the nasal duct and into the nose. That's why your nose runs when you cry.