Eye
The eyes and brain work together to make vision possible. The eyes detect light with nerve endings. Nerves then carry this information to the occipital lobe of the brain, which forms images from the information it receives. Numerous structures help the eye capture light and send signals to the brain.
The inner, back layer of the eyeball is the retina (Figure 2.24). The retina is made of nerve endings that are very receptive to light. These nerve endings allow the brain to see color, black and white, and shades of gray. In the center of the retina is an area rich in nerve endings. This area is responsible for forming sharp images.
The front of the eyeball’s surface is covered by the cornea. Light enters the cornea and passes through a lens. The lens focuses light on the retina. The iris, the colored part of the eye, lies in front of the lens, under the cornea. The iris appears round and has a black, circular opening called the pupil. The pupil allows light into the eyeball. The iris can change the pupil’s size. In bright light, the iris shrinks the pupil to let in a small amount of light. In darkness, the iris widens the pupil to let in more light.
The eyeball is protected by the bones of the skull. A strong tissue gives the eye its spherical shape. A firm gel fills the inside of the eye to help keep the eyeball’s shape.
In front of the eyeball are eyelids, skin-covered flaps that can close and protect the eye. Tear glands are located above each eye. The tears lubricate and clean the eye’s outer surface.