The eyes are the visual organs of the body, and rest in two sockets halfway up the front of the skull. They are made up of several important parts:
- The cornea, which is a clear lens covering the center of the eye. This focuses light on the back of the eyeball.
- The iris, or colored part of the front of the eye, which is attached to muscles which can contract or expand the iris in response to light levels, giving rise to the center opening, the pupil.
- The sclera, which forms the outer skin of the eyeball
- The vitreous humor, a thick transparent liquid which fills the eyeball and gives it its circular shape
- The retina, which is a surface of light receptive cells at the back of the eyeball, containing rods, which are sensitive to light intensity, and cones, which are sensitive to color.
- The optic nerve, which penetrates the back of the eyeball through the blind spot, together with the blood vessels that provide blood to the eye, and transmits nerve impulses to the brain.
In humans, the eye sacrifices some sensitivity to light, sensitivity to movement and the ability to see at distance for the ability to see in color. For example, cats, alhtough they have poor color vision, see much better in low light and can detect motion that is indetectible to a human.
There are several diseases of the eye, as well as others that cause symptoms in the eye, such as blurry vison, loss of tears and even blindness. These are often due to the effects of disease on the nerves, ducts and blood vessels that go into the eye.
Surprisingly, the eye has no sensitivity to pain despite the blink reflex. This can often be used to a physician's advantage as it is possible to use a needle through the eye to reach parts of the brain that are otherwise inaccessible.