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THE EYES HAVE ITTHE EYES HAVE IT
GLOSSARY
Blind Spot
The part of the retina that joins with the optic nerve. On this part of the eye, light cannot be detected.
Color Vision
The ability to see different wavelengths of light.
Light is a spectrum, a continuous sequence or range, of electromagnetic waves, which appear as different colors. Humans and other animals can see some of this range. The part we can see is called the visible spectrum.
Compound Eye
An eye made up of many separate units, each with its own surface area, lens, and optic nerve fiber. The images collected by all the parts of a compound eye are integrated in the animal's brain. Most invertebrates -- crustecea such as crab and shrimp, and insects such as dragonflies and bees -- have compound eyes. See Anatomy of a Compound Eye for more details.
Cones
One of two types of light sensors in the retina. Cones work best in bright light and detect color. In the human eye, cones respond to either red, green, or blue wavelengths of light.
Cornea
A clear, curved membrane that covers the front of, and allows light into, the eye. It protects the pupil, the iris, and the inside of the eye from dust and other matter. The cornea helps focus light onto the retina.
Eye
A sense organ that detects light. An eye can be a simple organ that detects only light and shadow, or a complex organ that detects shape, color, brightness, and distance. See Visual Basics for more information about eyes and vision. See Anatomy of a Single-Lens Eye and Anatomy of a Compound Eye for details about the parts that make up these eye types.
Eyecup
An eyecup is a cluster of light-sensitive cells forming a cup-shaped depression on an animal's body. The sides of the cup cast shadows on the cells inside it. This allows the animal to detect not only light and dark, but also the direction of the light source.
Eyecups can be found on seastars (starfish) and flatworms.
Eyespot
A light-sensitive cell on the surface of an animal's body. This simple eye form cannot gather enough information to see an image; it only detects changes in light and dark.
Eyespots can be found on simple organisms such as earthworms and leeches. When the eyespot is stimulated by sunlight, the organisim reacts by moving out of the light.
Eyespots can also be found on more complex animals that have well-developed eyes. A dragonfly, for example, has an eyespot on each side of its head in addition to its compound eyes. Since eyespots only detect changes in light and dark, the dragonfly may be using them as navigation aids -- by tracking the horizon where the light sky meets the dark ground.
Iris
A pigmented, muscular ring that surrounds and controls the size of the pupil. In dim light, the pupil is enlarged to allow more light into the eye. In bright light, the pupil is contracted to allow less light into the eye. Both actions are automatic, regulated by the reflex centers in the brain stem and spinal cord.
Lens
In a single-lens eye, the lens is a clear, flexible membrane. It can change shape to bend the light that passes through it, focusing the light onto the retina and creating a sharp image.
In a compound eye, the lens is an inflexible, crystalline cone that directs light down the ommatidium, toward its light-sensitive cells (retinulae). The lens in a compound eye cannot be focused.
Ommatidium
Plural: ommatidia. A a cylinder-shaped unit of a compound eye. Each ommatidium receives light from a narrow part of an animal's field of view and acts like a separate eye. An ommatidium has its own facet, crystalline cone (lens), light receptors (retinulae), and optic nerve fiber.
Orbit
A bony socket in the skull that holds and helps protect the eyeball.
Pupil
A hole in the eye, covered by a transparent layer called the cornea. Light enters the eye through the pupil. The amount of light that enters the eye is controlled by the muscles in the iris.
Receptor
Another name for sensor.
Retina
The innermost membrane of the eye, a network of photoreceptive (light-sensitive) nerve cells called rods and cones. The retina receives and reacts to incoming light and sends signals to the brain.
Rhabdom
In a compound eye, the rhabdom is a translucent cylinder formed by the surrounding retinulae (photoreceptive cells). The rhabdom acts a channel, similar to a lilght-shaft in a building, that allows each retinula access to the light entering the ommatidium acts a channel, similar to a lilght-shaft in a building, that allows each retinula access to the light entering the ommatidium
Rods
One of two types of photoreceptive sensors in the retina. Rods work best in dim light and allow black and white vision.
Sensor
Also called a receptor. A cell or group of cells that can detect stimuli. Rods and cones are photoreceptive sensors in the eye.
Stimulus
Plural: stimuli. An environmental change that is detected by a living organism's sensors and directly influences its activity, such as evoking muscular contraction or glandular secretion.
Tapetum Lucidum
A layered structure at the back of the eye that helps increase night vision in some animals by reflecting light that enters the eye back to the retina.
Vision
The sense that detects light. Most animals can detect the difference between light and dark. Animals with more advanced eyes and brains can see images of the world around them.
Vitreous Humor
A clear, jelly-like substance that fills the central core of the eye. It helps the eye maintain its round shape.

Oreodont photograph © M. Sloben 1998

The Eyes Have It | Kids' Habitat

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