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Common Vision Problems-

The Structure & Function of Eyes

How do we perceive our surroundings? What senses do we use? Do other organisms sense their surroundings in the same way or do they use very different mechanisms?

Today in the laboratory, we will examine a mammalian eye and examine examples of the compound eyes of insects. Are the eyes of an insect like those of a human? At first glance one, would say that although these sense organs have similar functions (i.e., sensing light), the eyes of humans and insects are not at all similar in structure.

There are two types of eyes in the animal kingdom, single-lens and compound eyes. Today, we will consider how these two types of light-sensing organs are similar and how they are different.

The Compound Eye of Invertebrates

Examples of compound eyes in animals are found in insects, crustaceans and some polychaete worms. The compound eye is made up of 1,000's of individual light sensing units called ommatidia (singular=ommatidium).


A figure of the structure of an ommatidium in a compound eye is shown below:


Each ommatidium contains a lens structure consisting of the cornea and crystalline cone that directs light on a rhabdom that consists of the overlapping membranes of a layer of photoreceptor cells called retinula cells. The membranes of these cells contain rhodopsin. Axons from the retinula cells transmit the sensory signals to the nervous system.

An example of the compound eyes in insects (house fly) is shown below:


Each ommatidium senses only a small portion of the field of view. The number of ommatidia per eye varies from species to species with only a few in ants, to 800 in fruit flies, to as many as 10,000 ommatidia in the compound eye of the horsefly. The compound eye provides information about patterns in the environment and is very good at detecting movement.

Some photographs which illustrate images derived from a compound eye:

The Single-Lens Eye

Some invertebrates such as jellyfish, polychaetes, spiders, and many mollusks as well as mammals use a single-lens eye to detect visual (light) stimuli. The single lens focuses the light image onto a collection of cells in a manner similar to the lens of a camera.



Vertebrates also use single-lens eyes. For example, humans have a very sensitive form of single-lens eye that can detect nearly a countless number of colors, can respond to as little as one photon of light, and can focus on and detect images of objects that are miles away. Below is a diagram of a human eye:


The exterior of the mammalian eye is covered by a white, rather hard covering known as the sclera; at the front of the eye is a softer, transparent covering called the cornea. Beneath the cornea is the pupil, an opening into the interior of the eye. The pupil is surrounded by a colored ring called the iris. The function of the cornea and the pupil is to allow light into the interior of the eye; the cornea is a cellular structure while the pupil is a hole that regulates the amount of light that strikes the retina. The iris is a muscular structure that contracts and relaxes to control the size of the pupil. The iris is pigmented to block light from entering the interior of the eye, except through the pupil.

The interior of the mammalian eye is filled with a transparent, jelly-like fluid called the vitreous humor; this thick fluid lends support to the eye yet still allows light to pass through it. The lens is suspended behind the pupil by radially-arranged black ligaments that function to adjust the shape of the lens. Light is focused on a thin layer of tissue by adjust the the thickness of the lens. When the lens is more rounded, the image of objects that are close is resolved; a more flattened lens resolves the image of objects that are farther away.


The retina is the pinkish layer of tissue in the back of the eye onto which light is focused by the lens. The retina contains the photoreceptor cells known as rods and cones due to their different shapes. The retina contains about 100 million rod cells that are more sensitive to light than the cone cells; rod cells are very important in night vision and in very low light environments, but play no role in color vision. There are approximately 3 million cone cells in the retina and are predominantly responsible for color vision. There are at least three different kinds of cone cell, each with a different type of opsin molecules that absorb light of different wavelengths. The photoreceptor cells are connected to the optic nerve by layers of transparent neurons. Where the optic nerve leaves the eye is an area with no photoreceptor cells, this area causes a blind spot in the field of vision; the brain fills in this blind spot with an image that is extrapolated from the rest of the field of view.

The eye does not actually SEE objects; the photoreceptor cells of the eye generate action potentials in response to light. These signals are relayed to the visual cortex in the brain. The brain then processes these signals allowing you to visualize the object.

But are there similarities between light-sensing organs of humans and insects that aren't readily apparent?

For years, scientists have debated whether the compound eyes of insects and other invertebrates are evolutionarily-related to the single-lens eyes of mammals or whether these two light-sensing systems evolved separately from each other. Recent work has suggested that compound eyes and single-lens eyes are related. Both Drosophila and mice share a homologous gene called Pax-6 whose expression is involved in the development of eyes in these distantly-related organisms. In fact, the Pax-6 gene of mice can be expressed in Drosophila and function in the development of compound fruit fly eyes.

A cartoon that suggests how Pax-6 may function in eye development is shown below:


from Zucker, C. Science(1994) 234, pg. 742.

A summary of the Zuker paper is available on the Web in Science News