|MadSci Network: Zoology|
The eye is such a wonderful and useful organ that most mammals have eyes that follow the same basic design. The inside of a dog's eye looks almost exactly like the inside of a human eye. If you already know what a human eye looks like on the inside, you can skip down to the next paragraph. The curved front of the eye is called the cornea. Behind that is a fluid called aqueous humor. The iris, the colored part of the eye, sits right in front of the lens, which focuses the light coming in through the eye to the back of the eye. The space between the lens and the back of the eye is the largest space in the eye, and is filled with a fluid called the vitreous humor. The back of the eye is lined with the retina, which is a light-sensitive surface that captures light and changes it into information. The optic nerve picks up the information and sends it to the brain. A good picture of the inside of the eye can be found at The Victoria Corneal Laser Centre (http://www.bbc.org/~cnelson/anatomy.html). Microsoft's Encarta Online also has a good entry explaining the parts of the eye.
The dog's eye is a little different from the human eye, but those differences would be hard to see. For one thing, the human retina is made up of two types of cells, rods and cones. Rods are most sensitive to movement, while cones transmit information about color. Dogs have no cones in their retinae, so they can't see colors. For more information on this, look at this answer on the Mad Scientist board. You wouldn't be able to tell that the cones were missing without a microscope, though.
Another difference in some animals is a reflective layer of cells on the retina called the tapetum lucidum. This is most common in nocturnal animals. It reflects light back through the eye, effectively amplifying whatever light is available so they can see much better in the dark. If you've ever seen a cat's eyes shine in the dark, you've seen this layer of cells in action. Cats, deer, and foxes all have this layer; do domestic dogs have it, too? This is something you can find out for yourself -- turn out the lights in a room and then look at your dog's eyes from different angles. Do they shine back at you? Can you see a person's eyes shine in a dark room?
Try the links in the MadSci Library for more information on Zoology.