Last week's article described how cats visualize and how their eyes differ from humans. This article will further discuss those differences.

Besides having more rods in their retinas in order to see better in dim light, a cat's eyes have further adaptations. For example, a cat's eyes are much larger in proportion to its body size than a human's. And, while human pupils are always surrounded by visible irises and whites, in dim light, a cat's pupils open to fill almost all of the visible portion of its eyes. These large, wide-open pupils allow even faint light to reach the cat's retina.

Another adaptation is displayed every time a cat's eyes are seen shining bright yellow-green in a car's headlights. Cats' eyes have a special reflective layer called the tapetum lucidum located directly behind their retinas. The tapetum reflects back light that has passed through the retina, redirecting the light for a second pass over the light sensitive retinal cells. When eyes are seen shining at the side of the highway at night, a person is looking through an animal's wide-open pupils at light reflected back from the tapetum. Researchers estimate that because of its tapetum and other special adaptations, a cat's eyes are about six times more sensitive to light than human eyes.

The location of a cat's eyes also aid in its success as a hunter. The eyes of prey species (such as mice and rabbits) are positioned far apart on the sides of the head. This arrangement allows prey animals to see in nearly a full circle around their bodies, thus enhancing their chance of escaping an attack.

In contrast, the eyes of the predatory cat are positioned close together at the front of its head. The cat's field of vision in one eye partially overlaps that of the other. When a cat looks at a mouse within this overlapping area of binocular vision, each eye sends a slightly different view to the brain. The brain interprets these two views as a three-dimensional image, allowing the cat to judge the distance between itself and its prey - essential information for a successful attack. In addition, cats see best at distances of about six to 20 feet - stalk-and-pounce range. But they cannot focus their eyes as close to an object as a human can. Cats probably do not see clearly focused images at much less than three feet.

Next week's article will address some of the more common eye problems in cats.

The writers are Dr. Steven B. Holzman, Dr. William R. Haagenson and Dr. Kathleen Tapley. They are associated with the Nassau-Suffolk Veterinary Hospital of Farmingdale.