The structure of cat eye is the same as human in general. Vision is based on the ability of organisms to detect light - visible part of electromagnetic radiation. The sense of vision - eye percept light due to retina that is a part of brain. Light reaches retina through a pupil. By means of special muscle a pupil is able to alter its' size, and, like an aperture in camera, regulates flux of light towards retina. Nocturnal animals have big eyes with big pupil; in day animals these are considerably smaller. Cats that both see in darkness and like to bask in the sun have a slit-like pupil because it's better than round and lessens the luminous flux towards sensitive retina.
The front transparent covering of eye - cornea and lens suspended on muscles and ligaments right behind pupil, refract luminous flux like an optic system in camera and focus the picture of object on retina. During examining objects at different distances the accommodation takes place - focusing of picture directly on retina through the change of form of pupil. The ability of such accommodation in cats and dogs is 2-3 times less than in Primates and humans.
Luminous flux reaches photoreceptors on retina. Photoreceptors are cells that contain colored substance - pigment, that under the influence of light become colorless, at this molecules of pigment change their form, which cause rise of electric potential. Photoreceptors differ by their form and are divided in two types: sticks and cones. Sticks contain one kind of pigment; therefore they provide twilight colorless vision. Cones contain pigments of three kinds, which provide color day vision. The ratio of number of stick and cones differ in different animals. Cones provide the ability to discern small features (keenness of vision). There is an area of the best vision in retina, which is pit-like in humans, and disk-like in cats. Here mainly cones are situated. Turning of head and eyes promote the image of an object to get in the area of the best vision on retina.
Many nocturnal animals, including cats, have special structures - tapetum near photoreceptors. Tapetum reflects light that was not absorbed by cells of retina back to photoreceptors and by this providing better vision in twilight. Eyes of animals, which have tapetum, can shine in darkness.
There are some other layers of nerve cells in retina but photoreceptors from which electrical signal passes along the optical nerve in brain. Tracts from right and left eyes cross so that information from both eyes comes in each of cerebral hemispheres. Spaces that a cat watches with its' right and left eye (fields of vision) overlap at the front on 45 % so that the animal can see the same object with both eyes simultaneously. This is the base of the animal ability to determine shape of object and distance at it. Cats being in one meter from the ground where they are going to jump are able to determine the distance to up to 3-5 cm.
Animal behavior controlled by vision much depends on training. This was demonstrated by a special research. Kittens till 8th week were brought in darkness, and then from 8th to 12th week - in a plant, which was a kind of a whirligig with two boxes. One kitten sat in the first box and another was fastened in such way that could walk on the floor and drove the first one. After four-week training a kitten that could walk himself was able to fulfill tests on determining the height of floor, landing on paws and so on; and the kitten that only looked around made mistakes in all tests.
Information from retina comes into an optic area of cerebral cortex where the most essential processing occurs. Nerve cells of cerebral cortex change their activity according to the subject that is shown to a cat - luminous line, a spot or a mouse. If cat looses vision in at an early age than the area of neurons concerned with vision reduces in cortex and at the expense of them increases the number of neurons concerned with discerning of acoustical and skin irritants. Whiskers of such cats become on 30 % longer then in cats able to see. The increase of the role of hearing, smell and other senses equilibrate the loss of vision so well that nothing differs their behavior from behavior of normal animals. However disturbances in cerebral visual structures will lead to mentality and nuances of mood not to reflect in beautiful cat eyes.
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).