What Happens When We See?
HOW WE SEE THINGS
Of course, it is the eyes with which we see things. When we think of the things with which we see, we think only of eyes, which give us our sense of vision, but there are certain forms of animal life which have no eyes but which have what are called eye spots or eye points, which are sensitive to light and which are merely spots. These eye spots may be located in any part of the body, and are often found in great numbers on the same body. These rude eyes are, however, not real eyes. They are, as has already been said, sensitive to light, but are found only in some of the very low forms of animal life which live in the water. A real eye is an organ in which the parts are so arranged that optical images may be formed.
As animal life becomes developed to a higher scale, the parts which contain the making of real eyes become more distinct although, of course, the eyes themselves are not so highly developed as in man. One of the first kinds of life which has eyes with a definite structural character are the worms, snails, etc., though their sense of vision is more or less dim.
When we come to the family of mollusks, however, low down in the scale of life though they are, we find them to possess eyes which enable them to see almost as well as animals which have a backbone, although this kind of eyes is constructed in a very different manner than the eyes of vertebrate animals referred to. As we ascend the scale of animal life in the study of eyes, we come next to the crustaceous, which is an important division of animal life that embraces the crabs and lobsters, shrimps, crawfish, and insects such as sand-hoppers, beach-fleas, wood-lice, fish-lice, barnacles. The eyes of such animals are quite developed, but the number that each will have varies. Some have only a single eye and others two, four, six or eight, but only certain kinds of this class of life have more than two eyes. The spiders generally have the most.
In vertebrates, which is the class of animal life to which we belong, the number of eyes is almost always two and no more. The eyes are formed in special sockets in the skull, which are called eye sockets or orbits. This arrangement of placing them in a socket is of great advantage because the eye is thus protected from chance of injury except from one direction—the front. These animals have also eyelids, eyebrows and eyelashes, which serve as a further protection to the eyes.
The principal parts of the eye are arranged in a globe-like ball called the eyeball. This eyeball is movable in the socket under control of various muscles. The eyeball is almost surrounded by a membrane which is opaque in most parts, but very transparent at the front. This transparent portion of the surrounding membrane is called the cornea, and is quite hard. This is the outside coat of the eye. The second coat of membrane consists of parts of various names and contains the iris. The third coat is the retina, which is the end of the optic nerve entering the eye full from behind and expanded into a membrane which spreads out over the second coat.
The retina or optic nerve receives optical impressions focused upon it by the crystalline lens. These impressions are carried along the optic nerve to the brain, and the brain then receives the sensation of seeing the image. The eyeball is hollow, and its three surrounding coats form what is practically the same as the interior of a camera. The crystalline lens of the eye acts the same as the lens in the camera. This crystalline lens is suspended within the eyeball right in front of the transparent opening in the front of the eyeball, and when the rays of light strike this lens it focuses them on the retina, which is the same as the film in your camera.