In order to understand the problems of eyes that cross, we must know some basic facts about eyes that that are not crossed.
The fact that we have two eyes is so taken for granted that the eyes are rarely given a thought unless something happens to one of them. For most purposes, our two eyes work together as a team so smoothly that we are not conscious of either eye separately unless, for example, we get a foreign body in one. Anybody who has been forced to wear a patch over one eye for a day or so, whatever the reason, can recall the difficulty encountered in performing the simplest acts-even going up or down the steps or walking through a narrow doorway. A person who is accustomed to having the use of two normal eyes which work together properly and who is suddenly deprived of the use of one of them, even temporarily, is in a sad fix indeed. Before considering this sudden helplessness, which descends upon us with the temporary loss of the use of one eye, let us first consider the mechanism whereby the two eyes work together.
Suppose you are walking along the street and suddenly decide to look to the right. You have no sooner made this decision than you have done just that- looked to the right-and done it almost instantly. Did you ever stop to think of the remarkable thing you have done? Or of what a marvelous mechanism is called into play so that you could see to the right? Probably not, because you have been able to look to the right or to the left or in any other direction you choose for as long as you as remember. Let us see how you do it.
Moving the eyes requires a chain of command exactly like that used in the army.
You can roll your eyes in any direction. Did you ever stop to wonder just how you do it? It is much easier to do it than to explain it, as a matter of fact. The eyes are moved by muscles in much the same way that you use rein to pull on the horse’s head. If you want to turn to the right, you pull on the rein running to the right side of the head; if you want to turn to the left, you pull on the rein on the left side of his head. If you can imagine a horse with three reins running on each side of his head instead of just one rein, you will have a fairly good picture of the muscles that move the eye. Our horse would have a total of six reins by which we might direct his head movements and thus guide him. There are six muscles that run the eye ball, like reins to our horse’s head, that move the eye.
We have so far discussed only the movements of two eyes and how they move together. Our eyes would be of little value to us if they only moved. They must see, and if our eyes are working normally, both of them must look at the same thing at the same time. This means that the lines of vision of the two eyes must always meet at whatever object we wish to inspect. It is just as if we have two cameras and wanted to take a picture of the same object with each camera at the same time and have the pictures so much alike that we could hardly tell one from the other. This is what actually happens because each of our eyes is merely a camera.
Suppose you look at a tree. Your right eye takes a picture of the tree and sends it to your brain. Your left eye also takes a picture of the same tree and sends it to your brain. The brain receives two separate pictures or images of the same tree-one from each eye. If you are to keep from seeing two trees when there is in reality only one tree, then the brain must put the two pictures together and make a single picture of them. This is done and the process of fusing the two pictures into a single picture is known as fusion.
If your brain could not fuse separate pictures of the same object coming to it from the two eyes, then you would see two of everything – you would see double. This would be annoying, of course, but it could be even worse if you decided to climb a tree! Obviously there is only one tree there, but if you saw two trees, you could not tell which was the real tree that could be climbed and which was merely the image or picture of a tree that wasn’t there at all. In short, you would be unable to place objects in the world around you accurately in space where they belong if it were not for the process of fusion.
The ability of the brain to fuse images of objects seen by the two eyes separately is something that develops after birth. A baby is born with two eyes and two legs. He can move his legs and he can move his eyes, but in neither instance can he move them accurately together in coordinated movements. He cannot walk and he cannot fuse! He makes many movements with his legs as if he might walk, but he cannot walk until he has learned to do so. He may at first learn to stand alone, then to toddle perilously about, and finally to walk gracefully and well. In the same way, the eyes make many movements at birth, but these movements are not always made together. Later the eyes begin to move together fairly well, and finally the brain learns to fuse images of objects seen by the child’s two eyes quite accurately. As the child finally learns to walk, so he finally learns to fuse, although he can do neither at birth. Both walking and fusing must develop, and to a certain degree be learned. A child learns to walk by using his legs together as a team, and he learns to fuse by using his two eyes together as a team. The average child can both walk and fuse at the age of 1 year but he can do neither as well as he will be able to do at an older age. The child usually learns to fuse accurately and well at about the age of 5 1/2 to 6 years. Once fusion is well developed, it helps the brain centers to control movements of the two eyes together with even greater coordination than is possible without fusion.
Fusion has developed comparatively recently in the evolution of man. As with any other recently developed faculty, in the case of fusion, a disturbance of function is more easily produced and abnormalities are much more common. Some children are born virtually without the fusion faculty, but since it is a common denominator for the success in the treatment of squint and lazy eyes (amblyopia), we must look carefully for any trace of it and develop it if it is rudimentary.
If you are about to cross a street and you hear an automobile horn blow suddenly on one side of you, you immediately look to that side. When you decided to look in that direction, you did not know exactly where the automobile was except that it was on one side. You therefore started your eyes moving toward that side until the auto came into view in the corner of one or perhaps both eyes. Once you have located the auto, even out the corners of your eyes, both eyes turn immediately to look straight at it. Once the auto was seen out of the corners of your eyes, the process of fusion was largely responsible for sending other messages to the brain to tell it exactly how far to move the eyes before stopping them. Thus it is that you may move your eyes, even when they are not closed, but when you open them and begin looking at objects around you, the process of fusion plays an important part in telling the brain just how far to move the eyes and in what position to stop them in order that you may see objects that you want to see and what is more important-that you may see only one of the particular object instead of seeing it doubled. Fusion is the magic cement that binds together the lines of vision and keeps them parallel.