Physiology: The Science of the Body

by Ernest G. Martin

longer than the length of the eyeball, and the crystalline lens by itself also has a long focus, but the two in combination give a focus that just corresponds with the length of the eyeball, so that the images of all objects at a distance of eighteen feet or more fall sharply on the retina. Since near objects focus farther away from the lens than far objects, the effect of this is to make objects nearer than eighteen feet out of focus. For them a longer eyeball would be needed, and it would have to become longer and longer the nearer the object was brought to the eye. We all know that when we look at near objects we make an adjustment in the eyes. This is known as accommodation; for a long time it was supposed that accommodation was actually secured by lengthening or shortening the eyeball to bring the focus right, but we now know that the eyeball does not change in shape when we accommodate. The same result is secured by another means, namely, by letting the crystalline lens bulge out and become thicker. It was stated incidentally a few pages back that thick lenses have shorter focuses than thin. So when the crystalline lens thickens it shortens the focus of the eye, and throws the image forward. This will locate the image of near objects on the retina, instead of behind it, as in the unaccommodated eye. The crystalline lens is not stiff like glass, but rather like a thick jelly; it is in a transparent bag, or capsule, which is fastened to the inside of the eyeball all around, and the pull on the capsule stretches it out pretty flat, making the lens thin. There are tiny muscles inside the eyeball, known as the ciliary muscles. These are so fastened that when they contract they pull the eyeball forward and inward, loosening the tension on the capsule of the lens. This then bulges out, taking up all the slack. When we learn, as babies, to accommodate for near objects we find out just how much the capsule must be loosened to give the proper adjustment for any distance. As people get along in years the crystalline lens often loses its elasticity so that it does not bulge when the capsule is loosened. After this happens vision for near objects is no longer clear. Relief is obtained by wearing reading glasses. These are merely glass lenses worn in front of the eyes selected so that their focus fits in with that of the lenses of the eye itself to bring the image of objects at reading distance sharply on the retina. It is usually necessary to replace these glasses from time to time as the crystalline lens becomes stiffer and stiffer and ordinary accommodation fails more and more.

There is a disease known as cataract in which the crystalline lenses become cloudy and finally completely opaque. Of course this means blindness, since the passage of light to the retina is interfered with. Relief is obtained by the simple expedient of removing the opaque lenses bodily. This is possible merely because the crystalline lens is not the chief lens of the eye. To be sure the cornea by itself will not focus on the retina, but a glass lens can be placed in front of it which will add itself to the cornea and the combined lenses will. There is no possibility of accommodation in a case like this, so the patient has to be furnished with bifocal lenses; the main part gives clear distance vision; the lower section gives clear vision at the reading distance. The patient has to get along with blurred vision in the regions between.

Not all eyes are exactly the right size so that distant objects shall focus sharply on the retina. In fact a large proportion of them are either too long or too short. It is clear that in an eyeball that is too long the image of distant objects will fall in front of the retina, but near objects that happen to be at just the right distance will focus exactly on the retina. The distance at which this happens depends, of course, on how much too long the eyeball is. Persons that have unduly long eyeballs are, therefore, nearsighted. The condition is called myopia. The correction for it consists in the use of lenses in front of the eyes that instead of shortening the focus shall lengthen it. Concave lenses, namely, those that are thick at the edge and thin in the middle, will do this, and these are the kind that are worn by near-sighted people.

When the eyeball is too short the image of distant objects falls behind the retina, and of course that of near objects tends to fall farther back yet. Since by