Natural Wonders

by Edwin Tenney Brewster

There are, then, in the skin, cold spots, hot spots, touch spots, and pain spots, from two to three times as many of each as there are fine hairs on the skin. Under each of these spots, is the end of a nerve, either branching like a little bush or ending in a sort of oval knob, much the shape of a foot ball. And just as the eye sees, but doesn’t hear, and the ear hears, but doesn’t see, so each of these nerve endings feels either pain, or cold, or heat, or pressure, but only one of them.

How many senses have we then? At least ten, which is twice the traditional five. Besides these there are thirst and hunger, which are certainly feelings, tho neither sight, hearing, touch nor any of the rest. Then there is that peculiarly unpleasant feeling which comes to us after we have dined less wisely than well, or have been rocked too fondly in the cradle of the deep, the sensation, I mean, which we call “sickness” or nausea. This makes thirteen senses. There are several more in addition to these, more or less vague affairs, which for the most part tell us only what is going on inside our own bodies.

If therefore, you ask how many senses we really have, I shall have to say that we had better call it ten. At least we have ten well defined sorts of feelings, which tell us what is going on outside our bodies—and after all, that is what senses are for. These, then, will be sight, hearing, touch, taste, smell, heat, cold, pain, equilibrium, and the muscular sense. Each of these has its own special place, in eye, ear, joint between two bones, or little spot in the skin. If we lacked any one of these (as indeed many creatures do) there would be something which it is important for us to know, but which would be forever impossible for us to find out.

XXVI
Eyes

I am not going to tell you about the wonderful structure of the eye, nor about how it works. That, if you have not learned something about it already in your school physiology, you will get sooner or later, certainly before you get thru the high school. This book is mostly about things that you do not learn in school.

I have, however, told you something about how the eye grows, how it buds out from the side of the brain, and then doubles in to form a cup; and how this cup becomes at length the nervous portion of the eye, the retina, which therefore, tho it lines the eyeball, is really part of the brain; and how this retina somehow or other, in a way that nobody understands, picks up the image of the things we see, and sends it along the optic nerves to the part of the brain which lies above the ear and round toward the back of the head. I think you know also how these optic nerves cross over, just as most of the other nerves do, so that the left eye reports to the right side of the brain, and the right eye reports to the left side. You know also how, in the end, both these reports get turned over to the left side of the brain, and remembered there; so that while we see with both sides of the brain, we remember what we have seen with one side only.

Aside from these matters, there are various little points about the eye which one can make out pretty well for himself. One of these is the reflections from the front of the eye. You know, if you look into a window in the day time, or try to look out of a window after dark, or look into a glass tumbler, or at the face of a watch, or in general, look at a glassy surface or at water, when it is lighter on your side than it is on the other, instead of seeing thru quite clearly, you see reflections from your side.

It is, naturally, the same with the glassy front of the eye. Look into another person’s eye, or into your own with a mirror, and you see reflections of windows, lamps, your own head, or any bright objects. You ought to be able to find three reflections of each bright spot. The largest, which is always right side up, is the reflection from the clear glassy front of the eye which covers the entire colored part from which we call our eyes blue or brown or gray or what not. If you look carefully, a little sideways, you will be able to get a still smaller picture, coming from the front surface of the lens of the eye, which lies just behind the round black hole in the center of the colored curtain. This also is always right side up. But there is still another, always up side down, which is the reflection from the back side of this same eye lens. These last two, you can get also by looking at a common spectacle lens, or by looking into the front of a camera.

For of course, the eye is really a little living camera. It takes a little picture like that in a camera, always upside down, at the back where the plate holder or the spool of films goes in a kodac. We can actually see this picture at the back of an animal’s eye; and what is more, people have sometimes taken out the lense of an ox’s eye, and taken a photograph with it as if it were a lens of glass. Indeed it is possible, tho the process is decidedly difficult, to take an ox’s eye from the butcher’s shop, keep it in the dark, let it look quickly at something bright, and then by treating it with the proper chemicals, actually to fix on the retina, as on a camera plate or film, the last object which the eye saw. There is a dark pigment in the retina, called the visual purple, which changes color in the light, and so forms the image.