Part 9

Five at least we are sure of, the traditional five. There is no need for anyone to tell us what our eyes, ears, and noses are for; nor that we taste with our mouths as well as talk and eat, nor that we feel touches anywhere over our skins. As to this last, however, I don’t think we always realize how completely the sense of touch is confined to the skin. Headaches, for example, we feel on the outside of the head; the brain itself can be pinched, cut, burned, and generally maltreated, and we not feel it so much as we feel a pinprick on a finger tip.

Do you remember how, a few years ago, when you had not so many things to do as you have now, you used to amuse yourself by whirling rapidly round in one direction, till you were dizzy and could hardly stand up. For a few moments, or till you whirled around in the other direction, you lost your sense of right-side-up-ness; you didn’t know sky from ground; and if you had been whirling especially fast or long, you probably fell down flat, and couldn’t get up again. When you dance, too, without reversing, by and by you get giddy, the floor begins to curl up at the edges, and you become uncertain as to which way is down.

The trouble is not with the eyes, because we get dizzy with our eyes shut. We really have a sense of up and down, which is neither sight, hearing, touch, taste nor smell; but a sixth sense. Its organ is a portion of the inner ear, the so-called labyrinth; and it tells us, not how things look or sound or taste or smell or feel, but whether we ourselves are right side up. Too much whirling upsets this sense of equilibrium, just as too bright a light dazzles the eyes, or too loud a sound stuns the ears. A shark will swim with his head cut off; but his ears being gone, he is as likely as not to swim upside down.

There, then, are six senses. Let us see if we can find number seven. While you are sound asleep at night, you do more or less turning over in bed, so that in the morning, you are lying in quite a different position from that in which you went to sleep. Yet altho you remember nothing of what you did in the night, when you awake in the morning, and before you open your eyes, you can tell exactly the position of every arm, leg, or finger. You know just how much each joint is bent. You have, in short, a sense of where the several parts of your body are, which is none of the six senses which we have recognized heretofore.

So, too, if you shut your eyes, and let some other person move any member into another position, you feel it move, and know all the while just where it is. Or suppose you are playing short-stop, and fielding a hot grounder. Your eye is on the ball, and your mind on the game; but your hands drop, and your back bends, and your knees sink, all to precisely the right degree to bring your hands where the ball is. You have a most accurate sense of where these hands are, tho you neither see nor hear nor touch nor taste nor smell them. A piano player relies on this same muscular sense, when without looking at the keyboard he skips unerringly from one note to another, never going too far nor stopping too short. In fact, this muscular sense is really one of the most accurate of all our senses. We can see a speck of dust much too small to feel; but a person used to using a microscope or doing other nice work, can make a movement with his fingers as small as the twentieth part of the width of a hair—and that is a good deal smaller than any unaided human eye can possibly see.

As you might guess from its name, this muscular sense is located partly in the muscles. Still more, however, do we depend on the joints. We do not feel one bone slip over the other, that would be plain touch; but we know with extraordinary accuracy just how far the joint has moved and just where it is at any moment.

Here then are seven senses; now for number eight. Take something not too sharp, a tooth-pick will do, or the point of a lead pencil, and touch the skin lightly on the back of the hand or on the forehead or near the elbow. You feel, of course, the touch. Now move the point ever so little, by the thickness say of a small pin, and touch again. You probably feel the same touch as before. Continue this touching, and before you have tried twenty times, you ought to discover a spot where the point feels suddenly cold. In short, some points on the skin feel pressure, which is the ordinary sense of touch. But other points feel cold, which is a different sense, an eighth sense, and not touch at all.

There is also a ninth sense, the sense of heat. If you are very careful indeed, and notice very closely, you can find heat spots, just as you found the cold spots. Only these are much harder to locate, so that you will probably not be able to make them out.

One more sense is located in the skin besides these three. That is the sense of pain. Tho to be sure other parts of the body feel pain also—the muscles and joints when we have rheumatism, and the teeth when we have the tooth ache. Pain is not touch. In the first place it feels very different, and in the second place, it is quite possible to lose one sense and keep the other. In certain diseases of the nerves, the ordinary sense of touch remains as before, but the sense of pain completely disappears. One can be pinched, cut, burned, in the most violent manner, feel the touch of fingers or knife or hot wire, and yet not feel the slightest pain. And the same thing is true, after one has taken the right amount of ether or chloroform; one isn’t asleep, and one can feel touches but one does not feel pain—and a wonderful blessing it is sometimes.

I wonder whether many of you have ever heard of cocain, or had it used on you when you got something in your eye, or had to have something done to the inside of nose or mouth. (Only you musn’t call it “co-cane,” in two syllables, as many people do and as it looks in print, but “co-ca-in,” in three syllables; for it is extracted from coca leaves, and the name is “coca,” with the “in” added on, like strychnin, atropin, protein, and the names of so many drugs and medicines). Cocain, then, looks a good deal like common salt, but fit grain or two in a drop of water, placed anywhere where the skin is thin, soaks thru to the ends of the nerves, and for ten minutes or so, puts an end to all feeling there. If now we apply cocain to the tongue in just the right strength, it takes away for the moment all sense of pain, while it leaves the sense of taste as before, and the sense of touch. One can bite his tongue and feel the bite, yet it does not hurt. One can drop hot syrup on it, taste the sweet, and yet not feel the burn. But if we make the cocain a little stronger, then when the pain goes, the taste goes with it. One can feel something in his mouth, but cannot tell sugar from salt; while a drop of strong acid, tho it burned a hole in the tongue, would neither taste sour nor hurt. A still stronger dose of the drug suspends all feeling—pain, taste, and touch alike.

So pain is a different sense from touch, just as much as taste is. Besides, if you take a sharp pin and make a line of pricks close together, much as you did when hunting for cold spots, taking pains to prick equally hard each time, you will be pretty sure to discover certain points where the prick hurts much more than it hurts the thickness of the pin away. This, of course, is a pain spot. Between these you hardly feel the pain at all.

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.

But how this image or picture gets to the mind is another question; a question, I am sorry to say, which nobody can altogether answer. We do know, however, that there are nerve endings in the retina, something like hot spots, cold spots, touch spots, and pain spots in the skin, only of course very much nearer together. Probably there are three kinds of these—red spots, green spots, and blue spots. Each spot sees one color; and by combining these colors in all sorts of ways, we build up the complicated pictures which we see. Still it is by no means impossible that there may be, not three, but six elements in our eye-pictures—white, black, red, yellow, green, and blue. Nobody really knows; and it all shows how little, after all, we have succeeded in finding out about ourselves, in spite of whole lifetimes of study of many hundreds of scientific men. Who knows but that some of you who read these pages may be the ones to discover some of these things which all the world thus far has not been able to learn.

There are still other curious facts about our sight which anybody can make out for himself. If you take any colored object, this book for example, put it behind your head; and then slowly bring it round in front, while you keep your eyes looking out steadily straight forward, you will notice certain very peculiar facts. In the first place, you will discover that you can see surprisingly far toward the back of your head. A horse can see all the way round, and if he did not wear blinders, could watch the people in the carriage behind and the road which stretches out in front, along with everything in between, all at once and about equally well. Many animals, in short, can see clear round their heads. We can’t; we can see only about half way round.

Then you will notice that you can see that something is there and moving, while it is still so far round to the side that you cannot at all make out either its shape or its color. Furthermore, you can see the color perfectly well, long before you can make out the exact shape. Indeed, you can make out the shape of ordinary letters well enough to read them only when you hold them exactly in front of the eye. The least little movement out of that one small spot mixes a whole page to a gray blur; curiously too, you can make out blue and green decidedly farther round toward the corner of the eye than you can tell red.

In short, then, we can see movement considerably farther round toward the backs of our heads than we can see color. We can see blue farther round than we can see green and green farther round than we can see red. But we cannot see shapes accurately, except right in front of our noses.

Now curiously enough, all animals that can see at all, can see something moving; tho they cannot see colors at all perfectly, nor make out the shape of anything. Many lowly sea creatures have eyes of this sort. A better kind of eye, like those of many insects, can see colors, but not make out much about shapes; while certain ants can see blue and green but are blind to red. Few indeed are the creatures that can see anything like as clearly as we see, looking hard at an object straight in front. Even a dog cannot do it, nor a horse.

XXVII

Seeing and Believing

Even we ourselves, we human beings, by no means always see so truly as we think we do. Take a look at this figure and say which of the two lines in the south-west corner continues the single line in the north-east corner. Then lay on a ruler or a strip of paper, and see which line really does run clear across the figure.

Or look at the figure below, and say which way the curved lines bend. Then take a straight-edge and test them.

Where, in this next figure, are the white bands widest, in the middle or at the ends?

Lay two strips of paper along the sides of a white band, one on each side, so as to cover up the slanting lines. Where now is the band widest?

Here is a square with some lines drawn across it. Are the lines straight? Are they the same distance apart everywhere?

Now I will add certain other lines to the same figure. Are the original lines still straight and the same distance apart everywhere?

Or look at the B’s, H’s, S’s, 3’s, and 8’s on this page, that look about the same size at top and bottom.

Then turn them up side down and see what they really are.

Or to take one more curious illusion, the lines of the figure on the next page are really flat on the paper, where the printer put them. But there is a point near the bottom of the page, about as far from the line nearest the bottom as that is from the ones nearest the top, where if you cover one eye and look at the lines with the other, each line will appear to stand straight up from the paper like a little post.

Or possibly you think your eyes always report correctly concerning colors? Then try looking at a landscape, with your head up side down, so that the view appears under your arm or between your knees. Are the colors the same as before. If not which is right?

Or try this experiment, take some brightly colored object—paper, cloth, or almost anything—in size anywhere between one and four inches across, lay it on a sheet of white paper, put the two in a strong light, and getting arm’s length or more away, stare steadily at the colored object for a half minute or so, until the eyes begin to tire. Then whisk away the colored object, continue looking at the same place, and notice what you see on the white paper, where nothing is. Or you can do what is really much the same thing, by looking at a window up against the bright sky, and after a moment turning away and shutting your eyes.

In all such experiments, one sees the outline of something that isn’t there, but in a contrasting color. We have, as you will recall, at least three sorts of color spots in the retina, red spots, blue spots, and green spots. By looking at a bright red we tire the red-seeing spots, so that everything looks blue-green. If we look hard at bright green, we tire the green-seeing nerves, and things look red-blue, which is purple. An eye tired of blue, sees yellow.

The curious thing about this is that about one man in thirty and one woman in three hundred is “born tired” to red. Such persons are said to be red blind. Otherwise they can see as well as anybody; but red things do not look colored at all. None of us can see red far round to the side out of the corner of the eye, as well as we can see green and blue. Color-blind persons have the corner of the eye all the way across, and cannot see red anywhere. They can see red things; but they cannot see them red. Railway train men and masters and pilots of vessels have to depend on red and green lights for signals. Such persons, now-a-days, are carefully tested for color-blindness; and all who cannot see red as the rest of us see it have to find some other occupation.

Why do we have two eyes? We can see outlines exactly as well with one; in fact, all the more difficult sorts of seeing, sighting a gun, using a microscope or telescope are done entirely with one eye. We can see colors exactly as well with one eye as with two. The only thing that we can’t see well with one eye is distance.

Try with both eyes open to put your finger rapidly on various spots arm’s length or so away. You can hit the mark every time. Now cover one eye—always when you want to use one eye, don’t shut the other; cover it, but keep it open. Also, by the way, if you are to use a microscope or a gun, don’t shut either eye; learn to keep both open, but to look with only one. With one eye only, then, try to put your finger rapidly on various points which you did not look at until after you had shut off the sight of the other eye. You can do it, but much less quickly and certainly than with both eyes. The nearer anything is, the more do the two eyes have to turn in, when both look at it at once. After eight or ten years of practice, as most of us have had, we learn to judge distances pretty accurately, just by the feel of this turning in.

All these peculiarities of our eyes, the judgment of distance, the different portions of the retina which see most clearly and which see colors, the various ways in which the eye is deceived, and the like, all these you can make out pretty easily for yourselves. There remains, however, one especially curious matter which you will hardly be able to discover, unless you take some little pains and follow directions pretty closely.

This is the so-called blind spot. We have, as you have seen, in each eye, a small spot in the center of the field of vision where the sight is especially sharp. This is, in fact, the only part of the eye that we can see to read with. Near this, between it and the nose, is another spot, about the same size, with which we cannot see at all.

We can prove this from the figure below.

Hold the page about a foot in front of the face, as if you were reading. Cover the right eye and look at the cross on the right, or cover the left eye and look at the dot on the left. Keep looking steadily but without too much effort, while you move the book back and forth, bringing it nearer to the face, or trying it farther away.

You should be able to find a distance at which the other mark, the one at which you are not looking, entirely disappears. You can see all round it, but the place itself is on the blind spot and is gone.

With some practice, one can make anything, not too large, disappear in the blind spot. Boys in college, when they are studying about the eye, sometimes amuse themselves in church by getting the clergyman on the blind spot, and so blotting him out. It really is queer enough. You cover one eye, and look with the other at the wall behind the preacher a little toward the side on which your own nose is. When you get just the right point to look at, the man simply disappears. You see the wall and the pulpit and the chairs or what not, on both sides. You hear the preacher’s voice. But the preacher is gone. You don’t even see a black spot where he was. Or if you are clever, you can cut off his head and leave his body; or cut off his body and leave his head hanging in the air.

All this, however, requires more control over the body and more steadiness of attention than boys and girls usually possess. I should not have told you anything about the wicked students if I were not sure that you will have forgotten all about the matter long before you get old enough to try it.

Meanwhile, don’t forget that as there is a blind spot in each of your eyes with which you simply cannot see what you know is there all the while, so there are many other things in heaven and earth which you cannot see, though they are there. Then don’t be too certain, when you happen to be blind to what other people see, that the people who do see are mistaken.

XXVIII

Some Other Senses