All are familiar with the "five senses" of our elementary physiologies, sight, hearing, taste, smell, and touch. A more complete study of sensation reveals nearly three times this number, however. This is to say that the body is equipped with more than a dozen different kinds of end-organs, each prepared to receive its own particular type of stimulus. It must also be understood that some of the end-organs yield more than one sense. The eye, for example, gives not only visual but muscular sensations; the ear not only auditory, but tactual; the tongue not only gustatory, but tactual and cold and warmth sensations.

Sight.—Vision is a distance sense; we can see afar off. The stimulus is chemical in its action; this means that the ether waves, on striking the retina, cause a chemical change which sets up the nerve current responsible for the sensation.

The eye, whose general structure is sufficiently described in all standard physiologies, consists of a visual apparatus designed to bring the images of objects to a clear focus on the retina at the fovea, or area of clearest vision, near the point of entrance of the optic nerve.

The sensation of sight coming from this retinal image unaided by other sensations gives us but two qualities, light and color. The eye can distinguish many different grades of light from purest white on through the various grays to densest black. The range is greater still in color. We speak of the seven colors of the spectrum, violet, indigo, blue, green, yellow, orange, and red. But this is not a very serviceable classification, since the average eye can distinguish about 35,000 color effects. It is also somewhat bewildering to find that all these colors seem to be produced from the four fundamental hues, red, green, yellow, and blue, plus the various tints. These four, combined in varying proportions and with different degrees of light (i.e., different shades of gray), yield all the color effects known to the human eye. Herschel estimates that the workers on the mosaics at Rome must have distinguished 30,000 different color tones. The hue of a color refers to its fundamental quality, as red or yellow; the chroma, to its saturation, or the strength of the color; and the tint, to the amount of brightness (i.e., white) it contains.

Hearing.—Hearing is also a distance sense. The action of its stimulus is mechanical, which is to say that the vibrations produced in the air by the sounding body are finally transmitted by the mechanism of the middle ear to the inner ear. Here the impulse is conveyed through the liquid of the internal ear to the nerve endings as so many tiny blows, which produce the nerve current carried to the brain by the auditory nerve.

The sensation of hearing, like that of sight, gives us two qualities: namely, tones with their accompanying pitch and timbre, and noises. Tones, or musical sounds, are produced by isochronous or equal-timed vibrations; thus C of the first octave is produced by 256 vibrations a second, and if this tone is prolonged the vibration rate will continue uniformly the same. Noises, on the other hand, are produced by vibrations which have no uniformity of vibration rate. The ear's sensibility to pitch extends over about seven octaves. The seven-octave piano goes down to 27-1/2 vibrations and reaches up to 3,500 vibrations. Notes of nearly 50,000 vibrations can be heard by an average ear, however, though these are too painfully shrill to be musical. Taking into account this upper limit, the range of the ear is about eleven octaves. The ear, having given us loudness of tones, which depends on the amplitude of the vibrations, pitch, which depends on the rapidity of the vibrations, and timbre, or quality, which depends on the complexity of the vibrations, has no further qualities of sound to reveal.

Taste.—The sense of taste is located chiefly in the tongue, over the surface of which are scattered many minute taste-bulbs. These can be seen as small red specks, most plentifully distributed along the edges and at the tip of the tongue. The substance tasted must be in solution, and come in contact with the nerve endings. The action of the stimulus is chemical.

The sense of taste recognizes the four qualities of sour, sweet, salt, and bitter. Many of the qualities which we improperly call tastes are in reality a complex of taste, smell, touch, and temperature. Smell contributes so largely to the sense of taste that many articles of food become "tasteless" when we have a catarrh, and many nauseating doses of medicine can be taken without discomfort if the nose is held. Probably none of us, if we are careful to exclude all odors by plugging the nostrils with cotton, can by taste distinguish between scraped apple, potato, turnip, or beet, or can tell hot milk from tea or coffee of the same temperature.

Smell.—In the upper part of the nasal cavity lies a small brownish patch of mucous membrane. It is here that the olfactory nerve endings are located. The substance smelled must be volatile, that is, must exist in gaseous form, and come in direct contact with the nerve endings. Chemical action results in a nerve current.

The sensations of smell have not been classified so well as those of taste, and we have no distinct names for them. Neither do we know how many olfactory qualities the sense of smell is capable of revealing. The only definite classification of smell qualities is that based on their pleasantness or the opposite. We also borrow a few terms and speak of sweet or fragrant odors and fresh or close smells. There is some evidence when we observe animals, or even primitive men, that the human race has been evolving greater sensibility to certain odors, while at the same time there has been a loss of keenness of what we call scent.

Various Sensations from the Skin.—The skin, besides being a protective and excretory organ, affords a lodging-place for the end-organs giving us our sense of pressure, pain, cold, warmth, tickle, and itch. Pressure seems to have for its end-organ the hair-bulbs of the skin; on hairless regions small bulbs called the corpuscles of Meissner serve this purpose. Pain is thought to be mediated by free nerve endings. Cold depends on end-organs called the bulbs of Krause; and warmth on the Ruffinian corpuscles.

Cutaneous or skin sensation may arise from either mechanical stimulation, such as pressure, a blow, or tickling, from thermal stimulation from hot or cold objects, from electrical stimulation, or from the action of certain chemicals, such as acids and the like. Stimulated mechanically, the skin gives us but two sensation qualities, pressure and pain. Many of the qualities which we commonly ascribe to the skin sensations are really a complex of cutaneous and muscular sensations. Contact is light pressure. Hardness and softness depend on the intensity of the pressure. Roughness and smoothness arise from interrupted and continuous pressure, respectively, and require movement over the rough or smooth surface. Touch depends on pressure accompanied by the muscular sensations involved in the movements connected with the act. Pain is clearly a different sensation from pressure; but any of the cutaneous or muscular sensations may, by excessive stimulation, be made to pass over into pain. All parts of the skin are sensitive to pressure and pain; but certain parts, like the finger tips, and the tip of the tongue, are more highly sensitive than others. The skin varies also in its sensitivity to heat and cold. If we take a hot or a very cold pencil point and pass it rather lightly and slowly over the skin, it is easy to discover certain spots from which a sensation of warmth or of cold flashes out. In this way it is possible to locate the end-organs of temperature very accurately.

Fig. 17.—Diagram showing distribution of hot and cold spots on the back of the hand. C, cold spots; H, hot spots.

The Kinæsthetic Senses.—The muscles, tendons, and joints also give rise to perfectly definite sensations, but they have not been named as have the sensations from most of the other end-organs. Weight is the most clearly marked of these sensations. It is through the sensations connected with movements of muscles, tendons, and joints that we come to judge form, size, and distance.

The Organic Senses.—Finally, to the sensations mentioned so far must be added those which come from the internal organs of the body. From the alimentary canal we get the sensations of hunger, thirst, and nausea; from the heart, lungs, and organs of sex come numerous well-defined but unnamed sensations which play an important part in making up the feeling-tone of our daily lives.

Thus we see that the senses may be looked upon as the sentries of the body, standing at the outposts where nature and ourselves meet. They discover the qualities of the various objects with which we come in contact and hand them over to the mind in the form of sensations. And these sensations are the raw material out of which we begin to construct our material environment. Only as we are equipped with good organs of sense, especially good eyes and ears, therefore, are we able to enter fully into the wonderful world about us and receive the stimuli necessary to our thought and action.

4. PROBLEMS IN OBSERVATION AND INTROSPECTION

1. Observe a schoolroom of children at work with the aim of discovering any that show defects of vision or hearing. What are the symptoms? What is the effect of inability to hear or see well upon interest and attention?

2. Talk with your teacher about testing the eyes and ears of the children of some school. The simpler tests for vision and hearing are easily applied, and the expense for material almost nothing. What tests should be used? Does your school have the test card for vision?

3. Use a rotator or color tops for mixing discs of white and black to produce different shades of gray. Fix in mind the gray made of half white and half black; three-fourths white and one-fourth black; one-fourth-white and three-fourths black.

4. In the same way mix the two complementaries yellow and blue to produce a gray; mix red and green in the same way. Try various combinations of the four fundamental colors, and discover how different colors are produced. Seek for these same colors in nature—sky, leaves, flowers, etc.

5. Take a large wire nail and push it through a cork so that it can be handled without touching the metal with the fingers. Now cool it in ice or very cold water, then dry it and move the point slowly across the back of the hand. Do you feel occasional thrills of cold as the point passes over a bulb of Krause? Heat the nail with a match flame or over a lamp, and perform the same experiment. Do you feel the thrills of heat from the corpuscles of Ruffini?

6. Try stopping the nostrils with cotton and having someone give you scraped apple, potato, onion, etc., and see whether, by taste alone, you can distinguish the difference. Why cannot sulphur be tasted?

CHAPTER VII

PERCEPTION

No young child at first sees objects as we see them, or hears sounds as we hear them. This power, the power of perception, is a gradual development. It grows day by day out of the learner's experience in his world of sights and sounds, and whatever other fields his senses respond to.

1. THE FUNCTION OF PERCEPTION

Need of Knowing the Material World.—It is the business of perception to give us knowledge of our world of material objects and their relations in space and time. The material world which we enter through the gateways of the senses is more marvelous by far than any fairy world created by the fancy of story-tellers; for it contains the elements of all they have conceived and much more besides. It is more marvelous than any structure planned and executed by the mind of man; for all the wonders and beauties of the Coliseum or of St. Peter's existed in nature before they were discovered by the architect and thrown together in those magnificent structures. The material advancement of civilization has been but the discovery of the objects, forces, and laws of nature, and their use in inventions serviceable to men. And these forces and laws of nature were discovered only as they were made manifest through objects in the material world.

The problem lying before each individual who would enter fully into this rich world of environment, then, is to discover at first hand just as large a part of the material world about him as possible. In the most humble environment of the most uneventful life is to be found the material for discoveries and inventions yet undreamed of. Lying in the shade of an apple tree under the open sky, Newton read from a falling apple the fundamental principles of the law of gravitation which has revolutionized science; sitting at a humble tea table Watt watched the gurgling of the steam escaping from the kettle, and evolved the steam engine therefrom; with his simple kite, Franklin drew down the lightning from the clouds, and started the science of electricity; through studying a ball, the ancient scholars conceived the earth to be a sphere, and Columbus discovered America.

The Problem Which Confronts the Child.—Well it is that the child, starting his life's journey, cannot see the magnitude of the task before him. Cast amid a world of objects of whose very existence he is ignorant, and whose meaning and uses have to be learned by slow and often painful experience, he proceeds step by step through the senses in his discovery of the objects about him. Yet, considered again, we ourselves are after all but a step in advance of the child. Though we are somewhat more familiar with the use of our senses than he, and know a few more objects about us, yet the knowledge of the wisest of us is at best pitifully meager compared with the richness of nature. So impossible is it for us to know all our material environment, that men have taken to becoming specialists. One man will spend his life in the study of a certain variety of plants, while there are hundreds of thousands of varieties all about him; another will study a particular kind of animal life, perhaps too minute to be seen with the naked eye, while the world is teeming with animal forms which he has not time in his short day of life to stop to examine; another will study the land forms and read the earth's history from the rocks and geological strata, but here again nature's volume is so large that he has time to read but a small fraction of the whole. Another studies the human body and learns to read from its expressions the signs of health and sickness, and to prescribe remedies for its ills; but in this field also he has found it necessary to divide the work, and so we have specialists for almost every organ of the body.

2. THE NATURE OF PERCEPTION

How a Percept is Formed.—How, then, do we proceed to the discovery of this world of objects? Let us watch the child and learn the secret from him. Give the babe a ball, and he applies every sense to it to discover its qualities. He stares at it, he takes it in his hands and turns it over and around, he lifts it, he strokes it, he punches it and jabs it, he puts it to his mouth and bites it, he drops it, he throws it and creeps after it. He leaves no stone unturned to find out what that thing really is. By means of the qualities which come to him through the avenues of sense, he constructs the object. And not only does he come to know the ball as a material object, but he comes to know also its uses. He is forming his own best definition of a ball in terms of the sensations which he gets from it and the uses to which he puts it, and all this even before he can name it or is able to recognize its name when he hears it. How much better his method than the one he will have to follow a little later when he goes to school and learns that "A ball is a spherical body of any substance or size, used to play with, as by throwing, kicking, or knocking, etc.!"

The Percept Involves All Relations of the Object.—Nor is the case in the least different with ourselves. When we wish to learn about a new object or discover new facts about an old one, we do precisely as the child does if we are wise. We apply to it every sense to which it will afford a stimulus, and finally arrive at the object through its various qualities. And just in so far as we have failed to use in connection with it every sense to which it can minister, just in that degree will we have an incomplete perception of it. Indeed, just so far as we have failed finally to perceive it in terms of its functions or uses, in that far also have we failed to know it completely. Tomatoes were for many years grown as ornamental garden plants before it was discovered that the tomatoes could minister to the taste as well as to the sight. The clothing of civilized man gives the same sensation of texture and color to the savage that it does to its owner, but he is so far from perceiving it in the same way that he packs it away and continues to go naked. The Orientals, who disdain the use of chairs and prefer to sit cross-legged on the floor, can never perceive a chair just as we do who use chairs daily, and to whom chairs are so saturated with social suggestions and associations.

The Content of the Percept.—The percept, then, always contains a basis of sensation. The eye, the ear, the skin or some other sense organ must turn in its supply of sensory material or there can be no percept. But the percept contains more than just sensations. Consider, for example, your percept of an automobile flashing past your windows. You really see but very little of it, yet you perceive it as a very familiar vehicle. All that your sense organs furnish is a more or less blurred patch of black of certain size and contour, one or more objects of somewhat different color whom you know to be passengers, and various sounds of a whizzing, chugging or roaring nature. Your former experience with automobiles enables you to associate with these meager sensory details the upholstered seats, the whirling wheels, the swaying movement and whatever else belongs to the full meaning of a motor car.

The percept that contained only sensory material, and lacked all memory elements, ideas and meanings, would be no percept at all. And this is the reason why a young child cannot see or hear like ourselves. It lacks the associative material to give significance and meaning to the sensory elements supplied by the end-organs. The dependence of the percept on material from past experience is also illustrated in the common statement that what one gets from an art exhibit or a concert depends on what he brings to it. He who brings no knowledge, no memory, no images from other pictures or music will secure but relatively barren percepts, consisting of little besides the mere sensory elements. Truly, "to him that hath shall be given" in the realm of perception.

The Accuracy of Percepts Depends on Experience.—We must perceive objects through our motor response to them as well as in terms of sensations. The boy who has his knowledge of a tennis racket from looking at one in a store window, or indeed from handling one and looking it over in his room, can never know a tennis racket as does the boy who plays with it on the court. Objects get their significance not alone from their qualities, but even more from their use as related to our own activities.

Like the child, we must get our knowledge of objects, if we are to get it well, from the objects themselves at first hand, and not second hand through descriptions of them by others. The fact that there is so much of the material world about us that we can never hope to learn it all, has made it necessary to put down in books many of the things which have been discovered concerning nature. This necessity has, I fear, led many away from nature itself to books—away from the living reality of things to the dead embalming cases of words, in whose empty forms we see so little of the significance which resides in the things themselves. We are in danger of being satisfied with the forms of knowledge without its substance—with definitions contained in words instead of in qualities and uses.

Not Definitions, But First-hand Contact.—In like manner we come to know distance, form and size. If we have never become acquainted with a mile by actually walking a mile, running a mile, riding a bicycle a mile, driving a horse a mile, or traveling a mile on a train, we might listen for a long time to someone tell how far a mile is, or state the distance from Chicago to Denver, without knowing much about it in any way except word definitions. In order to understand a mile, we must come to know it in as many ways as possible through sense activities of our own. Although many children have learned that it is 25,000 miles around the earth, probably no one who has not encircled the globe has any reasonably accurate notion just how far this is. For words cannot take the place of perceptions in giving us knowledge. In the case of shorter distances, the same rule holds. The eye must be assisted by experience of the muscles and tendons and joints in actually covering distance, and learn to associate these sensations with those of the eye before the eye alone can be able to say, "That tree is ten rods distant." Form and size are to be learned in the same way. The hands must actually touch and handle the object, experiencing its hardness or smoothness, the way this curve and that angle feels, the amount of muscular energy it takes to pass the hand over this surface and along that line, the eye taking note all the while, before the eye can tell at a glance that yonder object is a sphere and that this surface is two feet on the edge.