CHAPTER II: Sensation

Now that these points have been determined, let us proceed to a general discussion of the whole subject of Sensation.—Aristotle

§ 9. Sensations from the Skin.—The skin is part of our organic birthright. One of the great differences between the living and the not-living lies in the possession of a skin; stone and iron weather and rust, but even the naked amœba has its ectosarc, and flowers of tan their plasmoderm. The skin is also the oldest of the sense-organs, and the mother of all the rest; how old, we dare hardly guess; but we know that the chemical elements which make up living tissue took form early in the history of our planet, earlier than the heavy metals. So it is natural to begin our survey of sensations by questioning the skin.

The skin is a shifty witness; and to get positive answers, we must literally cross-examine it; we must go over its surface point by point and line by line, with all sorts of mechanical and thermal and electrical and chemical stimuli. The outcome is a little surprising; we find only four sensations, pressure, cold, warmth and pain. The organs of these sensations are dotted in a sort of irregular mosaic all over the skin, and the intervening spaces are insensitive. The organs of pressure, distributed over about 95% of the bodily surface, are nerve-skeins twined about the roots of the hairs; on the hairless areas of the body, we find the nerve-skein by itself. The organ of pain is probably a little brush-like bunch of nerve-fibrils just below the epidermis. The organs of warmth and cold are certainly distinct; the sensations are not degrees of one sensation, as the thermometer might lead us to suppose; but the precise nature of their nerve-endings has not yet been made out.

You may easily find pressure spots by fastening a short horsehair with sealing-wax at right angles to the end of a match, and applying the horsehair point to the back of the hand above a hair-bulb, that is, just to windward of the issuing hair; dot the horsehair about, here and there, till the sensation flashes up. You may find cold spots by passing the blunt point of a lead pencil slowly across the closed eyelid. Warm spots are more difficult to demonstrate. For pain, take the shaft of a pin loosely between finger and thumb of the right hand, and bring the point down sharply on the back of the left hand; you get two sensations; the first is a pressure, the second—which pricks or stings—is a pain.

As a rule, these organs are not stimulated separately but in groups. Itch, for instance, is due to the light stimulation of a field of pain-endings, and superficial tickle to that of a field of pressure-organs. The experience of heat, curiously enough, is a blend of warmth and cold; there are no heat spots. It may be observed in this way: if you apply a surface of increasing warmth to a region of the skin which has both cold and warm spots, you feel for some time only the warmth; but when the stimulus has reached a certain temperature, the cold spots, suddenly and paradoxically, flash out their sensations of cold; and the blend of warmth and of paradoxical cold is felt as heat. Cement a smooth copper coin to a handle, and apply it at gradually increasing temperature to the middle of the forehead just under the hair; you will presently find the heat. Or if you cannot do that, note the shiver of cold when you next step into an overhot bath.

When we compare these results with the show that the skin makes as a sense-organ in everyday life, we can hardly help bringing against it the charge of dishonesty. The pressure spots give us tickle, contact or light pressure, and pressure proper; the pain spots, itch, prick or sting, and pain proper. The cold spots give cold and cool, the warm spots lukewarm and warm; cold and warm spots together give heat; cold and pain give biting cold; cold and warm and pain give burning or scalding heat; and that is all. Yet the skin pretends to tell us of hard and soft, wet and dry, light and heavy, rough and smooth, yielding and resistant, sharp and blunt, clammy and greasy, oily and sticky, stiff and elastic, and so on. Where do we get all these experiences?

§ 10. Kinæsthetic Sensations.—We get them, for the most part, from the cooperation with the skin of certain deeper-lying tissues. Psychologists have long suspected the existence of a muscle sense. We now know that sensations are derived, not only from the muscles, but also from the tendons and the capsules of the joints. These tissues are, of course, closely bound together, and are all alike affected by movement of a limb or of the body. Their disentanglement, from the point of view of sensation, has been a slow and difficult matter. Psychology has here been greatly aided by pathology; for there are diseases in which the skin alone is insensitive, in which skin and muscles alone are insensitive, and in which the whole limb is insensitive; so that a first rough differentiation is made for us by nature herself. It is also possible artificially to anæsthetise muscle and joint; and psychologists have devised various forms of experiment whereby some single tissue is thrown into relief above the others.

Not only, however, are the sensations of these tissues aroused by movement; they also form the sensory basis of our perception of the movement of body and limbs. For this reason they have been named kinæsthetic, or movement-perceiving. They are of the following kinds.

First, we have from the muscles the sensation of physical fatigue. If the skin over a muscle is rendered anæsthetic, and the muscle is thrown into forced contraction by an electric current, we have, to begin with, a dull dead pressure; as time goes on, or if the strength of the current is increased, this pressure becomes dragging, the sensation of fatigue; and finally it becomes sore and achy, and passes over into dull pain. From the tendons we get a sensation which, when we are actively pushing or pulling, we call effort, and when we are passively holding or resisting we call strain; it, too, passes over into pain. Lastly, from the joints we have a pressure: something like the pressure you feel if you smear the right forefinger with vaseline, and turn it in the loosely closed left hand. Take a piece of elastic between the forefingers and thumbs; pull it out, and then relax it; at the moment of relaxation there is a pressure in the finger-joints, which is the specific joint-sensation.

Muscle and joint, then, yield sensations which are like those of pressure on the skin; and muscle and tendon yield sensations which are like those of pain from the skin; it is small wonder that the skin, the only portion of this whole sensory apparatus that is open to view, should ordinarily be credited with the entire number. In point of fact, there are very few of the experiences listed on p. 45 that do not imply the cooperation of some or all of the deeper-lying organs, the nerve-spindles of muscle and tendon and the nerve-corpuscles of the joints. Those that really belong to the skin owe their specific character to the context in which they are set; they change their meaning as a particular word changes its meaning from one sentence to another; think of the horribly clammy feel of a bit of cold boiled potato as you set your finger on it in the dark, and of its totally different feel when you have turned the light on and see what it is you are touching! Wetness, for instance, proves on analysis to be a complex of pressure and temperature; it is possible, when the observer does not know the nature of the stimulus, to arouse the feel of wet from perfectly dry things, such as powder, or cotton wool, or bits of metal; and it is possible to wet the observer’s hand with water and yet to arouse the feel only of a dry pressure or a dry warmth or cold.

So our very first adventure in psychology brings out, as clearly as we need wish, the difference between science and common sense. The skin is really living upon borrowed capital; it has added to its own sensations those derived from the subjacent tissues; but common sense, blind to what it cannot see, ascribes to it a ‘sense of touch’ that includes everything and examines nothing. More than this, common sense fails to draw the distinction between process and meaning which we discussed in § 6, and therefore ascribes to the sense of touch a variety of sensory experience that far outruns the facts. Hardness and softness and stickiness and oiliness and the rest are, no doubt, separate and distinct as meanings; but when we analyse the corresponding experiences, we find only the half-dozen sensations mentioned above.

§ 11. Taste and Smell.—The great physiologist Carl Ludwig once remarked that smell is the most unselfish of all the senses; it gives up everything it has to taste, and asks nothing in return. Taste is, indeed, an inveterate borrower; it borrows from smell and from touch, very much as the skin borrows from the underlying organs. When we have a cold in the head, we say that we cannot taste; but how is taste affected? The truth is that our nose is stopped, and we cannot smell.

If the surface of the tongue is explored with...