A Beginner's Psychology

by Edward Bradford Titchener

[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.