HOW SENSATIONS OCCUR

One particular physiological organ or group of organs is responsible for each of our five senses: the internal ear for sound, the retina of the eye for color, the papillae of the tongue for taste, and so on. Each of these special organs responds to a definite type of physical stimulation: the retina to vibrations of the ether, the internal ear to sound waves, the tongue to chemical solutions. Every simple psychological sensation has, then, a rather complicated causal history and it depends not only on the form of the physical stimulus but also on the peculiar make-up of its special organ.

Every physiological organ of the human body is, of course, subject to a wide range of individual differences. Some hearts are weaker than others, some livers more sluggish, and some lungs less efficient in their functions. It is only natural that the sense organs should differ also. And they do. Yet on the whole they are surprisingly uniform. All eyes react so typically to certain physical stimuli, and all noses to others, that physicists themselves call these stimuli by the names of the psychological sensations that they normally evoke. A light wave of 687 millimicrons, for example, is known as a red wave because when it impinges upon the normal retina it produces a red sensation, and a vibration of 527 is called a green wave for the same reason. A temperature of 12 degrees centigrade is marked as cold because when it stimulates the human skin it produces a cold sensation, and 45 degrees C. is hot because it produces the psychological experience of heat. Chemists are so familiar with the typical odors of certain substances that they accept these odors as invariable attributes of the substance. At times, however, our usually well behaved sense organs respond to the wrong type of stimuli and paradoxical sensations result. Neither ether vibrations nor sound waves are necessarily involved when we receive a severe blow on the head, yet such an accident can cause our ears to “ring” and our eyes to “see stars.” And a perfectly normal human being can feel a cold indistinguishable from normal cold when his skin is stimulated by a temperature of 45 degrees C. (or 113° F.). To understand this paradox it is necessary to familiarize ourselves with the anatomy and physiology of the skin.