The Sense Organs
O nce upon a time, a grade-school teacher would ask, “How many senses do we have?” And his pupils would confidently chorus back, “Five!” People who displayed a knack for predicting future events, or whose quick reactions seemed to give them a jump over most everybody else, were credited with having a “sixth sense.”
Scientists now recognize about twice that number—12 or 13 or more. Man, of course, has not grown a number of new senses in addition to the traditional five: sight, hearing, touch, smell, and taste. What has happened is that scientists have discovered many more specific kinds of sense receptor cells. For example, whereas touch was formerly thought of as just one sense, it has now been divided into no less than five different senses, each having its own special kind of receptor cell in the skin.
We can talk with a little more justification of the five sense organs, the five anatomical structures we associate with our senses—the eyes, ears, nose, tongue, and skin. But here, too, it does seem to be oversimplifying things to thus equate the nose—having only a tiny patch of olfactory (sense of smell) receptor cells—with the marvelously complex arrangement of sensing structures that make up the eye. Moreover, the other sense organs are not nearly so specialized as the eye. For example, a good case could be made for the nose being more valuable as an air purifier than as an organ of smell, or for the tongue being more valuable as an aid in digestion than as a source of the taste sensation.
Suppose we accept the proposition that pain is one of the senses of the skin; how then do we explain a pain from inside our bodies—say, a stomachache or a deep muscle pain? The answer, of course, is that there are pain receptors in many other places besides the skin.