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 various sorts of stimuli, and the nose is kept out of function by plugging of the nostrils, we find four sensations: sweet, bitter, sour, and salt. Think, then, how much ‘taste’ there would be in the meats and vegetables that deck our tables, if the nose were closed and condiments were not added! The sensation of sweet is strongest at the tip of the tongue; bitter at the root; sour along the sides; salt is fairly evenly distributed over all three areas; the middle region of the tongue is insensitive to taste. The sensory cells are grouped in flask-shaped structures, the taste-buds or taste-beakers, which are again gathered together in or about the papillæ of the tongue’s surface; some of these you can see, as red specks upon the dull pink mucous membrane, if you look at the tip of your tongue in a glass. There is only one instance of a blend of tastes; if sweet and salt are mixed, there appears a new taste, flat or vapid in character. Apart from these five things—sweet, bitter, sour, salt, vapid,—we ‘taste’ entirely by smell or touch.
Smell, on the other hand, has more sensations than we can count or name; more sensations, probably, than all the rest of our senses put together. We can make out certain great groups of odours: flower, fruit, spicy, musky, leek, burned, rank, foul, nauseous; we may take as examples vanilla, orange, cinnamon, sandalwood, onion, toast, cheese, opium, garbage. Realise that the flower odours comprise the scents of all the flowers, as well as those of vanilla, tea, hay, and suchlike things; or that the spicy odours comprise the scents of all the spices, as well as those of thyme, geranium, bergamot, cedarwood, and suchlike things; and you will get some idea of the variety of the world of smell. When we add that odours freely blend or combine to give new scents, you will understand that the number of smell sensations is enormous.
The sensory cells are found in two patches of mucous membrane, each about as big as the little-finger nail, which lie saddle-wise across the blind top of the nasal cavities. They cannot be stimulated directly; but particles carried into the outer nostrils by the breath-stream, or into the inner nostrils by the air-stream thrown back in the act of swallowing, eddy upward to them and thus arouse sensation. The second mode of stimulation plays, of course, into the hands of taste; we think we taste when we swallow; we forget that we have inner nostrils, though we know very well that we can sniff up a lotion and bring it down into the back of the mouth. But though the stimulation is thus indirect, the cells are extraordinarily sensitive; a mere trace of odorous substance will set up a sensation; and the nose is also keenly discriminative.
Yet in spite of the tens of thousands of sensations, and in spite of the extraordinary sensitivity of the cells, we often read that in man the sense of smell is degenerating! Of this there is not one particle of evidence. We could not, truly, live by smell, as dogs do; but then men have never been dogs; and even so there are cases on record—among the Botocudos of Brazil and the aboriginal tribes of the Malay peninsula—of savage hunters who track their game by scent. There is no atom of evidence that, since man was man, his sense of smell has degenerated. It is true, on the other hand, that the sense of smell has fallen into disuse. The reason is that smell is essentially a ground sense, as you may convince yourself any summer day that you lie out on the grass, or any time that you are willing to spend a few minutes on a dining-room floor; birds in general have a very obtuse sense of smell, and many of them perhaps lack sensations of smell altogether. When, then, mankind assumed the upright position, and the nostrils were lifted several feet above the surface of the ground, the sense was removed from its normal environment, and fell into disuse; sight and hearing took its place. But it may still be used. The late Sir Francis Galton, a cousin of Darwin’s, once made an essay, for instance, at an arithmetic by smell; peppermint stood for one, camphor for two, carbolic acid for three, and so on. “There was not the slightest difficulty in banishing all visual and auditory images from the mind, leaving nothing in consciousness besides real or imaginary scents. In this way I convinced myself of the possibility of doing sums in simple addition with considerable speed and accuracy solely by means of imaginary scents. Subtraction succeeded as well as addition.” Needless to say, it is not worth our while to do this sort of work; the very fact that odours have no settled system of names, like cold or pain, red or blue, shows that they have not been utilized in human life. It is fair to add, also, that sight and hearing are better suited than smell to our everyday needs; for smells very soon fade out and disappear; indeed, if they did not, the work of garbage collectors or of medical students in the dissecting room would be permanently disagreeable.
[§ 12]. Sensations from the Ear.—Sensations of hearing fall into two great groups, tones and noises. When we are speaking of tones, we naturally think of the keyboard of a piano. The piano tones are, in reality, not simple tones or sensations but compound tones; and we are able, after a little practice, to break up a compound tone into its simple constituents. You may get a fair notion of a really simple tone by blowing gently across the mouth of an empty bottle. The tone is dull and hollow, as compared with the bright solidity of a piano tone, but it has also a pleasant mellowness. With these two aids, the bottle tone and the piano keyboard, we may approach our study of tonal sensations.