Unfortunately, the rule has puzzling exceptions even in the cases of sour and salt. It is true that we get the taste of salt only from chemical salts: but there are chemical salts that taste sweet, others that taste bitter, and others again that have no taste at all. It is also true, apparently, that we get the taste of sour only from chemical acids, or from substances that contain acids: but there are acids that taste sweet, acids that are tasteless, and at least one acid (hydrocyanic) which is said to taste bitter. The suggestion has been made that the sour taste of the majority of acids may be accounted for by their ionisation in aqueous solu- tion, — may be ascribed, that is, to the setting free of the common ion hydrogen. Hydrocyanic acid is only partially ionised, and the tasteless fatty acids like palmitic, stearic and oleic are insoluble in water. — There is undoubtedly a close chemical relation between sweet and bitter; a very slight change of chemical constitution will change the one taste into the other. The groups of sweet-tasting and fitter-tasting substances are, however, extremely hetero- geneous.
In fine, then, much detailed work is needed, in taste as in smell, before any general law of the correlation of stimulus and sensation can be made out.
It has been pointed out that the inorganic sweet- tasting sub- stances are derived from elements of the III., IV. and V. groups, and that these elements are, so to say, double-faced, since they combine with acids as bases and with bases as acids to form salts. On the other hand, the inorganic bitter-tasting substances are derived from electropositive elements of the I. and II., and from electronegative elements of the VI. and VII. groups. Here is the hint of a principle, which may perhaps be carried over to the organic compounds, and it has, in fact, been maintained that all the sweet-tasting organic substances have this double, ± -character, while all the bitter-tasting — though closely related to them — have either the p/us or the minus sign. If the rule holds, we can readily understand that a slight change of the sweet-generating molecule will transform it into a generator of bitter. However, it is best to be on one's guard against premature generalisation.
§ 36. Mixtures and Adaptations. — We know, from every- day experience, that certain tastes are more or less antagonistic. Sugar moderates the bitter taste of coffee^ and chocolate, and the sour taste of unripe fruit_ Sour and salt offset each other, to a certain extent, in sour pickles and salad dressings. Salt corrects the too luscious sweetness of an overripe melon. On the other hand, bitter and salt may exist side by side, as in the taste of olives; and bitter and sour, as in that of a green peach.
Observations of this sort are, however, unsatisfactory. First of all, the act of eating or drinking brings the taste- stimulus into contact with the whole surface of the tongue. If, then, the particular stimulus contains salt and bitter, Taste we may have the sensation of salt set up at a part of the tongue which is especially sensitive to salt, and the sensa- tion of bitter at another part, especially sensitive to bitter: the two qualities will thus appear side by side, just as a blue and a yellow may appear side by side in the field of vision. To obtain assured results we must apply the mixed stimulus at one and the same point. Secondly, the jour taste-qualities require different_times for their arousal: salt comes first, then sweet, thensom^jmd, bitter.last. It is quite possible that, in ordinary life, these time-differences escape notice, so that we may regard two tastes as occur- ring together when really they occur in succession. And thirdly there is no guarantee, under the conditions, that stimuli are mixed in the right proportions. For all these reasons, we must have recourse to experiment.
A careful study of taste-mixtures in the laboratory brings out the following facts. With high intensities of stimulus, the two tastes seem not to influence each other; they simply oscillate. With low intensities, there is in most cases a partial compensation, which is least for sweet and sour, better for salt and bitter, better still for sour and bitter, sour and salt, and sweet and bitter. The antago- nism is not so clean-cut as it is in the case of sight; we rarely, if ever, obtain actual mmtrali^tion of two compen- satory tastes. Only in one instance, the mixture of sweet and salt, is there any reminder of the second law of colour mixture. If salt is added, little by little, to a weak sweet, there presently emerges a taste which is neither salt nor sweet, but flat and vapid.
We remarked in § 34 upon the curiously unitary character of the taste-blends. It is worth noticing here that the unitariness persists in spite of the antagonistic nature of the taste qualities.
§ 36. Mixtures and Adaptations 135 Think, for instance, of the flavour of a ripe peach. The ethereal odour may be ruled out by holding the nose. The taste com- ponents— sweet, bitter, sour — may be identified by special direc tion of the attention upon them. The touch components — tho softness and stringiness of the pulp, the puckery feelof the sour — may be singled out in the same way. Nevertheless, all these factors blend toother s^ i"<-i»T^tply th^t it ig \\*r<\ to ^\VP. i]p m^'c: belief in a peculiar and unanalysable peach- flavour- Indeed, some psychologists assert that this resultant flavour exists; that in all such cases the concurrence of the taste qualities gives rise to a new, basic or fundamental taste, which serves so to say as back- ground to the separate components. There is, however, no need to make any such assumption. It is an universal rule in psy- ch ologyjbjuy_\yijen sense-qualities combine to form what js called, a, perception, the_ result of their combination is not a sum but,- system, not a patchwork but a pattern. The parts of a locomo- ' tivetorm a system; the colours of a carpet form a pattern: in neither case is there a mere heaping together of materials. The same thing holds of perception. Hence, just as it would be absurd to say that the plan of the locomotive is a new bit of steel, or the pattern of the carpet a new bit of coloured stuff, so is it wrong to say that the peach-character of a certain taste-blend is a new taste quality. This character shows us the pattern of the blend, the specific way in which the components are arranged; it is not itself a sensation. — We shall return to the general question It was pointed out in § 29 that there is an intrinsic likeness be- tween the sensations of taste and of smell. This fact is clearly recognised in pharmacy. Thus, we are advised to take castor oil or cod-liver oil in claret or lemonade; the sour taste corrects the nauseating or hircine odour. Quinine, whict^tostesJjiHej; and has no soqell, is corrected by essence of orange peel, which has an aromatic smell and no taste In all --oris of children's medicines, a disagreeable odour is offset by a sweet taste, or a disagreeable taste by some pleasant odour. The result obtained is, of course, only partly due to the cancellation of sensations. When a child has fallen down and hurt itself, we try to turn its attention to A 1 36 Taste A 1 36 Taste something else: we tell it a fairy story, or give it a lump of sugar, and the crying stops. The same principle, of distraction of atten- tion from the unpleasant to the pleasant, plays its part in these medicinal mixtures. On the other hand, adults are less suggestible than children, and the corrections hold for us as for them; while an attempt to offset the stench of castor oil, say, by a popular melody or a comic picture would strike us as laughable. Un- doubtedly, then, sensations of taste and smell are sufficiently alike to exert a direct influence upon one another. This conclusion loses much of its strangeness if we remember that, phylogenetically, 1 taste and smell are simply two departments of a single chemical sense, the one differentiated for the reception of liquid, and the (other for that of gaseous stimuli.
Adarjtationto tastes is less obvious than adaptation to odours. It seems that the organ_of taste is more resistant, chemically more stable, than the organ of smffll. Apart frorh-this, however, our attention, in eating and drinking, is largely taken up with the ^mpl] anH tmirh components of the taste blends. Besides, we usually have at hand the materials (salt, sugar, vinegar, etc.) for raising the inten- sity of taste stimuli; we reach instinctively for the salt- cellar or the vinegar cruet as soon as we miss the taste of salt or sour. Nevertheless, there are times when the fact of adaptation stands out clearly enough. An orange that would taste sweet at the beginning of a meal tastes un- pleasantly sour if we take it after a sweet pudding. A stock soup that is at first disagreeably salt gets better after the first few spoonfuls. If we have the courage to attack a plate of early strawberries without sugar, we soon grow accustomed to the acid. — These observations are confirmed by the results of experiment.
In smell, the effect of adaptation is to increase our sensitivity for certain qualities, and to reduce or destroy § 36. Mixtures and Adaptations 137 it for others. Ln^taste^ where there are but four qualities, the negative result of adaptation is generally confined to the quality of the stimulus itself: adaptation to bitter weakens or abolishes the taste of bitter, but leaves the rest at least as strong as they were before. There are, how- ever, exceptions to this rule. If the tongue is painted with a fitting solution of cocaine hydrochlorate, we lose first the quality of bitter, and then that of sweet; if it is painted with gymnemic acid, we lose first the quality of sweet and then that of bitter; in both cases, salt and sour persist. The action of these substances upon the end-organs has not been explained. — The positive results of adaptation must be stated with some reserve, since there are great individual differences among observers. It seems, however, that adaptation to any one of the three tastes sour, sweet, salt affects the remaining two: a foregone sour, for instance, enhances a present sweet or a present salt, and so on.
The sense of taste appears, further, to show phenomena c^Jt of contrast, more or less akin to those of vision. A sour applied to the one side of the tongue brings out, for certain persons, the taste of a subliminal sweet applied to the other side. Contrasts may also be obtained, in laboratory practice, between salt and sour, and salt and sweet. On the other hand, subliminal bitter, applied at the same time as sweet, sour or salt, is usually sensed, if at all, as sweet; and supraliminal bitter is, from the first, strong and insistent.
Nothing is definitely known about after-images of taste. Many sweet stimuli leave a bitter taste in the mouth; but this may be due to the chemical relationship of the sweet-tasting and the bitter-tasting substances of which we spoke in § 35. It is note- 138 Taste 138 Taste worthy that, to many persons, distilled water, an intrinsically tasteless stimulus, tastes distinctly bitter j to others it may taste sour or sweet. Various explanations have been suggested. The taste may be simply an after-effect of adaptation: the mouth is never entirely free from particles of food. Or it may possibly result from the merely mechanical stimulation of the end-organs, just as a flash of light results from mechanical pressure on the eyeball. Or, again, it may be an associative process, an idea or, as it were, an illusion of taste. This and many similar points in the psychology of taste still await explanation.
§ 2>7- Theory of Taste. — The description given of the olfactory cells holds also for the specialised sensory cells which form the end-organs of taste: they are long, slender rod-cells, with large nucleus, set among supporting cells of the same sort as the columnar cells of the olfactory mucous membrane. The rod-cells are not, however, irregularly distributed between the supporting cells; they are gathered together into flask-shaped structures, which are known as the taste-buds or taste-bulbs. At the centre of the bulb stands a group of rod-cells, intermixed with a few supporting cells; the rod-processes converge periph- erally to the pore of the bulb. Next comes a wrapping of supporting or cover cells; while the outer wall of the bulb is composed of epithelial cells of special form.
The taste-bulbs occur in greatest numbers in the trenches surrounding the circumvallate papillae at the root of the tongue.1 They occur also along the edges of the tongue, posteriorly in the folds of the regio foliata and anteriorly in the fungiform papillae; and at the tip again in the 1 The author uses the Deyrolle model, La langue rue Ju cot'- droit. He knows of no models that show the sense-organs of nose and tongue in enlarged vertical section. The models must therefore l>e supplemented by charts (i?.^., th i>e of VVenzel's Anaiomischer Handatlas) or lantern slides.
fungiform papillae, the bright red specks which can be seen standing out from the dull pink of their surroundings. The central area of the surface of the tongue is insensitive to taste. In general, the root of the tongue is especially sensitive to bitter, the tip to sjveet, and the rmddle^ection of the edges to sour.
The distribution of the end-organs of taste in man shows great individual differences. In adult life, functional taste- bulbs are found on the surface of the tongue, with the exception of a central area of varying size, and on the soft palate; less constantly on the arches and veil of the palate and on the uvula; rarely on a portion of the hard palate. They also occur, curiously enough, in the interior of the larynx and on the epiglottis, regions that are not normally stimulated by sapid substances. Their presence here, and on the superior (posterior) surface of the veil of the palate, accounts however for the sweet taste of inhaled chloroform and the bitter of inhaled ether (§ 30). In children, the taste-bulbs \ extend over the whole surface of the tongue, and are also found in | the mucous membrane of the cheeks, — facts that explain, per- haps, the childish tendency to take big mouthfuls.
Why the central area of the tongue should lose its sensitivity, in adult life, is not easy to say. It is clear, however, if we consider the mechanics of chewing and swallowing, that there must be a stagnation of sapid liquid at the back and on the edges of the tongue; and it is clear that this stagnation is assisted by the trenches about the circumvallate papillae and by the folds of the regio foliata. These, then, are the important regions for tasting. They are more important even than the tip of the tongue, which / merely samples the substances that enter the mouth; and we find, / as a matter of fact, that insensitivity of the tip not infrequently t coexists with normal sensitivity of root and sides.
The reduction of the organ has analogies in other departments of sense. In hearing, for instance, the range of sensation is di- minished; the highest audible tone is more than an nrtnye higher. in childhood than in old age. We may suppose that the shortest 140 Taste 140 Taste fibres of the basilar membrane gradually lose their elasticity. In smell, again, the acuity of sensation is diminished; children are much more sensitive to odours than adults, though they do not appear to have a wider range of qualities. It is possible that, in the course of years, the olfactory mucous membrane is coated with minute particles of dust, etc., so that the cells are less easily stimulated.
The papillae of the same region do not all react in the jSame way to gustatory stimuli. Thus, of 39 fungiform papillae, stimulated with salt, sugar, hydrochloric acid and quinine, 4 proved to be wholly insensitive, while 31 were sensitive to sweet, 31 to salt, 29 to sour, and 21 to bitter; one was sensitive only to sweet, and one only to bitter. It seems probable, then, that there are four kinds of taste- bulbs, each one sensitive to a single quality of taste; and that all of these, or three, or two, or only one, may be /present in a given papilla.
This hypothesis agrees with the observed facts of taste mixture and of adaptation, and is also borne out by the pathological cases of loss of taste; the ageusia may be complete, or may affect some qualities more than others. On the other hand, it is impossible to say whether the distinction of the four classes of taste-bulbs is absolute. There are no anatomical differences that might help us to a decision. We must suppose that the substance of the taste cells has been chemically differentiated, for the reception of the , different forms of stimulus; but we cannot say whether the special- isation of function has been carried to the same point in all taste- bulbs.
It has been suggested that the extreme sourness of the orange eaten after sweet pudding is due to a contrast of feelings; the sour after the sweet is more unpleasant than a sour standing alone. Even, however, if we grant — and the point is more than doubtful — that contrast between feelings occurs, introspection shows that the sour quality is itself intensified; and the explanation is therefore to be sought in the sphere of sensation. The sweet-sensitive bulbs have been put out of function by adaptation to the sweet of I the pudding, so that the mixed, sweet-sour stimulus affects only / the sour-sensitive bulbs. Hence the orange naturally tastes sourer than it does under ordinary circumstances, when the sweet y and sour components are able in some measure to offset each other.
It is more difficult to account for the fact that a sour, etc. ap-i. plied to the one side of the tongue brings out the taste of a sub-U'\__ liminal sweet, etc. applied to the other side. We may, of course, challenge the fact itself. Liquids are apt to run on the surface of the tongue, and it is conceivable that the stimuli used in the ex- periments spread across the middle line. We know, however, that the terminal radiations of the n. lingualis which supply the one half of the tongue extend across the middle to the opposite half. The two groups of taste-bulbs, though locally distinct, are thus brought into connection at the periphery by their common nerve supply. — Nothing definite is known of the order of development of the taste qualities. A good deal has been made of the fact that sweets cloy and bitters whet the appetite, while salts provoke and sours quench thirst. This, however, is no argument for an original four- fold differentiation of the sense of taste: appetite is governed largely by smell. Some authors regard salt as a quality of late develop- ment, on the ground that the word refers to a particular substance, while sweet, bitter and sour are general terms, and that children and uneducated persons often confuse salt with sour. But we find that many primitive languages have no distinctive word for bitter; that some languages use the same word for sweet and salt; and that uneducated persons may also confuse bitter and sour! We have also seen that sour and salt have similar effects upon the organs of touch (§ 34). In view of the unitariness of the taste- blends, it is not surprising, then, that the two qualities should be confused by persons unskilled in introspection.
Taste References for Further Reading §§ 34~37- W. Wundt, Physiologische Psychologies ii., 1902, 52 ff.; C. S. Myers, Taste, in Reports of the Cambridge Anthropological Expedi- tion to Torres Straits, II., ii., 1903, 186 ff.; The Taste-names 0/ 1 Primitive Peoples, in British Journal of Psychology, i., 1904, 117; H. Zwaardemaker, Geschtnack, in K. Asher and L. Spiro, Ergebnisse der Physiologie, II., ii., 1903, 699; W. Nagel, Der Geschmackssinn, ml NagePs Handbnch, iii., 1905, 621 ff.
CUTANEOUS SENSES § 38. The Skin and its Senses. — In popular parlance, touch is ranked as a fifth sense beside sight and hearing, taste and smell, and the organ of touch is the skin. Neither the sense nor its organ is very strictly defined. We may say, however, that the word skin denotes the whole mem- branous investment of the body; it includes not only the skin proper, but also the red area of the lips, the lining of the cavities of mouth and nose, the conjunctiva and cornea of the eye. In so far as this surface is not occupied by organs of special sense, such as taste and smell, it represents the organ of touch. Hence the name touch applies to all the sensations aroused by contact of the bodily surface with objects of the material world. A thing is hard, soft, warm, cold, painful to the touch; it is by touch that we distinguish wet and dry, light and heavy, rough and smooth, yielding and resistant, sharp and blunt, clammy and greasy, motion- less and moving.1 Exception is made, again, only of those properties, such as odour and sapidity, that appeal to special senses. And here, naturally, there is no attempt at analy- sis; the sting of pungent odours and the bite of pungent tastes — qualities that really belong to touch — are referred to smell and taste themselves.
1 It is, of course, as plain to common sense as it is to psychological obser- vation that these distinctions are often drawn in terms of sight; we see that a thing is wet or heavy or in motion. What is now under discussion, how- ever, is the feel of objects that are actually in contact with the skin.
144 Cutaneous Senses Our experience with timbre and with the blends of smell and taste puts us on our guard in the present instance; we shall not, without question, accept wet and dry, rough and smooth, etc. as ultimate qualities of tactual sensation. Instead, however, of working through the list in detail, we may at once clear up a confusion that inheres in the pop- ular notion of touch, — the confusion between sensations from the skin and sensations from the tissues that lie beneath the skin. Pick up a pen from the table, or give a swing to your revolving bookcase, or try to open a window that has swelled with the rain: in every case you will find that the sensations from the skin are blended with internal sensations. Popularly, the whole complex is attributed to the sense of touch; psychologically, the two sets of sensa- tions are different and must be referred to separate senses.
Suppose, however, that the subcutaneous senses are ruled out: the question still remains whether the skin itself is the seat of one or of more than one sense. And introspection favours the second alternative. There is no resemblance, for example, between the hard smoothness and the chill of a bit of ice, or between the rough brittleness and the warmth of new-made toast; we may safely mark off the sense of touch from the sense of temperature. Nor is there, when we reflect upon it, any resemblance between touch and pain. Indeed, the two kinds of sensation are distinct in time as well as in quality: if we dip the hand into very hot water, or take up a very hot plate, we sense the contact appreci- ably earlier than the pain. It would seem, then, that the sense of touch must also be marked off from the sense of pain.
To go beyond this point, we must have recourse to ex- periment. The surface of the skin must be explored, § 38. The Skin and its Senses 145 accurately and minutely, with all sorts of stimuli, — me- chanical, thermal, electrical, chemical, — and the sensations which it yields must be described and classified. Much work of this sort has been done, and the psychology of the skin, though still unsettled in sundry details, has thus been put upon a firm basis. — It is found, first, that the surface of the skin is not uniformly sensitive. Sensations can be obtained only from definite spots or points; the remaining area is insensitive. The spots are fixed in their position, so that they always respond in the same way to the same stimulus; they un- doubtedly indicate the presence, in the substance of the skin, of separate sense-organs. It is found, secondly, that these spots are of four distinct kinds: they furnish the sensations of pressure, of warmth, of cold, and of prick. In other words, there are four cutaneous senses, — those of pressure, or of touch in the narrowest sense, of warmth, of cold, and of pain. We will take them in order.
What is ordinarily called the sense of touch thus turns out to be highly composite, — a mixture of the sensations derived from four cutaneous and from a number of subcutaneous senses. It follows, of course, that the terms used in current speech to denote qual- ities of touch are not directly available for psychology. We have words like pressure, contact, prick, sting, soreness, smart, ache, and we must somehow make them serve our scientific purpose. But the selection is not easy, and the same term may, as a matter of fact, mean different things in different books. Hence it is important that the descriptions given in these Sections be verified by actual test: the instruments required are exceedingly simple, and their manipulation is straightforward. The particular name will then stand for a special bit of elementary experience, a par- ticular feel; the analyses of cutaneous complexes will be analyses of concrete sense-material, and not a mere play of words; and, on L 146 Cutaneous Senses the other side, it will be possible by introspective reference to follow the accounts given by authors who employ a different terminology.
§ 39. The Pressure Sense. — If with the point of a pencil you brush one of the hairs that are sparsely scattered over the back of the hand, you obtain a weak sensation, of bright quality, which is somewhat ticklish, and which though thin and wiry yet has a definite body. This sen- sation, which we may term the sensation of contact, is physiologically a weak sensation of pressure. Wherever the skin carries hair, — that is, over about 95 per cent. of the cutaneous surface, — the hair-bulb is the organ of the pressure sense.
Pressure sensations may be studied systematically by means of the horse-hair point shown in Fig. 20. If you look at a hair on the ^1 back of the hand, you will notice that the shaft runs ob- liquely into the skin.
FIG. 20. Horse-hair point, for exploration of the.
cutaneous surface. A piece of horse-hair, about JUSt to Windward 01 2 cm. in length, is attached by sealing-wax to the ^ne hair direct! V Over end of a match.
the bulb, lies a pres- sure spot, which is easily found by a few trials with the horse-hair point. By applying the horse-hair to the pres- sure spot, with different degrees of pressure, it is possible to call out the pressure sensations at different degrees of intensity. You get, first of all, the wiry, bright sensation of the former experiment. As the pressure is increased, the sensation too becomes heavier, more solid: at times it has about it something springy, tremulous, elastic; at § 39- The Pressure Sense 147 times it appears simply as a little cylinder of compact pressure. Finally, at still higher intensities, the sensa- tion becomes granular: it is as if you were pressing upon a small hard seed embedded in the substance of the skin. The granular sensation is often tinged with a faint ache, due to the admixture of a pain sensation; and is sometimes attended by a dull, diffuse sensation derived from the sub- cutaneous tissues. It may, however, appear as pure pres- sure sensation.
If the hairless regions of the skin are explored with the horse-hair point, pressure spots will be discovered which yield the same sensations as the hair-bulbs. The organs of pressure are here to be found in very similar structures, known as the corpuscles of Meissner.
The end-organs of pressure may be stimulated by pressure from without, by traction or pull, and by the wrinkling or stretching of the skin itself. They respond, that is, to any decided change of the local level of pressure, whether the change is positive or negative, rise or fall. They do not, all alike, furnish the graded series of sensations which we have just described, but are, so to say, tuned to different intensities of stimulus, so that a pressure which evokes the granular sensation at one spot may call out only the weak, bright sensation from another. There is, however, no further difference in the nature of their response; all the sensa- tions of pressure belong to this single series.
Pressure spots are found over practically the whole extent of the skin. Their distribution differs in different regions. On the average, there are about 25 of them to the square centimetre; but this number may drop, for instance, to 7 on the upper arm, and may rise to 300 on the scalp.