On the other hand, there is_no evidence Jor the statement, often as it is made, that injnan jjiej^en^ejjfjmiej^ degenerating. BdtTTTn range of qualityand in discrimination of intensity (§ 66), it holds its own as against the other senses. Moreover, odours still have a high biological importance as appetisers: the smell of cooking makes the mouth water, as we say; and invalids may be tempted to eat by the aroma of the dishes set before them. The significance of smell for nutrition is masked by the fact that, in / man, stimulation of the organ from within the mouth, especially in the act of swallowing, is at least as important as its stimulation through the external nostrils; and here, as we have said, all the credit is taken by taste. Whether the sense of smell has any large share, primary or derivative, in the sexual life of man is a / disputed point. There are, no doubt, large individual differences in this regard; but, on the whole, the evidence is decidedly in the affirmative.
§ 30. The Olfactory Qualities 117 § 30. The Olfactory Qualities. — The sense of smell, like the senses of sight and hearing, includes a very large num- ber of qualities of sensation. It is impossible to say, at present, what this number is; we know too little about the world of odours to be able to undertake its systematic ex- ploration. Indeed, the number may always remain inde- terminable, since new odours are constantly added to the list. The progress of chemistry and of the arts that de- pend upon it means the continual discovery of odorous substances; and every experiment upon the cultivation of flowers and fruits may, in favourable climatic conditions, furnish a new perfume.
Under these circumstances, we can do no more than give a provisional classification of the smell qualities, based on their introspective resemblances. The following division into nine classes dates, in the main, from the great Swedish naturalist Linnaeus.
1. Ethereal or Fruit Odours. — All fruit and wine odours; the scents of the various ethers; the smell of beeswax.
2. Aromatic or Spice Odours. — All spicy smells: camphor, turpentine, cloves, ginger, pepper, bay leaves, cinnamon, cara- way, anise, peppermint, lavender, bitter almonds, rosemary, sassafras; thyme, geranium, bergamot; rosewood, cedar- wood, etc.
3. Fragrant or Flozuer Odours. — All flower scents; vanilla, tonka bean, tea, hay; gum benzoin, etc.
4. Atnbrosiac or Musky Odours. — Musk, ambergris, sandalwood, patchouli.
5. Alliaceous or Leek Odours. — Onion, garlic, asafoetida; indiarubber, dried fish, chlorine, iodine.
6. Empyreumatic or Burned Odours. — Roasted coffee, toast, tobacco smoke, tar, burned horn, carbolic acid, naphthalene, benzine, creosote.
1 1 8 Smell 7. Hircine or Rank Odours. — Stale cheese, sweat, valerian, root and stem of barberry and black currant, lactic acid, caproic acid.
8. Virulent and Foul Odours. — Opium, laudanum, French marigold, fresh coriander seeds, bed bugs, squash bugs.
9. Nauseous Odours. — Carrion flowers, stinkhorns, water from wilted flower stems, decaying animal matter, faeces.
All of these classes may be further subdivided, and in some cases the subdivisions may themselves be split up into still smaller groups. It is, however, unnecessary to go into more detail. The list is unsatisfactory, first, because there are many odours that cannot certainly be classed under any one of the nine headings; and, secondly, because the odours under certain headings (1 and 3, or 2 and 4) seem to be more nearly related than are particular odours under a single heading (2 or 6). Nevertheless, it serves to give an idea of the immense range and variety of the olfactory qualities.
Many of the stimuli mentioned in the list have sensory effects that extend far beyond the domain of smell. Thus the two ordi- nary anaesthetics, chloroform and ether, belong as scents to the ethereal group. But, further, inhaled chloroform tastes sweet, (and inhaled ether bitterj while both stimuli may, by diffusion, jive rise to sensations of cold and, by direct application, to sensa- tions of pain. Pungent odours (ammonia, pepper, mustard) arouse pricking or tingling sensations in the nose and throat. The smell of onions and of horse-radish brings tears to the eyes; in some cases, the smell of hay or of newly turned garden mould has an unpleasant effect upon breathing. Odours of the eighth and ninth classes may excite the sensation of nausea. — In view of these facts, it becomes necessary for us to raise the question of the es- sential nature of the olfactory stimulus, and of its mode of action upon the organ of smell.
§ 31. Olfactory Sensation and Olfactory Stimulus 119 §31. Olfactory Sensation and Olfactory Stimulus. — Sen- sations of smell are aroused, not by the transmission of wave-motions through air or ether, but by the actual con- tact of material particles with the sense-organ. The odor- ous particles may be given off by volatile substances in our immediate surroundings, or may be brought from a distance by currents of air. They are received into the nose in the act of inspiration: if we wish to get the full \ ^u^t fragrance of a flower, we sniff at it; so long as we hold the breath, we smell nothing. It follows that^ alljsmell stimuli must exist in the form of gas or vapour; solids and., liquids are odorous only if they are also volatile.
Sensations of smell may also be set up by way of the posterior nares: especially is this the case, as we have said, in the act of swallowing. If scented air is inhaled through the mouth, and expired through the nose, the scent will be clearly perceived, though there is some loss of intensity due to the adhesion of odorous particles to the moist lining of mouth and throat.
It is possible, though it is by no means easy, to drive out all the air from the cavities of the nose, and thus to bring an odorous liquid into direct contact with the organ of smell. Experiments of this sort have been made, but with uncertain result. Even if we grant, however, that liquid stimuli can arouse sensations of smell, it would still remain true that the normal olfactory stimulus has the gaseous form.
There can be no doubt that the action of stimuli upon the organ of smell is chemical in its nature, so that a sub- stance is odorous or inodorous by virtue of its chemical con- stitution. Many attempts have been made to express this fact in precise terms, — to discover precisely what sort of molecule is able to arouse an olfactory sensation. No sin- gle or general law has as yet been found. The following results show, however, that the prospect is not hopeless, S-Vl— «-tt 1 20 Smell In the first place, it is agreed by most investigators that the chemical elements are inodorous. True, exceptions to the rule (chlorine, bromine, iodine) at once suggest them- selves. It is probable, however, that these substances become odorous only in combination with the hydrogen of the air in the nasal cavities. If the rule holds, our field of search is so far restricted; we may neglect the atom, and turn our attention solely to the molecule. ~^~*" Secondly, all odorous substances (with one exception) are derived from the trivalent, divalent, and univalent ele- ments of the fifth, sixth, and seventh groups.1 The single exception is given with the great group of the hydro- carbons. The real odorous substance in their case may, however, be a product of oxidation. Again, therefore, if the rule holds, our field of search is restricted; we may confine our attention to molecules which contain certain elements from group V., VI., or VII.
It must be confessed that these two rules, even if strictly valid, do not take us very far. Detailed study of the chemical composi- tion of substances of like odour, and of the odour of substances of like chemical composition, does not, as yet, take us much farther. One point is worth mentioning: it has been found that the homologous series of organic chemistry furnish, within limits, series of related but progressively diverging odours, so that like- ness or difference of smell runs roughly parallel to likeness or dif- ference of chemical constitution. An illustration will make this rule clear. The series of fatty acids begins with formic (CHjCX), acetic (C2H402), propionic (CjH^CX), butyric (C4HsOL.), valerianic (C5H, „(>.), caproic (C6H1L,02). All these substances have related 1 A statement and explanation of the periodic law, and a table of the ele* ments arranged in accordance with it, will be found in any good encyclopaedia. The important elements are: V. nitrogen, phosphorus, arsenic, antimony, bismuth; VI. oxygen, sulphur, selenium, tellurium; VII. fluorine, chlorine, bromine, iodine. Their serial positions should be noted.
§32. Smell Mixtures §32. Smell Mixtures odours, which become increasingly different with increasing dis- tance between the terms of the series. Moreover, the odour, which in formic acid is weak, grows stronger and stronger as the series advances. Presently, however, the olfactory quality rather abruptly lapses: the higher acids — palmitic (C16H3202), margaric (C17H3402), stearic (C18H3602), etc. — are almost or entirely in- odorous.— As things are, theory can do little with these and simi- lar facts; but it is clear that, if such uniformities occur, a chemistry of smell must in the long run be possible of achievement.
In fine, then, we cannot correlate olfactory quality with the | configuration of the molecule, as we can correlate visual quality j with the wave-length of light, and auditory quality with the wave- / number of sound, though we may hope that some day a chemical 1 correlation will be made out.
§ 32. The Dependence of Olfactory Sensation upon the Com- position and Time-relations of Stimulus. — Sight and smell are both chemical senses. We may, therefore, expect to find a certain resemblance in the mode of behaviour of their sense-organs. How far the resemblance goes, we shall discover only by experiment; but we may safely look to sight for guidance in our first investigations of smell. — Two colours that are mixed in accordance with the first or second law of colour mix- ture either neutralise each other or produce a new, in- termediate colour. What happens if we mix two odours?
We may proceed in two ways: we may conduct the odours separately to the two nostrils, by means of the olfactometer; or — if chemical combination does not occur FIG. 18. Double Olfactometer (solid stimuli).
Smell — we may make a mechanical mixture of the odorous sub- stances before smelling. In both cases, we obtain results analogous to those got by the mixture of colours.
First, there are undoubtedly odours which, if mixed in the right proportions, neutralise each other. Bridal bouquets often have gardenia mixed with their orange- blossoms, in order that the aromatic scent may weaken the too powerful fragrance. Tooth-powder of orris root is used to counteract the foetor ex ore. In medical practice, and in the operating room, recourse is had to this principle of compensation: balsam of Peru offsets the smell of iodoform, and carbolic acid the stench of pulmonary gan- grene. Laboratory experiments yield the same result: the odour of red india-rubber, for instance, neutralises the odours of cedarwood, gum benzoin, paraffin, beeswax, tolu balsam, etc.
Secondly, there are odours which, if mixed in the right proportions, give rise to a resultant odour, a new olfactory quality. Most of us have noticed that the addition of a few fragrant leaves to a bunch of flow- ers may alter the scent of the whole bouquet; that the mixture of two toilet perfumes may produce a perfume different from either; that the attempt to overpower a foul or nauseous odour by a perfume will sometimes set up a scent more Double Olfactometer (liquid stimuli).
sickening than the first. Laboratory experiments bear out this conclusion: new odours arise, for example, from the mixture of musk and opium or listerine, iodine and ylang ylang or camphor, valerianic acid and lavender or hya- cinth. In all such cases the resultant odour is simple and unanalysable; it resembles the component odours, but it cannot be resolved into them.
Whether odours have a constant mixing value, inde- pendent of their mode of origin, — whether, that is, we have in smell an analogue of the third law of colour mix- ture, — cannot be said with certainty. The trend of evi- dence appears to be towards the affirmative.
The likeness between these results and those of colour mixture is evident. Nevertheless, there are striking differences. Thus, smell mixtures are, in general, much less stable than colour mix- tures. Experiment shows that two odours rarely neutralise each other completely foTmore than a lew seconds; it is easy to obtain an unsaturated odour otThe quality of the stronger component, butm^t easy to get actual extinction^ This seems to mean that C the^chemical equilibrium of the olfactory cells is less stable than that /of the retinal cones. In the same way, the new odour resulting from a twofold mixture is often transitory in character, giving place either to the odour of a single component or to an oscilla- tion of the two. This is due, in many instances, to the fact that the sense-organ becomes more quickly adapted to the one stimulus than to the other, or that the substances mixed are not equally volatile; but in others it also seems to point to a chemical insta- bility of the olfactory cells. Resultant odours of a more per- manent kind may be secured by the mixture of a number of components. The flower perfumes of the perfume industry are, as a rule, quite complicated mixtures: heliotrope, for example, is derived from the mixture of vanilla, rose, orange-flower, ambergris and almond.
Again, it is impossible to draw a sharp line of division between 124 Smell complementary odours and odours that combine. We should naturally expect that the members of the same or of related classes would mix, and that the members of diverse classes would cancel one another. Some ten years ago, a statement to this effect would have found support in the composition of toilet per- fumes, in pharmaceutical practice, and in the results of psycho- logical experiment. Recent work has proved, however, that no such rule can be laid down: odours of the second and eighth classes, for instance, may combine as readily as odours within either group, and odours taken from the same class may behave as complementaries. Evidently, there is in smell no such clean- cut principle of antagonism as we have found in sight.
The fact of adaptation to stimulus is, perhaps, more in evidence in the case of smell than it is even in that of vision. Odours of the most insistent kind fade out, if only the stimulation is kept up without intermission, in N/Wa comparatively short time. Workers in tanneries, /J^ cheese warehouses and fish markets, garbage collectors, jl^ habitual smokers, patients with iodoform dressings, medi- J cal students in the dissecting room, — these persons are, as a rule, quite unconscious of the odours that surround them. All of us have, probably, at one time or another, been asked to go into a certain room and "see if we don't smell fire," and have noticed that, after a few vigorous sniffs, we were wholly unable to say whether we did or did not. Laboratory experiments simply make these observations more precise. Thus, heliotrope be- comes inodorous if smelled for about 5 min.; asafoetida in i£ min.; stale cheese in 8 min.; and so on.
Here, too, there are marked differences between smell and sight. The fading out of a given sensation does not mean the arousal ot its complementary; there is no negative after-image of smell; adaptation to india-rubber does not leave us with a scent of cedarwood or tolu or beeswax. The effect of adaptation is simply to increase our sensitivity for certain odours, and to re- duce or destroy it for others. Thus, it has been found that a partial adaptation to cedarwood or tolu or beeswax renders the nose more sensitive to the smell of india-rubber, while partial adaptation to glycerine soap or cocoa butter or Russian leather has no such effect. On the other hand, adaptation to iodine leaves us insensitive to the odour of eau de Cologne, absolute alcohol, heliotrope, oil of caraway. In this way, a continuous adaptation of the sort mentioned above may materially change the world of odours: the user of perfumery, the smoker, the hospital attendant, will be peculiarly susceptible to certain scents and peculiarly obtuse to others. There is, of course, a possibility that the sense of smell as a whole may be blunted by the repeated application of the same stimulus.1 § 33. Theory of Smell. — The organ of smell2 is ex- tremely simple. It consists of a patch of brownish mu- cous membrane, not much larger than one's little-finger nail, which lines the roof and part of the walls of the extreme upper portion of the nasal cavities. This ter- minal pouch is so narrow and so remote that the air current of respiration does not reach it; the olfactory epithelium can be stimulated only by diffusion or by eddies from the main stream. The olfactory cells are set amongst columnar supporting cells; they are very slender, possess a large nucleus, and are prolonged peripherally as rod-shaped processes ending between the columnar cells at the free surface of the epithelium.
1 It is said, in recent text-books, that smokers possess only about \ of the normal sensitivity to odours. The statement is apparently taken from H. Griesbach, who in 1899 published a comparative study of the senses of hearing, smell and touch in the blind and the seeing (Archiv f. d. gesammtt Physiologic, lxxiv., 577; lxxv., 365, 523). But Griesbach worked only with india-rubber!
2 The author uses the Deyrolle model, Coupe mediane du nezgrossi.
1 26 Smell This simplicity of structure suggests at once that the organ of smell must respond to olfactory stimulus in the same sort of way as the eye to light, and not as the ear to sound. For every sensation of tone we find a separate structure in the cochlea. On the other hand, all the sensations represented in the colour pyramid are derived from the six antagonistic processes in the cones {Bk- W, B- Y, R-G) and from the cortical grey. Our day- light vision, rich as it is in sense qualities, depends simply upon four chemical reactions, three reversible and one constant. Now a theory which, like the theory of vision, reduces the manifold of psychological elements to a small number of elementary psychophysical processes is termed a theory of components. Black, white, grey, and the in- variable R, G, B and Fare the components of our visual theory: psychophysically, they are the elements of vision, though psychologically they are no more elementary than orange or violet or purple. It is important to bear this distinction in mind.
We may expect, then, that the right theory of smell will be a component theory. This expectation is borne out by the fact, already mentioned in § 32, that adaptation to a particular odour leaves us insensitive to some, while it does not impair our sensitivity to other odours. The odours that are killed by adaptation to iodine, for instance, evidently require for their arousal the same psychophysi- cal processes: they stand to iodine in much the same re- lation that rose, lilac, mauve, heliotrope, purple bear to violet. If, therefore, we could work over the whole range of olfactory qualities, and find out which are weakened or blotted out, and which are left intact, by adaptation to the various odours taken singly, we might hope to discover the psychophysical elements of olfactory sensation. In- deed, the programme need not be made so comprehensive: if we could work systematically with even a few odours, selected from all the nine classes and their recognised sub- divisions, it is probable that the outlines of a component theory would emerge from our results.
The work is, however, exceedingly laborious, and con- sumes a great deal of time. Something has been done; very much more remains to do. It has been calculated, on the basis of our present knowledge, that 30 or 40 spe- cific chemical processes must be assumed for the sense of smell, — many more than for daylight vision. It is unlikely that there are 30 or 40 kinds of olfactory cells. But whether there are, say, 10 sorts of cells, each the seat of 3 or 4 processes, or 3 or 4 sorts of cells, each the seat of 10 different chemical processes, we have no possible means of deciding.
While these phenomena of adaptation afford the strongest sup- port to a component theory, they do not by any means stand alone. It is clear that the results of smell mixture — resultant odours and compensations — point in the same direction, as does also the mere fact that odours may be grouped, by their intro- spective resemblances, into a number of distinct classes. Further evidence comes from pathology. In cases of partial anosmia, which occurs both as congenital defect and as the consequence of influenza, diphtheria, etc., the patient is insensitive to some and sensitive to other odours: thus, the musky odours or the vanilla- group of the fragrant odours may be destroyed or weakened, while all the rest persist in their normal character. Cases of par- osmia, or subjective perversion of the sense of smell, fall into similar groups, which, so far as they have been investigated, ap- pear to correspond with the fourth, fifth, sixth and ninth olfac- tory classes.
128 Smell References for Further Reading §§ 29-33. Die Physiologie des Geruc/is, 1895, by H. Zwaardemaker, professor of physiology at Utrecht; J. Passy, Revue generate sur les sensations otf actives, in Annie psychologique, ii., 1896, 363 ff.; W. Wundt, Physiologische Psychologies ii., 1902, 46 ff.; W. Nagel, Der Ge- ruchssinn, in Nagefs Handbuch, iii., 1905, 589 ff.
TASTE § 34. The Gustatory Qualities. — For the most part, sen- sations of taste come to us blended with sensations of smell, touch and temperature. These blends have a curiously unitary character: it is only by directing the attention, in the light of past experience, first to one and then to another aspect of the given whole, that we can distinguish the separate components. Thus the flavour of a peach, or of black coffee, seems to be simple and unique; but we may happen to notice the aroma before we begin to taste, and in this way take an involuntary first step towards analysis. At times, the difference be- tween smell and taste comes to us with a sort of shock; the bitter taste of unsweetened chocolate, for instance, is in sharp contrast to the aromatic odour. Again, we may remark that our food to-day is more savoury than it was yesterday, when our nose was stopped up with a cold; or we may discover that the repulsive flavour of certain ■** medicines, such as castor oil, is avoided by the simple expedient of holding the nose. In all these cases, and in many others like them, everyday experience plays into the hands of psychological analysis. Smell and taste are, after all, separate senses with separate sense-organs; and while a blending of their sensations is the rule, occasions are bound to arise when we taste without smelling or smell without tasting.
Taste There is no such natural separation of taste from touch and temperature. It is, however, not difficult to observe that in oily and fatty tastes we have something that is precisely like the feel of greasy fingers, and in pungent and biting tastes something that is precisely like the prick- ing of pungent odours in the nose or the bite of mustard- plaster upon the skin. The cold of ice-cream in the mouth is the same as the cold of icy water to the hands; and when a too hot soup scalds the tongue we have — apart from the impairment of taste itself — the same sensations as when we step into a too hot bath. Having made these observations, we are able to single out, by the attention, the touch and temperature components in ordinary tastes.
If the taste-blends are thus analysed, and the foreign constituents referred to the sense-departments to which they properly belong, there remain only four qualities of taste: sweet, bitter, sour and salt. Here is poverty in- deed, as compared with the wealth of sight, hearing and smell! — and a poverty all the more striking, since taste makes so brave a show of variety in everyday life.
This result depends, not only upon introspective analysis of the taste-blends, but also upon a systematic exploration of the organ of taste with very various kinds of stimulus. Before the experimental tests were made, the lists of gustatory qualities put forward by dif- ferent authors were, as we should expect, widely different. It would, however, be a mistake to suppose that they have grown steadily shorter as analysis has advanced. None of them are very long. Smell, indeed, seems to have been practically eliminated al- most from the outset, though some physiologists speak of aromatic tastes, foul tastes, etc., and it is odd that the rule of holding the nose during experiments on taste was laid down for the first time by the French chemist M. E. C'hevreul as late as 1824. On the other hand, the touch and temperature components evidently :1 §35. Gustatory Sensation and Gustatory Stimulus 131 presented great difficulty. We find oily tastes, pungent tastes, smooth tastes, astringent tastes, etc., figuring in the scheme of taste qualities; and contrariwise we find sour and salt transferred, on ac- count of their astringent and burning character, from the sense of taste to that of touch. Here, then, are give and take, addition and subtraction: Linnaeus brought the number of tastes up to 10, but a recent investigator1 who reduces them to 2 (sweet and bittei) is merely repeating what had been said sixty years earlier.2 It was long supposed that nausea is a taste quality; this view was taken, for instance, by so great a man as Johannes Mtiller, the father of modern physiology, on the ground that the sensation aroused by pressure on the base of the tongue — putting your fin- ger down your throat — cannot be identified with any quality of touch.3 At the present time, many psychologists incline to the view that the alkaline and the metallic tastes must be regarded as elementary qualities of taste; but tests made with the nose close prove that the irreducible factor in both cases is due to smell. — If smell is ruled out, the ordinary taste-blends may be analysed as follows. Sour is at first_astringent; rhpr^jis it hemmes stronger, burning; finally, purely painful. Sail- is^ attended hy a weak hnr.ii- ing, which. rlnes ppt rise to positive pain. Sweet brings with it the perception of smoothness and softness; _at high intensities of stim- ulus, it pricks or gives a sharp burn. Bitter suggests something fatty; at high intensities it may burn.
§ 35. Gustatory Sensation and Gustatory Stimulus. — In order to be sa-plcf, a substance must be, to some extent, soluble in the saliva of the mouth. If this condition is ful- filled, it may exist in any form, as solid or liquid, vapour or gas.
There are, however, soluble substances which are taste- 1 W. Sternberg, Geschmack und Chemismus, in Zeitschrift f. Psychologie u. Physiologie d. Shinesorgane, xx., 1899, 387.
3 Handbuch der Physiologie des Menschen, ii., 1840, 489.
/ 132 Taste less. We are thus thrown back, as in the case of smell, upon the question of chemical constitution, and must try to work out a correlation between stimulus and sensation in chemical terms. Now chemistry uses the terms salt, acid, sugar as class-names for related groups of compounds. All three words — as well as the phrase 'bitter principles,' which is employed in pharmacy and in organic chemistry — are borrowed from the sense of taste; and we can say off- hand, from ordinary experience, that acids generally taste sour, salts salty, and sugars sweet. A little enquiry brings out the further fact that the bitters with which we are most familiar are alkaloids. Can we, then, correlate the four taste qualities with these four types of chemical com- bination?