R. W. Moncrieff, B.Sc., F.R.I.C. THE CHEMICAL SENSES l : "• l ',• . .• �. :.. ,-... "·;. � . THE CHEMICAL RUBBER co ClfVElANO. OHIO © R. W. Moncrieff, 1967 First published 1944 Reprinted in U.S.A. 1946 Second Edition 1951 Third Edition 1967 TO MOTHER / Set in 10 on 12 pt. Times New Roman and printed by Neill & Co. Ltd., Edinburgh on paper supplied by Inveresk Paper Merchants. Bound by Mansell (Book­ binders) Ltd., London. Contents Page CHAPTER I. The chemical senses-their part in life Taste and Smell, Fundamental Aims of Animals, Hunting, Evasion of Dangerous Enemies, Poisoning, The Smells and Tastes of Poisons, Nutritional Wisdom, Nutrition, Repro­ duction, The Influence of Civilization, JEsthetic Enjoyment, Religion, Medicine, Chemistry, Hygiene, Perspiration, Public Health, Water Supplies, Unwanted Odours in Foods, Industrial Odours, Activated Carbon: a Deodorizer, Odour as a Warning, War Gases, Terminology. CHAPTER 2. Structure of the chemical sense organs and their connections 39 Location of the Receptors, The Nose, The Olfactory Region, The Olfactory Nerve, Electron Microscopy of the Olfactory Region, The Olfactory Bulb, Taste Receptors, The Taste Nerves, Electron Microscopy of the Gustatory Region, The Common Chemical Sense Receptors, The Vomero-Nasal Organ, Comparison of Olfactory, Gustatory and Chemical Sense Receptors. CHAPTER 3. Sensation Irritability. Reflex Action, Classification of Sensations, Dif­ 66 ferentiation, The Nervous Impulses, Fechner's Law, Reaction Time, Adaptation, Consciousness, Integration, The Berger Rhythm, EEG Records, Depth Encephalography, Olfactory Memory, The Salivary Reflex, Unlike Reactions, The Pattern of Sensation. CHAPTER 4. Olfaction Respiration, The Stimulus, The Sorptive Nature of the 122 Stimulus, The Mechanics of Adsorption, Abnormal Stimuli, Human Acumen, Minimum Stimulus, Fatigue, Selective Fatigue, Odour Likeness, Self-and Cross-Adaptation, Odour Intensity as a Property of Matter, Comparisons of Unlike Intensities, Zwaardemaker's Concept of Intensity, Counter­ action, Masking, Dilution, Different Responses, Drugs, Ap­ praisal of Odour, Chemical Change in the Body, Anosmia, Parosmia, Pigment, Olfactory Apparatus of Pike, Mechanism of Olfaction in Catfish, Olfactory Reactions in the Brain of the Hedgehog, The Electro-Olfactogram, The Pattern of Smell. V CONTENTS Page vi CHAPTER 5. 'The measurement of odour 208 Intensity, Zwaardemaker's olfactometer, Fournie's Olfacto­ scope, The Allison and Katz odorometer, The Fair and Wells Osmoscope, Elsberg's Blast Injection Method, The Stream Injection Method, Wagenaar's olfactometer, Barail's Osmo­ meter, Pressure Osmometer, The Pendant Drop Method, Adsorption Oxidation Cell, Tanyolac;'s Electro Odo-Cell, Adsorption Smell Detector, Comparison of Instruments. CHAPTER 6. Gustation 245 The Four Tastes, The Sour Stimulus, The Salt Stimulus, The Sweet Stimulus, The Bitter Stimulus, Minimum Stimuli, Abnormal Stimuli, Masking, Taste Blindness, Physiological, Fundamental Substrates of Taste, Electrical Responses in the Nerve Fibres, Temperature, Critical Frequency, Temporal Patterns of Taste Responses, Theories, Enzyme Theory of Taste. CHAPTER 7. The common chemical sense 298 Function, The Stimulus, Reactions, A Separate Sense, Classi­ fication of Stimuli, Pungent Spices, Lachrymatories, Sternuta­ tories, Suffocants, Skin Irritants, Liquid Stimuli, Differentia­ tions, Theories. CHAPTER 8. Chemical sensibility in lower animals 322 Animal Classification, Protozoa, Sponges, Crelenterates, Platyhelminthes, Mollusca, Annelids, Crustaceans, Arachnids, Insects, Fishes, Amphibia, Reptiles, Birds, Mammals, Com­ parative Reactions to Sugars. CHAPTER 9. Classification of odours General, Rimmel's Classification, Zwaardemaker's Classifica­ 371 tion, Henning's Olfactory Prism, Crocker and Henderson's Arrangement, Amoore's Stereo-chemical Classification, Various. CHAPTER 10. Chemical constitution and odour 384 General, The Elements, The Electrochemical Series, Inorganic Compounds, Hydrocarbons, Alcohols and Phenols, Ethers, Carboxylic Acids, Esters, Aldehydes, Acetals, Ketones, Ketals, Anhydrides, Lactones, Halides, Amines, Nitrogen Com­ pounds, Sulphur Compounds, Heterocyclic Compounds, Macrocyclic Compounds, Ionones, Methyl Ionones, Structural Requirements for the Violet Odour, Irones, Steroids, Steroids as Olfactory Sex Attractants, Vitamins, Mixed Compounds, Isomerism, Stereoisomerism, Conclusions. CONTENTS Page vii General, Salty Compounds, Sour Compounds, Sweet Com­ pounds, Bitter Compounds, Alkyl Groups, The Phenyl Group, CHATPhTe EHRyd ro11xy. l GTraoustpe, Tanhed Acloknosxtyi tGurtoiounp, Esters, The Nitro 486 Group, The Amino Group, Nitramines, Amides, Hydrazides, Ureas, Nitrogen Compounds, The Halogens, Sulphur Com­ pounds, Saccharin, Dulcin, Suosan, Homologues, Isomerism, Stereoisomerism, Taste of Perfumes, Conclusions. Vapour Pressure, Solubility, Infra-Red Absorption, Ultra­ CHAPTER 12. Physical properties of odorous 544 Violet Absorption, Ultra-Violet Irradiation, The Raman mEffaetcetr, iaSlpsr ay Electricity, Talc Scattering, Diffusion, Dia­ magnetism, Dilution, Adsorption. Province of Odour Theories, Wave or Particle?, Master Theories, Olfactory Facts, Ogle, Woker, Fabre, Heyninx, CHAMPaTrcEhRan d1, 3.H enTnhinego, riBesa cokfm oand,o uDru rrans, Zwaardemaker, 568 Niccolini, Dyson, Wright's Vibrational Theory, Beck and Miles, Enzyme Theory of Odour, Rainer's Information Theory, Davies's Puncturing Theory, Moncrieff, Summary, Molecular Limits. Essential Oils, The Garden of Provence, Expression, Distil­ lation, Enfleurage, Extraction, Adsorption; Concrete, CHAAPbTsoEluRte , 14Is.o laPtee; rfAutmtare s oaf nRd oessesse, nOciel s of Neroli, Oil of 601 Lavender, The Tuberose, Oil of Cloves, Ylang-Ylang Oil, Table of Essential Oils, Resins, Odorous Constituents of Oils and Resins, Variation in Odour of Natural Products, Green Leafy Odours, Animal Perfumes, Ambergris, Civet, Musk, Synthetic Musk, Castor, Fixatives, Scent, Perfume Com­ pounding in Bulk, Aerosols. Flavour Factors, Water, Beverages (other than Alcoholic), Alcoholic Beverages, Bread, Fruits, Vegetables, Foods of CHAAPnTimEaRl O 1ri5g.i n, FSwlaeveotsu, rS painceds, fTooobda cco, Choice of Flavour, 650 Colour and Flavour, Measurement of Spoilage of Foodstuffs, Removal of Undesirable Flavours, Packaging Odours. 699 AUTHOR INDEX 717 SUBJECT INDEX CHAPTER 1 THE CHEMICAL SENSES-THEIR PART IN LIFE Taste and smell Man is commonly said to be endowed with five senses, sight, hearing, touch, taste and smell. Of these, sight is known to be caused by light waves or corpuscles, whilst hearing and touch are caused by pressure differences. Sound waves in the air impinge on the ear and bring about rapid changes in pressure on a diaphragm which vibrates and transmits the sensation of hearing to the mind; touch is caused by pressure applied to practically any part of the skin. Taste and smell, however, are not induced by any known waves in the "ether" or in the air, but are caused by contact in the case of taste and nearness in the case of smell of certain specific substances. Such a substance as salt, sugar, vinegar, quinine, wine or a rose-petal was believed to have a chemical action on the sensory organs in the nose and mouth, hence we spoke of taste and smell as the chemical senses. A more precise analysis of the senses shows that smell may be divided into two modalities: first, smell as of roses, night-scented stocks, or mignonette or the less attractive odours of cooking cabbage or the glue works; secondly an irritation which affects notonlythe nose, but also the mouth, eyes, anus and genital apertures, in fact any part of the mucous membrane of the body with which the irritating sub­ stance can come into contact. This second sense is spoken of as the common chemical sense and is set in action by such irritants as ammonia or acid fumes. In the ancestry of man the common chemical sense was the first of the chemical senses, and taste and smell are developments or differentiations of it, which, however, occurred at a very early stage of development, somewhere amongst the inverte­ brates. These considerations will be examined in greater detail later, for in this first chapter it is proposed to consider the part played in life by taste, smell and the irritating chemical sense. Fundamental aims of animals Biologically the fundamental aims of animal life are (I) self-preservation; (2) preservation of the species. THE CHEMICAL SENSES 2 It is easy to see how taste and smell have been developed to serve these aims. Self-preservation involves hunting for food, the rejection of poisonous or unsuitable food and escape from dangerous enemies. Preservation of the species requires the survival of some individuals until sexual maturity is reached, the meeting of the two sexes, fertili­ zation by the male either of the female or of the eggs she has laid, in many cases incubation of the eggs and in the higher animals care of the offspring until they can fend for themselves. The part played by the chemical senses in these activities of life may now be considered. Hunting Whether man hunts for the cinema or for the cooking-pot the neces­ sity of approaching the quarry without giving warning of approach is still the same, and most difficult does the hunter find it to arrange matters so that he is not perceived by smell, although well able to avoid being heard or seen by the animal. This does not apply if man is hunting man as in warfare, for smell is much less sensitive in man generally than in animals, and even aboriginal trackers rely more on woodcraft than on smell. Man is said to be microsmatic, keen­ scented animals are said to be macrosmatic. When man is hunting animals he uses as much as possible the sense of vision in which he is far superior to most animals, and he places the hunted animal at the biggest possible disadvantage by keeping his position relative to the animal such that the wind blows from the animal to him. The way in which scent is carried by the wind is conclusive proof that it is a function not of the "ether" but of the air. On other occasions the keen olfaction of the animal world may be turned to good account by man the hunter in his use of bait; the deer for the tiger, the putrefying meat which, dragged behind a boat, will draw sharks from miles away, the worm or the maggot and the ground bait used by the fisherman. At the other end of the evolutionary scale the protozoa, each con­ sisting of a single cell, simplest of all animal life, push forward tenta­ tively a pseudopod, a transitory bulge in their jelly-like bodies. If they come in contact with microscopic plants and animals they engulf them but reject inert particles. Such protozoa as the microscopic amreba of fresh-water ponds and the paramecium which roam about bumping into things, taking an avoiding reaction and going off in a fresh direction, have no special sense organs, no cells of taste or smell, but if the water contains irritating chemicals or unsuitable food, another direction is taken. The protozoa have only the most THEIR PART IN LIFE 3 rudimentary chemical sensibility, certainly no taste or smell, yet the chemical sensibility such as it is, directs the hunt for food. In the search for food flatworms have a highly developed chemical sense, and a piece of meat placed in a spring will soon attract large numbers of small planaria. Earthworms show food preferences, preferring carrot leaves to celery, and celery to cabbage, and appear to be guided by taste. Among the insects bees are directed to flowers at Necroplwri least partly by smell, and other insects like the to dead bodies and dung on which they feed. When a dog finds the dead body of a mole and rolls on it, it may be that it will by doing so disguise its own scent and improve its hunting ability, although it may also be simply that the dog derives purely resthetic pleasure from the odour of the dead mole. Evasion of dangerous enemies This is the other side of the picture, the rabbit scents the weasel, the fox scents the hounds and is off. The skunk occupies a peculiar position, for the foul scent he spreads is designed to keep his enemies away. The same thing applies to the stink badger of the Philippines (Mydaus Marchei Huet) and n-butyl mercaptan, one of the foulest substances known, has been found in the secretions of this animal. Even the protozoa appear to have warning by chemical sense of the approach of an enemy and try to make themselves scarce. Poisoning How do the beasts reject the poisonous herbs, the birds the poisonous berries? There is little instinctive knowledge of this sort in us; we have to be taught as children which berries to avoid, and a little expert knowledge is generally required before mushrooms are gathered, eaten and enjoyed. The relation between the odours and edibility of fungi and the value of odour for identification have been (Phann. Zentral., studied by Herrman 1918, 59, 1-4, 7-10, 19-21). It is well known that many fungi are edible, but very wisely most people leave well alone what they are not sure of; there are, how­ ever, advocates of the use of some fungi as food and one of these, Bakonyi, has described (German Patent 700537, 1940) a method whereby the objectionable taste and odour of otherwise edible fungi may be removed. This is accomplished by exposing a layer of the fungi to the action of a strongly fermenting yeast or lactic acid mash for from 6 to 10 hours. Some of the odorous substances synthesized by fungi and responsible for their odours have been THE CHEMICAL SENSES (Arch. Mikrobiol., i4n vestigated by BlPineen iacnildli uKmr ivic S19c4ro0p, u1l1a,r io3p9s1i-s 405). Varieties of produced esters and a which had an odour like that of Roquefort cheese produced a ketone, although a very old culture also gave the order of a carbylamine. The safety of the animals lies in their much keener smell and taste. is noteworthy that the olfactory region, that part of the It nose where the sense organs lie, is always pigmented, yellow in man, darker brown in macrosmatic animals, and the pigment seems to be connected with smell. is significant that the animals lacking It in pigment, for example, white animals, are much more subject to poisoning. In parts of Virginia farmers will rear only black pigs as Lachantes tinctoria white eat, and are poisoned by roots of which the black pigs leave alone, and in the Tarentino only black sheep are Hypericum cristum. reared as white ones are poisoned by Darwin was of the opinion that the black and the white animals ate the same, but only the white were poisoned, but it has since been shown that only the white animals eat the poisonous food. Attempts to stock woods with white rabbits have failed, although as the rabbits are so conspicuous, other factors come into play here. Albinism even in man is associated with complete or partial anosmia. Nor­ mally an animal relies on smell to avoid poison, without smell it is lost. Himalayan rabbits are born perfectly white and remain so for several months, until when they are weaned, pigment makes its appearance just as the necessity to discriminate in food arises. The smells and tastes of poisons is a source of wonder that the nose can cope so efficiently with the It hundreds of thousands of new synthetic substances that are now made and that have never occurred in nature. They could hardly have been regarded as likely constituents of the environment with which the animal, human or otherwise, must keep in balance. The animals' senses provide the necessary information for the balance to be maintained, they signal the incidence of some unfortunate or potentially dangerous condition and the animal voluntarily, or more often involuntarily, takes suitable action to avoid the danger. The countless unnatural synthetic chemicals that we have made are accepted for test by the nose and their smells are observed as effort­ lessly as if practised. The real reason for this is that the initial stimulus is one of adsorptiqn, which is one of the most fundamental and widespread of known physical processes. Exceptionally the nose is deceived and dangers can and do occur although only very THEIR PART IN LIFE 5 unusually. The smell of dioxan (an unnatural synthetic) is not alarming (it is mildly ethereal and sweetish), but if inhaled, it can cause lethal damage to the kidneys; the light gases such as propane and butane that the oil refineries supply to the gasworks are very nearly odourless and they have for safety's sake to be given a little odour by adding one part of tetrahydrothiophene per million of finished gas. The exceptions, and carbon monoxide (p. 391) must not be forgotten, are few and far between. Furthermore, they are all synthetic; there is no naturally occurring toxic vapour that is odourless; if there had been then animals and men that were ex­ comp. physiol. (J. posed to them would have perished. Psycho!., And so with taste: in the words of Richter 1950, 43, 358-373). "Tasteless toxic substances could not have existed widespread in nature in readily available forms at any time in evolutionary history, since in the absence of a taste warning every animal or man that ingested them would have perished. It is more likely that they belong to a group of compounds to which in evolutionary history, man and animals have never been ex­ tensively exposed: that is, substances that are not commonly present in plants or in the bodies of other animals, or in the streams, lakes, springs, and other sources of drinking water; or they may belong to a group of man-made compounds which do not occur in nature." There are, nevertheless, some arsenic compounds that do occur free in nature. For centuries arsenic was the poisoners' preferred weapon, but the introduction of Marsh's and kindred tests made it too easy to detect in the corpse and its use now is restricted to a few brash amateurs. In some experiments that Richter carried out he found that for humans arsenic trioxide had no taste at all as a powder, although it had a bitter taste in solution; however, it dis­ solves only slowly and as a fatal dose consists of only about 0· 1-1 gram (i.e. 1 ·5-15 mg./kg. body weight) it was formerly used with outstanding success by professional poisoners. They were taking advantage of a gap in the body's defences. The sense of taste was caught napping. Urginea scilla, Red squill is another poison that occurs naturally; it is a large bulb with pink scales, that grows in Mediterranean coastal regions and has been widely used as a rat poison chopped up with bait of some sort; it was widely believed to have only a low toxicity to man, but there have been cases of poisoning that have thrown doubt on this; all humans, but apparently not all rats, can taste it.