JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ionone and rose oil are appreciably soluble. Apparently his proposal to continue the experiments was not carried out and no confirmation of his results has been recorded. On the other hand, his evidence has not been refuted. THE OSMOCEPTOR THEORY At the conference on "Basic Odor Research Correlation" held in New York in April, 1953, the eminent American biochemist, James B. Sumner, very briefly outlined his new theory of olfaction (Ann. N.Y. Acad. Sci., 1954, 58, Art. 2, 68-72). Here, it is interesting to recall the rather similar theory advanced by another distinguished chemist, Ruzicka, about thirty- five years ago (Chemiker-Zeitung, 1920, 44, 93-94 129-131). His observa- tions led him to conclude that, in addition to volatility, specific solubility in air and ability to react with certain constituents of the olfactory mucous membrane are the two properties most essential for power to stimulate the olfactory nerves. These supposed constituents he named osmoceptors. He further supposed that there are two kinds of osmoceptor, namely, the primary and the secondary. Any odorous substance, on reaching the membrane, combines first with primary osmoceptors only. If there are sufficient primary osmoceptors to combine with the quantity of odorous substance present, or if such osmoceptors can be re-formed quickly enough by the organism, the quality perceived remains constant. But if all the primary osmoceptors are saturated, the substance may then react with secondary osmoceptors and the quality of odour may change, the change being slow if the concentration of the substance is low, and rapid if it is high. If the change should be very rapid, a "mixed" odour is perceived. Substances which react with primary but not with secondary osmoceptors cease to be perceived when all the primary osmoceptors are saturated, and the fatigue thus caused lasts until free primary osmoceptors are again available. Fatigue for one odorous substance presumably extends to substances having similar "osmophoric" groups but not to other substances. MODELS OF THE OLFACTORY SYSTEM Useful models of biological systems have been constructed from time to time. Perhaps the only one of these regarded as capable of illustrating the mechanism of olfaction is that of De Jong and Saubert which was described shortly before the Second World War (Proc. Sci. Section, Kon. Akad. Wetensch. Amsterdam, 1937, 40, 302-306, and Protoplasma, 1937, 28, 329-344). This colloidal model can be said to be not too remote from nature. Its designers began by supposing that the first stage--the only stage which they considered--in the process is a reversible change (possibly in permeability) in the olfactory cells resulting from penetration by gaseous molecules of odorous substance. Although the cell constituents probably chiefly affected are phosphatides--the phosphatides are both hydrophilic and lipophilic 210

MORE TRUTH ABOUT ODOUR and are very sensitive to the action of organic non-electrolytes even in very low concentration--De Jong and Saubert, for technical reasons, studied instead oleates which behave in much the same way as do the phosphatides. Emulsified oleates, in the form of minute droplets known as coacervates, were examined microscopically and it was shown that the vapours of some organic substances caused shrinking of the droplets when the concentration was low but swelling when it was high. When the vapour passed out of the coacervate, restoration of the original volume occurred. It is not difficult to conceive how certain olfactory phenomena can be correlated with these findings. For example, neutralisation (or compensation) would occur when the shrinking produced by one odorous substance was counteracted by the swelling produced by another, acting simultaneously. Again, the gradation in quality often observed in homologous series is paralleled by a change from swelling to shrinking. Thus, methyl, ethyl and propyl alcohols cause swelling whilst butyl, amyl, hexyl and heptyl alcohols cause shrinking. Substances which cause shrinking when in low concentration but swelling when in high or conversely may be those which have one odour quality when dilute and another when concentrated. And the model could doubtless be used to throw light on other related phenomena. Since the present article was written, Davies and Taylor have described "A model system for the olfactory membrane" in Nature, 1954, 174, 693-t394. Davies, moreover, had previously published a paper entitled "Olfactory stimulation. Some ideas and possible model systems" (Internat. Per. fumer, 1953, s, 17-22). It is not to be expected that study of any or all of the works discussed above would lead to a rapid solution of the many complicated and difficult problems of olfaction. Nevertheless, those who now attempt to solve such problems are not likely to succeed if they will not study carefully the work of their predecessors. The whole truth, perhaps, will never be known, but the true searcher will be happy if he can reveal even a small part of it. Such an investigator was Cloquet, the author of "Osphr•siologie," who wrote in that book long ago: "Quel que soit, au reste,-le mode d'action de ces organes, notre intention ne sera point de vouloir remonter & l'essence de la sensation, ni de d•montrer comment elle peut •tre eprouv•e. C'est une cause plac•e hors de la sphere de nos recherches, et derobae, probablement pour tourjours, aux moyens d'investigation que l'homme a resus avec la vie. L'6tre anim• pr•sente une foule de probl•mes qu'il n'est point donn• aux sciences accessoires •t la m•decine, ni •t la madecine elle-m6me de resoudre." 211