SMELL--THE PHYSICAL SENSE 59 When we compare the structure of the olfactory organ with those of vision and hearing, the closer approach to the latter is again evident. The olfactory receptors terminate in a hairy structure and they have to convert energy, brought to them by rapidly moving gas molecules, into an electrical impulse. In both of these characteristics they resemble the auditory receptors closely. Let us postulate that the olfactory receptor works in a physical way: that odoriferous gas molecules are adsorbed on to the hairs of the vesicles at the end of the nerve fibres and that the energy represented by the heat energy liberated by the adsorption is converted by the vesicle to an electrical impulse. If these postulates are true it should be possible to explain the various phenomena of oilaction by reference to the known properties of nerve systems, of the adsorption of gases and of the construction of the olfactory area. The observed phenomena of oltaction given in Part I have there been arranged in such order that they are easily assigned to sections whose fundamental basis is similar. We have groups concerned with the properties of the nervous system, the properties of the sensory receptors, the properties of gases, and the properties of solutions. Referring to these under the same reference letters and in the same order as given in Part I, we have the following explanations: (a) Adaptation and Fatigue. Adaptation has been found to be a character- istic property of the brain cells, of the nerve fibres and of sensory end-organs (particularly of those of the skin, where it is much more rapid than in the nerve fibres). It is reasonable to expect the olfactory receptors to resemble other end- organs, and, like them, rapidly to come to an equilibrium state with any given level of stimulation by a particular odour. Provided this level is below that giving maximum olfactory response, further increase in odorant concentra- tion should make the substance perceptible once more. This is often made use of by a perfumer when studying a compound in order to copy it: he lets the smell diffuse over the olfactory area, mentally indexing the odours as they are perceived one after the other. During this performance, the more volatile smells are recognised first because they more rapidly reach their maximum concentration then, as the olfactory area comes into equilibrium with, and adapted to, them and they are no longer recognised, the next set of not quite so volatile materiMs are now recognised until adapta- tion sets in after they have reached their maximum adsorption, and so on. This process of adaptation is essential for the recognition of the odour ingredi- ents. If the perfumer now wishes to check his assessment, he clears his nose by passing air over the olfactory area and starts again, this being possible because the clearing has removed the stimulants from the olfactory area, which is then once more in a "reactive" state.

60 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS We see that adaptation is one of the most important properties of the sensory system and enables the recognition of individual ingredients in a mixture of smells, in the same way that it must also do in a mixture of sounds. It follows that if the nose is in a constant odour concentration, such as there is in cosmetic and perfumery laboratories, it rapidly ceases to smell the ambient odours. (b) Anosmia. The ability to smell the "normal" gamut of odours depends on one's having the usual distribution of undamaged olfactory receptors, nerve fibres and cerebral receiving areas. That some persons are unable to differentiate between odours accepted by others as different, and yet can differentiate between other odours, suggests that individual odours have specialised nerve systems, for then if part of one system is damaged no indication of the stimulation of the receptor can be appreciated, whilst odours can still be perceived by the undamaged systems. Relevant to this are Adrian's • findings, suggesting that the various sections of a rabbit's olfactory area are especially sensitive to varying types of stimulating molecules. (c) Paralysis. Formaldehyde reacts with the reactive amide and sul- phydryl hydrogen of protein molecules to form hydroxy-methyl with one, or methylene bridges with two, reactive hydrogen molecules, e.g., -- NH-- q- HCHO---• -- N(CH•OH) -- and --SH q-HCHO q-HS----+--S--CH•-- S-- q-H•O. If the sensory hairs on the vesicles of the olfactory area have points of special activity in adsorbing odorant molecules, removal of the number of active groups in them by chemical reaction will reduce the extent of the adsorption of odiferous compounds on the hairs, and t•ence increase the threshold concentration required to smell them. If sufficient formaldehyde is present, it could be that very few reactive groups remain, and the sense of smell will be temporarily paralysed until the hairs are renewed. The ability of formal- dehyde to paralyse the perception of all types of odour indiscriminately points to the possibility of the amino and sulphydryl groups being some of the active adsorption centres of the olfactory hairs. (d) Smell in Insects. In an insect's olfactory organ moisture is not necessary for stimulation of the sense of smell, and the olfactory hairs are not bathed in moisture. Now, if the odorant is adsorbed directly on to the hairs, which action stimulates the nerves, the presence of moisture is not necessary in order that an odorant shall be perceptible. In an animal's olfactory area, the moisture always present will produce an atmosphere saturated with water which will compete with the odorants for the active centres of the adsorbent surface. This will limit the kinds of molecules which will be adsorbed on it, those with a heat of adsorption on the surface less than that of water being adsorbed in extremely small amounts and consequently not smelled.