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.

SMELL--THE PHYSICAL SENSE 61 (e) Smell in Animals. Animals vary in the sensitiveness of their response to odorants for instance, the rabbit's olfactory sense is thought to be more sensitive than that of man. The rabbit has a larger olfactory area than man, and each vesicle of the rabbit has about twice the number of hairs. Thus, assuming that the density of the vesicles in unit area to be the same as in man, the rabbit will have a much greater adsorbing surface than man, and would be expected to have a more sensitive sense of smell. (f) (i) Molecular Structure of Odorants. Adsorption varies very con- siderably from point to point over a surface: we have seen that De Boer finds that the first few molecules of gas adsorbed on graphite have about twice the heat of adsorption of the subsequent molecules, showing that adsorption takes place at a few active centres much more strongly than on the remainder of the surface. Hair fibres are constructed from 18 amino acids arranged in various proportions and combinations. It is probable that the hairs attached to a common vesicle have a similar molecular con- struction, and that there is a systematic change in hair structure from vesicle to vesicle, so that various areas of the olfactory organ will respond efficiently to varying odorants. This is supported by the evidence of Adrian TM, who has found that there are parts of the olfactory area where response to a given stimulant is greater than that of the rest of the olfactory area. We have seen that the surface varies and that parts of it will have a greater attraction--i.e., heat of adsorption--for particular molecules than for others. The perception of the type of odour will depend on the part of the brain receiving the message from the olfactory receptors, which in turn will depend on the particular vesicles stimulated. Therefore we would expect stereoisomers to differ in odour, particularly if the active atoms of the mole- cule are spatially shielded by inactive atoms in only one of the isomers. Since the extent to which a molecule will be adsorbed on a surface depends on both its own entire molecular structure and on that of the surface, the members of a homologous series will differ in odour, and--as their vapour pressure drops as the series is ascended--they will also alter in type of odour, the more volatile (high vapour pressure) being rapidly perceived and not persistent, whilst the less volatile will be more slowly perceived and more persistent on an equi-molar basis. (f) (ii) Molecular Structure of Odorants. As a homologous series is ascended, the saturated vapour pressure decreases and thus at a given low pressure the proportion of the surface covered unimolecularly is increased and the vapour produces a stronger smell. During the ascent of such a series the speed of the molecules decreases, consequently the rate at which they diffuse to the olfactory surface also decreases, until a point is reached when these effects outweigh the benefits of the falling saturated vapour pressure. Thus, a series will first increase its intensity and then decrease it as it is ascended.