SMELL--THE PHYSICAL SENSE 49 ing its original state by mechanical removal of the odorant molecules by the air-stream flowing over it. In the former case it would be a chemical sense analogous to sight, in the latter a physical sense analogous to hearing. If a chemical sense, then a limited small number of receptor chemicals would be expected, giving rise to primary odours from which all other odours could be compounded in this case it would probably be impossible to differentiate between the ingredients of a composite odour for the same reason that the eye cannot differentiate between the primary colours making up a composite colour. If smell is a physical sense, we should expect to find no such primary odours and to be able to differentiate between the ingredients of a composite smell in the same way that the ear can analyse a composite sound. It will be advantageous to consider in greater detail the sensory nerve systems, the visual and auditory senses, and the structure of the olfactory area before attempting to explain any observed phenomena. We will also consider the properties of gases relevant to an adsorption theory of smell. The following are brief descriptions of these. (a) The Sensory Nerve System. A sensory nerve system consists of (i) the receptor organ, (ii) the nerve which transmits, via a limited number of junctions, the response to the stimulus given to the receptor, and (iii) the receiving area of the brain specialising in the interpretation of the message. Receptor organs are of several kinds. Those responsible for touch, light pressure, heat and cold are all constructed on the same basis as Pacinian corpuscles, which are those responding to pressure by change of shape they are large receptors made of concentric shells arranged like layers of onion with a soft core through which the nerve passes, to end at the far side. There are other kinds of touch receptors, such as Meissner corpuscles, which are ellipsoids, the nerve ending inside the soft centre in a complex •nesh of fine fibres. Yet another end-organ is associated with hair follicles, in which the nerve ending is grouped basket-like round the base of the hair so that the nerve cell is stimulated by mechanical displacement of the hair. This latter receptor device is used in the ear to convert sound into nerve signals, and in the vestibular apparatus which controls the balance of the body. The nerves are bundles of fibres rarely greater than 0.01 mm. in diameter, each fibre being an extension of the nerve cell which sends out two fibrils-- one to the receptor, the other towards the brain. Nerve fibres are of two types: medulated and non-medulated. A medulated fibre consists of a senfi- fluid central core surrounded by a sheath of concentric layers of protein (myelin sheath) which in turn is covered by a homogeneous membrane called "the neurilemma." Non-medulated fibres resemble medulated, except that the myelin sheath is absent. When the nerve is excited, an electric current passes along it, the active region of the nerve becoming negatively charged with respect to the rest

50 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of it. It has been found that all messages from whatever type of receptor have the same characteristics: each nerve fibre conducts an impulse of specific intensity at constant speed, both factors being directly proportional to the fibre diameter: thus a fibre of 0.001 mm. diam. conducts at about 5 metre/second, and one of 0.02 mm. diam. at 120 metre/second. The impulse builds up to its maximum intensity rapidly and dies away more slowly: its size is independent of the strength of the stimulus, which must be above a minimum in order to excite the nerve: it is an all-or-none reaction. It has been found that there is a value of the product of the time multiplied by intensity which must be reached in order to produce a response. The lowest intensity which •511 give a response, regardless of the time for which it is applied, is called the "threshold," and any lower intensity, however long applied, will have no effect. Intensity of stimulus is proportional to the frequency of impulse: thus, a greater stimulus causes a stream of impulses to be transmitted which are closer together than are those from a weaker stimulus. An impulse takes about 0.001 second to reach its maximum and lasts about 0-002 second. When it has passed, there is a period which varies between 0.015 and 0.1 second, during which the nerve cannot conduct another impulse: this is called the "refractory period" and covers the time taken for the intensity of the impulse to die away. It is considerably longer than the time taken for the impulse to build up to its maximum intensity. The sensitiveness of a nerve to a low intensity response can be increased by having several receptors supplying it, when it is found that stimulation of any one of the receptor end-organs activates the nerve which cannot now accept a message from another one of its end-organs until its refractory state has died away sufficiently. As a stimulus given to a nerve increases above the threshold, at first a small number--and then a larger number-- of fibres respond, because the fibres differ in excitability, those most readily excited responding at the threshold and the less easily excited as the stimulus increases, until a point is reached where all of the fibres respond and any further increase in stimulus gives no additional response. If a stimulus is increased in intensity gradually, the intensity can reach relatively enormous proportions without promoting activity (thus a gradual build-up in the intensity of an electric stimulation of a nerve to forty times the threshold value has been found to give no response), and thus ability of the nerve to accommodate itself to slow change is called "adaptation." Nerve fibres are rapidly adapted, and thus a stimulus of nearly constant intensity soon ceases to elicit a response, the nerve accepting the new level of the stimulus as its normal and responding again only when this level is changed. Receptor organs have the same characteristics as the nerve fibres those responding to the movement of hairs, and others in the skin and the eye, are particularly rapidly adapted to the new level of the stimulus. Hence sensations arising from the stimulation of these receptors rapidly fade away