The sense of smell, or olfaction, is less well understood than any of the other senses: sight, hearing, touch and taste (although the latter is partially connected with the sense of smell). In the past few years research in Japan and England has increased our knowledge of the physiological processes by which our bodies experience odours, and of course such discoveries enrich our understanding of how essential oils can affect both body and mind so powerfully.

The olfactory nerves lie in the upper part of the nose. Unlike the nerves involved in touch, hearing, etc., they are directly connected to the brain. Indeed, they have been described as 'brain cells outside the brain'. Because of this, the sense of smell is the most immediate of our senses. It is also the most fleeting perception of odours is sharpest when the odour is first smelt but decreases in intensity very quickly. This can be seen in two phenomena known as 'fading' and 'tiring'.

Fading takes place when we are exposed to one smell for a period of time, and can be illustrated by the example of a cook who does not smell the food being prepared, because he or she has been working with this food in the kitchen for some time. Somebody coming into the kitchen will notice the delicious aroma, and if the cook went out for a while and came back into the kitchen later, the smell would be noticeable because his/her nose had been away from that smell for a while, so it would react as to a new stimulus.

Tiring takes place when we are exposed to a succession of smells within a relatively short time. Very soon we lose the ability to distinguish clearly between one smell and another and they all begin to smell rather alike. You may have experienced this when trying out perfumes in a shop. If you try to sample more than three or four essential oils in succession the same phenomenon occurs.

Now, let us take a more detailed look at what happens when an essential oil (or any other odoriferous matter) is inhaled.

Essential oils, and others odorous substances, are very volatile, i.e. they evaporate into vapour easily when exposed to the air. In this form they are breathed into the nose. The inside of the nose is always moist, and the particles of the aromatic vapour dissolve in this moisture. The olfactory nerves can only detect aromatic particles when they are in this liquid form. From the main body of each olfactory cell, several fine filaments called cilia extend into the layer of moisture (mucus) in the nose, and the tips of these filaments are equipped with receptors which detect the presence of any aromatic particles. Information about these particles passes along the cilia to the body of the cell. From here longer nerve fibres transmit this information to the brain, passing through the bony plate at the top of the nose (the cribriform plate). The brain identifies the particular smell, and we become conscious of perceiving it. All this happens almost instantaneously.

In recent years, the invention of electron microscopes with enormous powers of magnification, have revealed more about the way in which the cilia detect odorous particles. The receptors at the tips of the cilia are varied in shape. The molecules that make up the particles of anything that has a smell are also different shapes and sizes according to their origin, and when the smell-receptors come into contact with molecules that match their own shape, this correspondence triggers off the process that transmits information to the brain. The brain identifies the smell according to which type of receptor has picked it up. This does not mean that the information transmitted indicates 'rose' or 'tomcats': -such distinctions come from associated areas of the brain where memories of earlier smell experiences are stored. What the cilia can transmit is that the smell is sweet or acid, flowery, woody, heavy or light and so forth. Most odours are quite complicated, and made up of many different elements. The variety of shapes of the smell-receptors enables us to register all these complexities, and the total input is then interpreted by the brain as that particular odour.

However, that is not the whole story, for the nose can detect far more different smells than the ears can detect sounds. Taste and sight are even simpler, as all taste and all vision is registered through only three or four different kinds of nerve cells. But the nose registers as many as ten thousand different types of smell sensation, and there do not appear to be ten thousand different kinds of receptors, so it seems likely that, as well as their shape and size, the rate of vibration of individual smell molecules plays a part in distinguishing between the myriad of smells that exist.

In understanding the physical actions of essential oils on the body, it helps to know that the area of the brain in which smell is registered, is linked by nerve-pathways to the hypothalamus, a structure at the base of the brain which is involved in regulating many important body activities. These include the endocrine system, which controls the secretion of hormones affecting growth, sex, metabolism and other functions; the autonomic nervous system which controls most of the unconscious activities that maintain life, such as digestion, rate of heartbeat and breathing and so forth; control of body temperature and hunger. Just how impulses from the part of the brain registering odour affect the hypothalamus, we do not know, but the connection can be easily observed. Smelling good food makes us feel hungry; 'bad' smells, such as decaying meat can make us vomit and certain odours may arouse sexual feelings. Maybe one day we will know why this is, but for the understanding of Aromatherapy, it is enough to know that these reactions do happen.

Why and how odours affect our emotions and memories is even less clear, though some ideas about this are discussed in the entry for the MIND. It is sometimes asked whether people who have no sense of smell can benefit from aromatherapy treatment. In fact they can, because essential oils are absorbed into the bloodstream, either through the skin or via the lungs when they are inhaled, so they can have a beneficial action on the body even if the recipient cannot smell them. It is doubtful, though, whether the same mental or emotional response to the oils could be experienced.