'Then I shall tell you, otherwise I cannot answer your question. Attend closely, please. Air is sucked in through the nostrils and passes the three baffle-shaped turbinate bones in the upper part of the nose. There it gets warmer and filtered. This warm air now travels up and over two clefts that contain the smelling organs. These organs are patches of yellowish tissue, each about an inch square. In this tissue are embedded the nerve-fibres and nerve-endings of the olfactory nerve. Every nerve-ending consists of an olfactory cell bearing a cluster of tiny hairlike filaments. These filaments act as receivers. "Receptors" is a better word. And when the receptors are tickled or stimulated by odorous molecules, they send signals to the brain. If, as you come downstairs in the morning, you sniff into your nostrils the odorous molecules of frying bacon, these will stimulate your receptors, the receptors will flash a signal along the olfactory nerve to the brain, and the brain will interpret it in terms of the character and intensity of the odour. And that is when you cry out, "Ah-ha, bacon for breakfast!" '
'I never eat bacon for breakfast,' I said.
He ignored this.
'These receptors,' he went on, 'these tiny hairlike filaments are what concern us. And now you are going to ask me how on earth they can tell the difference between one odorous molecule and another, between say peppermint and camphor?'
'How can they?' I said. I was interested in this.
'Attend more closely than ever now, please,' he said. 'At the end of each receptor is an indentation, a sort of cup, except that it isn't round. This is the "receptor site". Imagine now thousands of these little hairlike filaments with tiny cups at their extremities, all waving about like the tendrils of sea anemones and waiting to catch in their cups any odorous molecules that pass by. That, you see, is what actually happens. When you sniff a certain smell, the odorous molecules of the substance which made that smell go rushing around inside your nostrils and get caught by the little cups, the receptor sites. Now the important thing to remember is this. Molecules come in all shapes and sizes. Equally, the little cups or receptor sites are also differently shaped. Thus, the molecules lodge only in the receptor sites which fit them. Pepperminty molecules go only into special pepperminty receptor sites. Camphor molecules, which have a quite different shape, will fit only into the special camphor receptor sites, and so on. It's rather like those toys for small children where they have to fit variously shaped pieces into the right holes.'
'Let me see if I understand you,' I said. 'Are you saying that my brain will know it is a pepperminty smell simply because the molecule has lodged in a pepperminty reception site?'
'Precisely.'
'But you are surely not suggesting there are differently shaped receptor sites for every smell in the world?'
'No,' he said, 'as a matter of fact, man has only seven differently shaped sites.'
'Why only seven?'
'Because our sense of smell recognizes only seven "pure primary odours". All the rest are "complex odours" made up by mixing the primaries.'
'Are you sure of that?'
'Positive. Our sense of taste has even less. It recognizes only four primaries - sweet, sour, salt, and bitter! All other tastes are mixtures of these.'
'What are the seven pure primary odours?' I asked him.
'Their names are of no importance to us,' he said. 'Why confuse the issue.'
'I'd like to hear them.'
'All right,' he said. 'They are camphoraceous, pungent, musky, ethereal, floral, pepperminty, and putrid. Don't look so sceptical, please. This isn't my discovery. Very learned scientists have worked on it for years. And their conclusions are quite accurate, except in one respect.'
'What's that?'
'There is an eighth pure primary odour which they don't know about, and an eighth receptor site to receive the curiously shaped molecules of that odour!'
'Ah-ha-ha!' I said. 'I see what you're driving at.'
'Yes,' he said, 'the eighth pure primary odour is the sexual stimulant that caused primitive man to behave like a dog thousands of years ago. It has a very peculiar molecular structure.'
'Then you know what it is?'
'Of course I know what it is.'
'And you say we still retain the receptor sites for these peculiar molecules to fit in to?'
'Absolutely.'
'This mysterious smell,' I said, 'does it ever reach our own nostrils nowadays?'
'Frequently.'
'Do we smell it? I mean, are we aware of it?'
'No.'