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breath would all be deposited on the receptor sheet with the same general 

 pattern of concentration and timing. 



It is unlikely that the patterns in the nose would have as much detail 

 and variation as those on the retina, but the great size of the receptor 

 sheet does suggest that some of the information might be given in this way, 

 at all events in a macrosmatic animal. And in the rabbit there are certainly 

 distinct regional differences in the effectiveness of different odours and in 

 the timing of the discharge they set up. In the absence of simultaneous 

 records from multiple sites we could not expect evidence of detailed chro- 

 matographic patterns, but if we make simultaneous records from two 

 positions in the olfactory bulb there is often enough difference to show 

 which of four or five smells has been used to stimulate. 



One can think of a variety of reasons for these spatial and temporal 

 differences, differences in the receptors, in the nature of the surface or in 

 the course followed by molecules of different weight and solubility in their 

 passage through the organ. In man the organ is much less complex and 

 we experience the same distinctive olfactory sensation whether the air is 

 drawn in through the nose or drawn through the mouth and expelled 

 through the nose. It is unlikely therefore that a microsmatic nose gets 

 much information from the way the excitation is distributed, but a macros- 

 matic nose might get a good deal more. 



But it is unlikely that these spatial and temporal patterns could be more 

 than a minor guide in distinguishing one smell from another and so we 

 are left with the second method, that of individual receptors sensitive to 

 particular molecules as the colour receptors in the retina are sensitive to 

 particular wavelengths. In the rabbit they can be demonstrated without 

 much difficulty, for when the discharge from a single mitral cell is recorded 

 it is usually found to be produced only by one substance or group of 

 related substances, ethereal, esters, aromatic hydrocarbons, etc. The 

 specificity is a fixed property of the unit, unaltered by time, anaesthesia 

 or repeated stimulation and specific units have been found for many kinds 

 of smell and from mitral units in several parts of the bulb. 



From my own work, done some years ago, I should find it very difficult 

 to say how many different varieties of receptor there are, but we shall hear 

 more about it at this meeting and I think it will be clear that the part 

 played by specific receptors in the olfactory discharge would certainly 

 give far more information about the nature of the smell than could be 

 gathered from the general distribution of the excitation pattern. 



It seems, then, that as far as the vertebrate sense organs are concerned, 

 our recognition of what we hear depends entirely on the general 

 excitation pattern, the recognition of what we see depends mainly on the 

 pattern but is aided by specific colour receptors, and our recognition of 

 what we smell depends mainly, perhaps entirely, on specific receptors for 



