4 LORD ADRIAN 



particular smells, though it may be aided by the characteristic pattern. 

 This conclusion seems reasonable enough, for sound waves can only exert 

 their mechanical effect, hght waves can have different photochemical 

 actions according to their wavelength and odorous particles will have a 

 much greater variety of chemical effects which could be distinguished by 

 specific receptors, though their physical properties would influence their 

 distribution over the receptor surface. 



We can only distinguish four tastes, but we can distinguish a vast 

 number of smells : differences in the excitation pattern, if they help at all, 

 would be unlikely to separate more than a few general categories and, 

 although specific receptors could give much more details, the evidence 

 seems scarcely enough to account for the whole range of olfactory dis- 

 crimination. We can account for it, however, if we remember that the 

 essential role of the central nervous system is to integrate all the incoming 

 signals and to produce the appropriate response. Our recognition of what 

 is happening around us is guided by all the information available whatever 

 its source. Olfactory recognition, like visual recognition, need not depend 

 on a single method of sensory analysis, it may depend on several methods 

 taken together. If smells can be separated in five categories by the spatial 

 pattern and five more by the temporal and another 50 by the specific 

 receptors, the number of different smells that might be distinguished 

 would be very large. We could pick them out as we pick out a book in 

 a library catalogue by a class mark, a letter and a number. 



All this is guesswork and it cannot be more than that until we know 

 more about the reception of the taste and smell signals in the brain ; 

 and we shall not have a complete picture of the chemical sense organs 

 until we know how they are arranged in the invertebrate as well as in the 

 vertebrate. Fortunately we shall hear about all these problems at our 

 meeting. So it is high time now for me to stop making the guesses and 

 to call on our speakers to tell us the facts. 



