SMELL AND TASTE I97 



its molecules. It is probable that the action of molecules of 

 odorous substances causes a similar molecular change in the 

 carotenoids of the nose and starts a message travelling along 

 the nerve to the brain. 



When the receptor cells of any sense are stimulated they 

 generally respond only to the "correct" stimulus; for 

 example, the eye receptors respond only to light and those 

 of the ear only to sound : conversely the eye is deaf and the 

 ear is blind. The messages that pass along the nerves from 

 the receptors are, however, always of the same nature and 

 can be recorded as changes in electric potential. The brain 

 must therefore sort them out so that any message coming 

 from the eye means "light", or from the ear means "sound". 

 This is well illustrated when you get a blow in the eye and 

 "see stars" — the pressure resulting from the violent blow 

 has stimulated the receptors in the retina, but because the 

 message received by the brain comes along the eye nerve it 

 interprets the message as light and not as pressure. The 

 restriction of the receptors to responding to only one sort of 

 stimulus is carried further, as we shall see, in the sense of 

 taste, where each kind of taste has its own receptor. Whether 

 there are separate receptors for each of the infinite variety 

 of smells we do not know, but it does seem necessary that 

 there should be, because it is difficult to imagine how dif- 

 ferent substances stimulating the same receptor could be 

 distinguished by the brain from the message received : on 

 the other hand, although we cannot classify smells it is pos- 

 sible that the receptors in our noses can; there may be a 

 limited number of basic smells and the variety may be pro- 

 duced by blending two or more with the result that most of 

 them are "chords". The nose might then perceive them 

 somewhat as the ear perceives musical chords, the brain 

 receiving a number of messages from as many receptors and 

 combining them to give perception of a single specific odour. 

 It may not be without significance that the language that 

 perfumers use in describing the scents produced by their art 

 borrows many of its terms from that of musicians. 



The sense of smell is of great importance to most mammals 

 in seeking and testing their food, in recognizing each other 

 and the trails left in passing, and the scent deliberately laid 



