XVII 



SMELL AND TASTE 



The sense of smell is the third of the distant-receptors that 

 we have to consider, but it is so closely associated with the 

 sense of taste that the two must be dealt with together. Both 

 smell and taste are chemical senses and function by minute 

 particles of odorous or tastable substances coming into 

 contact with the cells in the end organs of the nerves serving 

 these senses. We have less detailed knowledge of these senses 

 than we have of sight and hearing for several reasons ; in 

 the first place our sense of smell, though it may be very 

 acute in detecting some odours, is not a sense of vital impor- 

 tance to us and is not nearly so highly developed as it is in 

 many other animals. Secondly there is no objective method 

 by which we can classify or measure smells — at the most we 

 classify them as pleasant, unpleasant or indifferent — and for 

 the rest we describe them in terms of other senses such as 

 sweet, sour, heavy or smooth, or liken them to the odours 

 of familiar things, as when we say a substance has the smell 

 of violets or of rotten eggs. 



In vertebrates the olfactory nerve-endings lie in the nose 

 and in mammals they lie in the upper part of it. The cavity 

 of the nose is divided into right and left halves by a partition 

 in the middle, and much of the cavity is filled by three thin 

 sheets of bone (the turbinal bones) attached to the outer side 

 and rolled up like scrolls with the ends facing out and in. 

 The degree of development of the scrolls varies widely in 

 different kinds of mammals ; in some they are very com- 

 plex but in others comparatively simple or even almost 

 non-existent. Their chief function appears to be that of 

 warming and moistening the air breathed in, and of filtering 

 out from it any particles of floating dust. The lower ones at 

 least are not particularly concerned with the sense of smell, 

 for the nerve endings of the sense are concentrated in the 



