176 PHYSIOLOGY CHAP. 



conditions in virtue of which they are able to excite the olfactory 

 end-organs. 



V. The qualities of odours are extraordinarily numerous. No 

 one, as Nagel justly points out, can say that they know all the 

 substances capable of exciting specifically distinct sensations of 

 smell; many people are not acquainted with certain very 

 characteristic odours familiar to chemists, e.g. formaldehyde, 

 picric acid. We cannot as a rule recognise the components in a 

 mixture of many odours. It is also possible to make a gradual 

 transition from one to another of two very different odours by a 

 series of mixtures, in which the two components are present in 

 different proportions. In this respect smell differs very markedly 

 from taste, in which, as we have seen, there are few specifically 

 distinct qualities of sensation, so that the components are easily 

 recognised in any mixture. 



Granting all this, it is not surprising that we have as yet no 

 true scientific classification and scale of odours. We are not even 

 able to distinguish the different qualities of odours by different 

 names, and to express them we employ the names of the vegetable 

 or animal substances from which they emanate. Lastly, we 

 cannot differentiate odours into elementary and compound. 



It has, however, been attempted by different methods to 

 classify odours in certain groups or categories. Haller proposes 

 to arrange them in three groups, according as they are pleasant, 

 unpleasant, or indifferent : odores suaveolentes, odores inter mediae, 

 odores foetores. The first class includes particularly the ethers 

 and essential oils : among the foul smells are certain gases of very 

 simple composition (sulphuretted hydrogen, carbon bisulphide, 

 certain hydrogen carbides, etc.), as well as certain decomposition 

 products (indole, skatole, etc.). But it is impossible to distinguish 

 the two classes sharply from one another and to determine the 

 odours belonging to the intermediate class, because these dis- 

 tinctions are based exclusively on subjective appreciations which 

 vary considerably in different individuals. Moreover, some 

 gases, e.g. chlorine, bromine, iodine, ammonia, which have a bad 

 smell when concentrated, are, on the contrary, indifferent or even 

 pleasant when suitably diluted. 



At first sight the classifications of odours proposed by Frohlich 

 seems better. As the nasal mucous membrane is supplied by two 

 pairs of nerves, the olfactory and the nasal branches of the 

 trigerninal, the first of which alone is the specific nerve of smell, 

 while the second serves touch, temperature, and pain, the sensations 

 generated here must also be placed in two categories, i.e. those 

 resulting solely from excitation of the olfactory nerve, and those 

 which are due to excitation of other sensations as well. The 

 former are pure olfactory sensations, e.g. those produced by 

 ethereal oils, resins, balsams, etc., which never give rise to reflex 



