1036 THE SENSES 



branch of fifth) of the olfactory mueous membrane, are still per- 

 ceived, though not distinguished from each other. In fact, the 

 so-called pungent odour of these substances is no more a true smell 

 than the sense of smarting they produce when their vapour comes 

 in contact with a sensory surface like the conjunctiva, or a piece 

 of skin devoid of epidermis. 



It was at one time believed that odoriferous particles could not 

 be appreciated unless they were borne by the air into the nostrils; 

 but this appears not to be the case, for the smell of substances 

 dissolved in physiological salt solution is distinctly perceived when 

 the nostrils are rilled with the liquid ; and fish, as every line-fisher- 

 man knows, have no difficulty in finding a bait in the dark. 



The substances which can affect the olfactory mucous membrane 

 can be divided into four groups : 



1. Those which act only on the olfactory nerves, the odours 



proper. 



2. Substances which act at the same time on olfactory nerves, 



and on nerves of common sensation (tactile nerves) 

 e.g., acetic acid. 



3. Substances which act at the same time on the gustatory 



nerves. 



4. Substances which act only on the nerves of common 



sensation (tactile nerves) e.g., carbon dioxide. 

 Zwaardemaker has classified the pure odours as follows: 

 (i) Ethereal odours, as those of fruits; (2) aromatic odours, as of 

 camphor or bitter almonds; (3) fragrant odours, as of flowers; (4) am- 

 brosial odours, as of amber or musk; (5) garlic odours, as of onion, 

 garlic, asafcetida; (6) empyreumatic, or burning odours, as of burnt 

 coffee or tobacco smoke; (7) caprylic or goat odours, as of sweat; 

 (8) repulsive odours, as the odour of the disease ozaena; (9) nauseating 

 odours, as of faeces or putrefying material. 



The most interesting form of inadequate stimulation is electrical 

 excitation of the olfactory mucous membrane, which causes a sensation 

 like the smell of phosphorus. The sensation is experienced at the 

 kathode on closure and the anode on opening. As to the manner in 

 which the multitudinous adequate stimuli excite the olfactory nerves, 

 we can only suppose that they act as chemical stimuli. Smell and 

 taste are pre-eminently the ' chemical ' senses, as sight and hearing are 

 pre-eminently ' physical ' senses. But little is known of the relation 

 between the chemical constitution or physical properties of substances 

 and the quality of the odoriferous sensation which they excite, although 

 Haycrafthas pointed out some interesting relations between the atomic 

 weights of certain elements and their power of exciting odours. The" 

 number of distinct odours which can be perceived is so great that it is 

 scarcely conceivable that each is subserved by special olfactory fibres. 

 Marked changes occur in disease, and all odours need not be affected 

 to the same extent. Some may be almost normally perceived, while 

 relative or complete loss of smell exists as regards others. These and 

 other facts have given rise to the idea that there are several groups of 

 olfactory fibres, each concerned in the appreciation of a particular 

 odour or group of odours. Yet it has not proved possible to reduce 

 them to a limited number of fundamental odours and their combina- 

 tions. 



Acuteness of smell may be measured by arrangements called olfac 



