1076 THE SENSES 



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 filled with the liquid; and fish, as every line-fisher- 

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

 odoriferous substances, even when air-borne, are dissolved in the 

 nasal secretion before they can affect the olfactory end-organs, and 

 it may be due to the peculiarities of this solvent that it is so difficult 

 to imitate a normal stimulation of the olfactory organs by solutions 

 experimentally introduced into the nose (Parker). 



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 sen- 



sation (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 ozsena; (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 

 Haycraft has 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 combinations. 



Acuteness of smell may be measured by arrangements called olfac- 

 tometers. Zwaardemaker 's olfactometer consists of a piece of india- 

 rubber tubing fitted inside a glass tube, through which air is drawn 



