SMELL 



197 



gram per litre of air, can be detected. Training renders the sense 

 more acute. The working chemist relies on his sense of smell to 

 a great extent to help him in the identification of compounds. 

 The tea blender and the wine expert can detect very slight differ- 

 ences in " flavour." 



It is worth while noticing that receptors all depend for stimu- 

 lation on the existence of an alteration in external energy. This 



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OLMCTOK.Y EPITHELIUM 



OPEN I MS OF 



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TUBE 



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CESOPHAGUS 



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Fm. 42. Antero-posterior section through nasal fossae, mouth and neck. The 

 arrows show the direction of the air currents during inspiration. 



is specially marked in the case of this ancestral chemical sense. 

 Our accustomed environment presents no stimulus. Air has no 

 smell and water no taste. The introduction of a trace of foreign 

 body alters the energy content of the environment and stimulation 

 follows. It is a common experience to find that people do not 

 experience sensations that have, for the time being, become 

 permanent in their environment. A room may be stuffy to an 

 incomer but quite comfortable to the tenants. The physiological 



