THE SENSES 965 



Wien has elaborately investigated the question how the sen- 

 sitiveness of the ear varies for tones of different pitch. A tone of 

 50 vibrations a second, in order to be just heard, must have an 

 intensity corresponding to about 100 million times as much energy 

 as is needed for a tone of 2,000 vibrations. It is only on the extra- 

 ordinary sensibility of the ear for the range of tones used in ordinary 

 speech that the possibility of understanding speech depends when 

 the circumstances are unfavourable e.g., at a great distance, or 

 in the presence of much stronger accompanying noises. 



Smell and Taste. 



Smell was defined by Kant as ' taste at a distance '; and it is 

 obvious that these two senses not only form a natural group 

 when the quality of the sensations is considered, but are closely 

 associated in their physiological action, especially in connection 

 with the perception of the flavour of the food. The olfactory end- 

 organs in the mucous membrane of the upper part of the nostrils, 

 the so-called regio olfactoria, have been already described 

 (p. 8 1 6). In cases of anosmia, in which the olfactory nerve is 

 absent or paralyzed, smell is abolished ; but substances such as 

 ammonia and acetic acid, which stimulate the ordinary sensory 

 nerves (nasal branch of fifth) of the olfactory mucous mem- 

 brane, are still perceived, 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 filled with the liquid ; and fish, as every line- 

 fisherman 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) ambrosial odours, as of amber or musk ; (5) garlic odours, as 



