848 A MANUAL OF PHYSIOLOGY 



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 membrane, 

 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 pro- 

 duce 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 normal saline 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. 



Beaunis has classified the substances which can affect the olfactory 

 mucous membrane as follows : 



i. Those which act only on the olfactory nerves : 



(a) Pure scents or perfumes, without pungency. 

 (ti) Odours with a certain pungency, e.g., menthol. 

 t . 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 only on the nerves of common 



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

 Electrical excitation of the olfactory mucous membrane causes a 

 sensation like the smell of phosphorus. The sensation is experienced 

 at the kathode on closure and the anode on opening. 



Taste. The sense of taste is not so strictly localized as the 

 sense of smell. The tip and sides of the tongue, its root, 

 the neighbouring portions of the soft palate, and a strip in 

 the centre of the dorsum, are certainly endowed with the 

 sense of taste ; but the exact limits of the sensitive areas 

 have not been defined, and, indeed, seem to vary in different 

 individuals. 



The nerves of taste are the glosso-pharyngeal, which innervates 

 the posterior part of the tongue ; and the lingual, which supplies its 

 tip (see p. 722). The end-organs of the gustatory nerves are the 

 taste-buds or taste-bulbs, which stud the fungiform and circumvallate 

 papillae, and are most characteristically seen in the moats surround- 



