244 OUTPOSTS OF THE INTELLIGENCE SERVICE 



That only four kinds of taste can be recognised is readily under- 

 stood when consideration is given to the four types of receptors, 

 each with a lower threshold for one particular form of chemical 

 stimulation. Some substances are able to stimulate more than one 

 group of gustatory end-organs. For example, saccharine pene- 

 trates and stimulates the receptors at the back as well as those at 

 the tip of the tongue. It is, therefore, perceived as both bitter 

 and sweet. In the same way, acetate of lead is sweet and sour ; 

 acetone, sweet and bitter ; potassium sulphate, bitter and sour ; 

 magnesium chloride, bitter and salt, and so on. Then other tasty 

 solutions are mixtures of substances, each stimulating at a different 

 part of the tongue, i.e. a compound stimulation. Proof of this 

 fourfold taste coniplex has been obtained in man by stimulating 

 the afferent nerve fibres for taste in the chorda tympani of an 

 individual with a fistula in his ear. Trials were made at different 

 times and by various direct means of stimulating the nerve, and, 

 in every case, the subject reported sensations of sweetness, sour- 

 ness, bitterness or saltness only. One must conclude, in view of 

 Adrian's work, that we differentiate between tastes not so much 

 because a different stimulus is applied, but because it is applied 

 at a different place, i.e. a primary analysis takes place at the end- 

 organ. As far as one can judge, the nature of the nervous impulse 

 is the same for all tastes and, therefore, final analysis must take 

 place in the sensory centre. That is, we appreciate the difference 

 between sweet and bitter, say, in the same way as we differentiate 

 between a touch on the arm and a similar stimulus applied to the 

 calf of the leg. 



7. Smell. The sense of smell added to that of taste contributes 

 in large measure to the pleasures of the table and serves as an 

 excellent substitute-stimulus for the fiow of saliva and gastric juice 

 (conditioned reflex). Flavours, as a matter of fact, are olfactory 

 and not gustatory stimulants. If we lose our sense of smell, say 

 by a cold in the nose or by experimentally preventing the entrance 

 of gases into the upper nasal passage, much of our food seems to 

 become tasteless. We are unable in these circumstances to tell 

 whether Ave are chewing raw potato or raw apple. 



Smell is the ancestral chemical sense and may be classed, 

 especially in the lower animals, as a distance receptor. In civilised 

 man this sense, unless rendered acute by training, is merely 

 vestigial. 



The areas of nasal mucosa associated with this perceptive 

 mechanism arc small rectangular strips in the upper part of each 

 nasal cavity, just above the superior turbinate bone. In ordinary 

 respiration, air docs not pass directly over the olfactory mucous 



