148 SMELL, TASTE, ALLIED SENSES 



parently easily stimulated in this way. The head and 

 especially the eight barbels about the mouth of Amiurus 

 are richly supplied with taste-buds. These organs, like 

 those on the human tongue, are apparently extremely 

 sensitive to metals probably because of the slight electric 

 currents produced by these bodies, for, the fishes respond 

 with great readiness to a weak constant current from a 

 dry cell. If such a current is led into an aquarium 

 through a water-filled glass tube and out again by a sim- 

 ilar tube, the water acting as a conductor, catfishes can 

 be readily stimulated by bringing such tubes close to 

 them. If the current is sufficiently reduced (a little less 

 than a microampere) the fishes will approach the open 

 ends of the tubes and nibble at the current as though it 

 were a bait, thus giving evidence that the organs stimu- 

 lated are the gustatory receptors (Parker and Van 

 Heusen, 1917). Hence the electric stimulus seems in every 

 way to duplicate the stimulus normal for the organ of 

 taste, a solution of a sapid substance. 



9. Distribution of Tastes on the Tongue. The four 

 well-recognized tastes, as the preceding sections show, 

 are normally excited by very different stimuli. The sour 

 taste is dependent upon the cation, hydrogen. The saline 

 taste is called forth by a number of anions: chlorine, 

 bromine, iodine, and the sulphate and nitrate ions. The 

 bitter taste has as stimuli the alkaloids, such cations as 

 magnesium, ammonium, and calcium, and possibly the 

 anion of picric acid. The sweet taste depends upon such 

 organic compounds as the sugars and alcohols, and on 

 saccharine, on lead acetate, and on hydroxyl and gluci- 

 num ions. The four tastes, therefore, are excited by 

 entirely independent groups of stimuli and it is a matter 



