SMELL, TASTE, AND CHEMICAL SENSE IN VERTEBRATE- 229 



The fact that a sugar solution with a relatively high osmotic pressure does 

 not stimulate the integumentary end-organs of certain fishes, though the.-. 



organs are stimulated by solutions of acids, alkalis, salts, etc., of much lower 



osmotic pressure, shows conclusively that this form of stimulation is not due 

 osmotic pressure, but must be the result of the chemical action on (he nerve- 

 terminals of the dissolved substances, a conclusion in agreement with the v 11- 

 founded view as to the stimulus for taste in man and other higher vertebistt 



Quinine, as a representative of the bitter substances, shows considerable 

 diversity in its powers to stimulate. The mouth of Am turns was stimulate! by 

 a golution as weak as w/150 and that of Ammocoetes by a still weaker one, m/640. 

 Sheldon's records (1909) also show that the mouth of Mustelus is stimulated by 

 a weak solution of quinine, though the weakest possible solution is not apparently 

 recorded. When applied to the trunk and tail of Mustelus and Amiurus, quinine 

 solutions called forth no response, though they were stimulating to the corre- 

 sponding parts of Ammocates. This difference in the capacity of the integument 

 of different species of fishes to be stimulated by quinine has already Ix'en recorded 

 by Nagel (1894, p. 183). Notwithstanding these differences for the exterior 

 of fishes, all thus far tested have been found sensitive in the mouth to quinine, 

 even in relatively dilute solutions, though not so excessively dilute as can be tast 1 

 by the human being, m/25,000. 



Of the inorganic stimuli, acids, alkalis, and salts, the least efficient judging 



from the concentration needed for the minimum stimulus, is salt. In only on< 



location, the mouth region of Amiurus, was it possible to demonstrate a response 



from a solution of salt at a concentration (n/50) at which acid was ineffective. 



Aside from this exception, the relation of salt to acids and alkalis in the reactions 



of fishes is much the same as in human taste w here the weakest solution of salt 



that can be tasted is n/50, very much stronger than the weakest alkali, n/400, 



or the weakest acid, n/1,000. Not only are the salt stimuli for the fish and for 



human taste thus quantitatively closely related, but since sodic chloride and 



potassic chloride give almost indistinguishable results with the fish, it is fair to 



conclude that the stimulus of the salt solution is the chlorine ion and not the 



sodium or potassium ions. This agrees with what is known of the salty taste 



in man where the chlorine ions are the effective elements and not the sodium or 



potassium ions which in the earthworm (Parker and Metcalf, 1906) are the real 

 stimuli. 



The acid and alkaline stimuli for the tail, mid-trunk, and mouth of the fishes 

 are. as just indicated, usually much stronger than salt. In the matter of strength 

 t hey form together a fairly well defined group in which in particular combinations 

 acid is sometimes more stimulating than alkali or the reverse. In their effective- 

 ness at low concentrations they agree well with the condition in human taste 

 where the alkali is stimulating at a dilution of n/400 and the acid at one of 

 n/1,000. Not only is there this general agreement from a quantitative stand- 

 point, but, as already pointed out, the effective element in the stimulation of tin 



