THE COMMON CHEMICAL SENSE 103 



part of the body where a mucous surface is in contact 

 more or less with the exterior. In man, then, the recep- 

 tors for irritants have a much wider distribution over 

 the body than the olfactory receptors have in that they 

 are found on almost every exposed or partly exposed 

 mucous surface. 



2. In Lower Vertebrates. In other mammals than 

 man, in birds, and in reptiles the receptors for irritating 

 substances are probably distributed in much the same 

 way as in man and are confined to the exposed or semi- 

 exposed mucous surfaces. In the amphibians and the 

 fishes, however, this system of receptors shows a pro- 

 digious expansion in that in these animals it is found 

 covering their whole exteriors. The well known experi- 

 ment of stimulating the frog's foot with solutions of 

 acids and other such substances is based upon this peculi- 

 arity and the sensitiveness of the skin of this and other 

 amphibians and of fishes as worked out by Nagel (1894), 

 Parker (1908a, 1908b, 1912), Sheldon (1909), Cole (1910), 

 Crozier (1915, 1916), and others show quite clearly that 

 sensitiveness to solutions of chemicals is a common 

 property of the skin in all these aquatic vertebrates. 



As early as 1894 Nagel discovered that the integument 

 of the dogfish Scyllium was extremely sensitive to a great 

 variety of chemical substances. He likewise found that the 

 skin of the goosefish Lophius and of the lancetfish Amphi- 

 oxus were also generally open to chemical stimulation. 



Nagel 's observations on Amphioxus were confirmed 

 in 1908 when it was shown that the skin of this fish was 

 sensitive to solutions of acids, alkalis, alcohol, ether, 

 chloroform, turpentine, oil of bergamot and oil of rose- 

 mary but not to solutions of sugar. It was also demon- 



