THE COMMON CHEMICAL SENSE 105 



Amiurus. If a bait in the form of a piece of meat or the 

 like is held close to the flank of one of these fishes, the 

 animal is very likely to turn suddenly and snap it up. This 

 is not a surprising response, for the sides of these ani- 

 mals are well provided with taste-buds. They are also 

 supplied with lateral-line organs. Both these sets of 

 receptors may be eliminated by cutting on the one hand, 

 the branch of the facial nerve that is supplied to the taste- 

 buds of the side of the body and, on the other, the lateral- 

 line nerve that is distributed to the lateral-line organs 

 of the same region. After the fish has recovered from 

 such an operation, it will no longer respond to a bait held 

 near its flank, but the skin of this region is still per- 

 fectly open to stimulation by sour, saline and alkaline 

 solutions. As the only receptors left after the operation 

 just described are the free-nerve terminals of the spinal 

 nerves, these terminals must be the receptors for chem- 

 ical irritants. This conclusion is in accord with the fact 

 that this type of ending is the only one that occurs in 

 many portions of the skin of the dogfish, of the foot of 

 the frog, and of the partly exposed mucous surfaces of 

 the higher vertebrates such as those of the mouth and 

 the nose. Moreover when these endings are rendered 

 inoperative by cutting their nerve trunks, as Sheldon did 

 on the dogfish and as has often been done on the nasal 

 cavities of mammals, irritating substances are no longer 

 effective. Free-nerve endings of spinal or cranial nerves 

 are, therefore, quite certainly the type of nerve-terminal 

 concerned with the reception of chemical irritants. 



4. Eelation to Other Senses. In discussing the relation 

 of the receptors for chemical irritants to other sense 

 organs some of the earlier workers suggested a compari- 



