MELANOPHORES OF THE HORNED TOAD 283 



3. Mechanical stimuli 



Mechanical stimulation, such as tapping or pressing the skin, 

 does not produce any effect upon the melanophores. Mechan- 

 ical stimuli of a more violent nature cause profound changes in the 

 condition of the melanophores. Such stimuli, however, do not 

 act directly upon the pigment cells to which they are applied, 

 but affect the pigment of the entire body by producing a state 

 of nervous excitement. Such stimuli are better classed with 

 noxious stimuli, to be considered later. 



4. Faradic stimuli 



The application of a faradic stimulus to the surface of the skin 

 of the horned toad fails to produce a change in the state of the 

 melanophores. Apparently the dry horny layer of the skin 

 forms a very poor conductor for the electric current. If the 

 electrodes are applied to the under surface of the skin, the 

 melanophore pigment of the region becomes completely contracted 

 in the course of five minutes. Faradic stimuli, if sufficiently 

 violent, may produce effects upon the melanophores of the entire 

 body. Such stimuli are conveniently classed as noxious stimuli. 



5. Noxious stimuli 



In this category may be placed a variety of stimuli which have 

 a common effect upon the state of the nervous system of the 

 horned toad. Noxious stimuli are those which may be considered 

 harmful, painful, or otherwise unpleasant and produce in the 

 horned toad a state of nervous excitement. Whenever such a 

 nervous condition is established in this animal there ensues a 

 complete contraction of the melanophore pigment. A descrip- 

 tion of some of the conditions which produce nervous excitement 

 has already been given on page 279. For experimental purposes 

 the most convenient noxious stimulus is the electric current. 

 By placing platinum-point electrodes in contact with the mucous 

 surface of the mouth or cloaca of the animal and passing a faradic 

 current between these points, a complete contraction of the pig- 



