XVI THE NERVOUS SYSTEM 303 



The Croaking^ Reflex. — If the side of a frog be stroked 

 with the finger, the animal often responds by croaking. The 

 same reaction frequently occurs when the frog is picked up 

 in the hand. It takes place more often in the male frog 

 than in the female, especially during the breeding period, 

 when the croaking mechanism is very readily put into 

 activity. A frog that is seized may continue to croak vigor- 

 ously for some time, and when it has ceased to croak it may 

 be induced to continue by rubbing its side with the finger. 

 The croaking reaction, however, is very variable, and in 

 certain individuals it may not be performed at all. And the 

 same individual reacts quite differently at different times, 

 according to what seems its own caprice. 



It is quite otherwise with frogs from which the cerebral 

 hemispheres have been removed. It was found by Goltz 

 that animals thus operated on croak with mechanical regu- 

 larity whenever one strokes their side or back. They never 

 croak when the hind legs or ventral side of the body is 

 stroked, and they croak only onc^c after each stroke on the 

 back or side. The experiment succeeds well with both 

 sexes, but the croak of the male is naturally much louder 

 than that of the female. " I know of scarcely any physio- 

 logical experiment," says Goltz, " which succeeds in so cer- 

 tain and regular a manner as this croaking experiment. . . . 

 As I was delivering a discourse upon this subject before the 

 meeting of the naturalists at Hannover, in the year 1865, I 

 placed upon the table, for purposes of demonstration, sev- 

 eral frogs operated upon several months previously, which 

 I had brought with me from Konigsberg. In the course 

 of the session I asked Herr Von Wittich, who was jiresent 

 in the audience, how often each of the frogs, which were 

 resting silently and still, should croak. The answer five 

 times was given, and each of the frogs, upon being given five 



