304 THE BIOLOGY OF THE FROG CHAP. 



strokes, croaked exactly five times, to the evident gratification 

 of the auditors, whereupon they all lapsed into silence." 



The croaking of a brainless frog is a reflex set in opera- 

 tion by a particular localized external stimulus. In a normal 

 specimen this reflex is subject to control from the higher 

 nerve centers. It may be entirely checked by impulses 

 down the spinal cord from the brain. The normal frog 

 shows a spontaneity and freedom of action which is largely 

 destroyed in individuals which have recently lost their 

 cerebral hemispheres. The latter often croaks without any 

 apparent exciting cause, while a brainless frog croaks only 

 upon the application of an external stimulus to certain parts 

 of the body. 



The utility of the croaking reflex is not apparent. It has 

 been suggested by Baglioni that, since in croaking air is 

 pushed back and forth between the mouth and the lungs, 

 the swelling thus produced enables the animal to push 

 against anything that seizes it and make its escape, the 

 sound being merely an incidental accompaniment of the 

 process. Any one who picks up a frog in the hands may 

 readily convince himself that the croaking movements are a 

 means of enabling the creature to slip from his grasp. In 

 fact, these movements often occur in the female without 

 producing any sound whatever. It is a significant fact that 

 the reflex is evoked by stimuli upon those parts which, as 

 the body swells, press against whatever seizes the animal. 

 It is not brought about by seizure of the head or hind legs ; 

 the swelling in such cases would be of no avail. The fact 

 that the croaking reflex is brought about by gently stroking 

 the side or back may indicate nothing but the extreme readi- 

 ness with which the swelling reflex is initiated. When the 

 animal is seized and there is a constant pressure stimulus, 

 there is a repeated swelling of the body and accompanying 



