XVI THE NERVOUS SYSTEM 305 



croaks. If the animal is stroked once on the side, it croaks 

 once and then stops because the stimulus is stopped. 



The Clasping Reflex and the Recognition of Sex. — The 

 tendency of the male frog to clasp the female is one of the 

 strongest of its instincts, but it appears only for a short time, 

 during the breeding period in the spring. At this time the 

 male will clasp another male frog, one's fingers, or in fact 

 almost any object that is placed between its fore legs, but 

 objects other than females are after a time rejected, while the 

 duration of ordinary copulation is commonly several days. 



The clasping efforts of the male frog afford a typical 

 illustration of instinctive action ; nevertheless they occu r 

 in entire independence of the hi gher Tie rvp rpnt-prg The 

 Abb^ Spallanzani showed that a male frog mav have its 



head cut off during CO J^ulatjon withmit- rpasing to rljpg 



t enaciously to the female. Goltz went still farther and 

 cut off the head of a male, then cut the body through 

 between the third and fourth vertebrae, and removed the 

 viscera from the body cavity ; the section of the frog that 

 remained after these operations consisted of the first three 

 vertebrae, the pectoral girdle, and the fore legg>«_ Yet whe n 

 t he skin of the in ner ^^nrfirpwvMhp fnrp legs was rubbed 

 \yith_Lhs_fingers, this se gment would sho w the same claspin g 

 e fforts as a normal male frog . The clasping of the male 

 frog is, therefore, a reflex actio nwhose center_lies_in .the 

 b rachial region of the spinal co rd^ If the skin on the 

 inner surfaces of the fore legs and on the breast be re- 

 moved, the clasping reflex is destroyed. It is probable, 

 therefore, that the reflex is initiated through the stimulation 

 of the sensory organs of these regions of the body. 



Notwithstanding the almost mechanical character of this 

 spinal reflex, the frog discriminates between two such similar 

 objects as the males and females of its own species. When 



