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 occur 

 in entire independence of the higher nerve centers. The 

 Abbe Spallanzani showed that a male frog may have its 

 head cut off during copulation without ceasing to cling 

 tenaciously 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 legs. Yet when 

 the skin of the inner surfaces of the fore legs was rubbed 

 with the fingers, this segment would show the same clasping 

 efforts as a normal male frog. The clasping of the male 

 frog is, therefore, a reflex action whose center lies in the 

 brachial region of the spinal cord. 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 

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