ANIMAL BEHAVIOUR 



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of a stimulus a sensory or perceptory nerve-cell from 

 which impulses pass in to the central nervous system ; 

 (2) a ' motor ' nerve-cell which connects the central 

 nervous svstcm with a muscle, or, it may be, with a 



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gland ; and (3) between these two a ' communicating ' 

 nerve-cell connecting them within the nervous system. 

 But the pre-established arrangements arc usually more 

 complex, and several " reflex arcs ' become interlinked. 



FIG. 49. ATTITUDE OF A HEN PROTECTING HER BROOD AGAINST A DOG 

 (From Darwin's Expression of Emotions.) 



The reflex way in which a crab throws off a limb that 



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has been crushed or violently seized is nowadays simple 

 physiologically as compared with the way in which the 

 same is accomplished by a prawn. 



Reflex actions are uniform reactions to a particular 

 kind of external or internal stimulus ; they are exhibited 

 by all animals of the same species in approximately the 

 same way, though some individuals are quicker than 



