THE NERVOUS SYSTEM 61 



great heat, for example, the body proteins would coagu- 

 late, but death would be painless. 



In taking possession of the final common path, 

 stimuli observe a definite order of succession. The 

 stimulus which secures possession of the final common 

 path at any moment is always the stimulus which 

 phylogenetically is the most important. For example, 

 stimuli threatening death or fatal injury will take 

 precedence over stimuli presaging slight physical 

 discomfort or the acquirement of food. Thus, in the 

 simultaneous arrival of a flea bite and a heavy blow 

 a man would be unconscious of the bite and would 

 feel only the blow. Moreover, a physically or phylo- 

 genetically less intense stimulus already in possession 

 of the final common path can at any time be dis- 

 possessed by a stronger stimulus or one of more phylo- 

 genetic importance. Thus, an animal in enjoyment 

 of its food, or a hunter in search of small and harmless 

 game, would each drop his occupation and flee in terror 

 at the sudden apparition of a powerful advancing 

 enemy. Thus, the schoolboy may have his desire 

 for learning dispossessed by a pin prick administered 

 by a neighbor; but if the schoolhouse were to catch 

 on fire, the pin prick would be unnoticed and primitive 

 fear would assume control of his motor mechanism. 



The Threshold 



The order of precedence of stimuli is, to a large 

 extent, determined by what is known as the threshold, 

 or the amount of resistance which each stimulus has 

 to overcome before it excites action. Each incoming 

 impulse, somewhere in its path to the brain cell, or 



