Some Aspects of Attention 289 



one such device. The comparatively slight amount of 

 chemical energy coming from a bit of food may have its 

 effectiveness for the nervous system greatly increased through 

 its reception by a structure adapted to use the whole of it 

 to advantage. Light stimulation involves a quantity of 

 energy that is insignificant in comparison with tljgLjgrosser 

 forces acting on an organism; yet falling on the retmaT^the- 

 energy is economized and magnified through the stored--up 

 chemical forces it sets free. Thus a weak stimulus may by 

 a sense organ be made powerful to determine reaction. 

 Another arrangement to the same effect is the peculiarity 

 of the nervous system whereby, through an arrangement 

 akin to the summation of faint stimuli, a moving stimulus, 

 one acting successively upon neighbor ng points of a sensitive 

 surface, produces an effect disproportionate to its intensity. 

 A moving stimulus is a vitally important stimulus ; it means 

 life, and hence may mean food or danger. The response to 

 it is in most cases adapted rather to its importance than to 

 its physical strength A third arrangement for the securing 

 of reaction to vitally important stimulation lies in the existence 

 of preformed connections in the nervous system, which bring 

 it about that the path of the excitation produced by one stimu- 

 lus is clear to the motor apparatus, while that of another is 

 closed. Reactions of this sort we call instinctive. The 

 nesting bird responds to the sight of building material rather 

 than to that of objects offering equally strong stimulation 

 to the optic nerve ; the cat sits at the mouse hole, the parent 

 animal responds to the faintest cry of the offspring, because 

 these stimuli have the right of way by virtue of inherited 

 nervous connections. 



Finally, a weak stimulus may determine reaction and be 

 victorious over a stronger one because of nervous pathways 

 formed throught he individual's own experience. The conse- 



