158 NERVOUS CORRELATION 



activity is likely to be the only kind that will serve the 

 animal. When the distance receptors are stimulated, 

 on the other hand, the adjustment need not be immediate. 

 If the stimulus is indicative of danger the reaction may 

 be delayed somewhat and yet the organism may survive. 

 This introduction of a possibility of delay in response to 

 stimulation permits a great extension of nervous action 

 in that it opens the way for the making of a choice as to 

 what the response is to be. In simple reflex actions, 

 as has been repeatedly* said, no choice enters, the response 

 follows immediately and automatically upon the stimula- 

 tion. The possibihty of choice implies, on the other hand, 

 that the response need not be immediate and automatic 

 but may be delayed somewhat and may be subject to 

 regulation. We can think of only one basis for this regu- 

 lation which would be of any value whatever to the or- 

 ganism, namely, what has happened to it before; so that 

 choice, to be worth anything, must be founded on past 

 experience. A very little reflection will show clearly how 

 vast is the extension of an animal's range of adjustment 

 if it can call previous experience into play as a part of the 

 mechanism for guiding its present behavior. 



Machinery for Using Past Experience. — In an 

 earlier paragraph the fact was brought out that distance 

 receptors communicate directly with the front ganglion 

 of the nervous system, the brain. Some of the nerve- 

 cells of this front ganglion have a peculiar property not 

 found in the other nerve-cells in the body, namely, that 

 when nerve impulses come into them they are not passed 

 out immediately as is the case with the other nerve-cells, 

 but are stopped and held. The result of this property is 

 to interrupt reflex action and prevent its completion. 

 These nerve-cells have the additional peculiarity that the 

 nervous disturbances that are thus held within them 

 make some kind of permanent impression upon them, so 

 that under suitable conditions impulses previously re- 

 ceived can be passed on again. Thus a reflex action that 



