12 BRAINS OF RATS AND MEN 



actually do? What part do they play in the practical 

 problems of getting on in the world? 



One phase of this problem is satisfactorily an- 

 swered. Some of these nerve fibers are the an- 

 nunciator wires that run from the doorways at the 

 surface of the body. A beam of sunlight falls upon 

 the body. There are only two doors by which it may 

 enter. If it enters the eye, here it presses a button 

 that activates a circuit in the optic nerve, the signal 

 is registered upon a central switchboard, and we see 

 the light. Or if it falls upon the skin, another button 

 is pressed and we feel the genial warmth. Similarly, 

 the ear alone can announce the impact of a sound 

 wave and the nose the odor of my morning coffee. 



The nerves and their sensory end-organs are 

 analyzers, through whose action alone we are able to 

 adjust ourselves to what is going on about us. The 

 actual adjustment is effected by another set of 

 nervous circuits which go out to the appropriate 

 muscles. But what determines which muscles will 

 respond to any particular sensory excitation — 

 whether I drink the coffee or give it to the goldfish? 



The answer to this question is to be sought in the 

 central switchboard mechanisms of the brain. And 

 for this answer we are still searching. We know that 

 the burned child dreads the fire, and instead of 

 reaching out again to grasp the pretty flame, he 

 shrinks in terror. But we do not yet know how he 

 does it. We believe also that this particular switch- 



