LAWS OF SENSORY IMPULSES 437 



First law. A sensory fiber wherever stimulated, car- 

 ries only one kind of impulse. Thus a touch nerve fiber, 

 whether stimulated in its end organ or elsewhere always 

 produces the sensation of touch, never that of sight or 

 hearing. 



Second law. In consciousness an impulse is located as 

 coming from the natural source of such an impulse. Thus, 

 in an amputated leg, pressure upon the cut end of the 

 nerve is felt by the brain as though it came from the 

 original end organs of that nerve. In this way may be 

 explained many of the instances where pains are felt in 

 the toes or fingers even after these parts have been 

 amputated. 



Third law. The increase in any given sensation from a 

 definite kind of impulse does not take place uniformly 

 with the increase in stimulation. On the contrary the 

 amount of stimulation has to increase by a definite quan- 

 tity before any increase in sensation is felt. A man named 

 Weber investigated this relation between stimuli and sen- 

 sation, and the law is often called Weber's Law on that 

 account. An illustration will make clear its application. 

 If we hang a thirty-gram weight to the finger we get a 

 certain feeling of pressure. If we now add a half-gram 

 weight no increase in pressure is felt, and it is only when 

 we add a whole gram that the increase is actually felt. 

 If, now, we start with sixty grams we have to add two 

 grams before the increase is felt; that is, one thirtieth of 

 the original weight. In this case, the stimulus increase 

 necessary to sensation is one thirtieth of the original weight. 

 The amount of increase in a stimulus necessary to in- 

 creased sensation varies greatly with different fibers. It 

 is possible to reduce by practice the increase in stimulus 



