STIMULATION, CONDUCTION, RESPONSE 19 



or other forms of energy are all capable of producing 

 changes in protoplasm. Its power to respond to stimuli 

 is dependent upon its physical and chemical structure. 

 Not only does it respond at the point of contact, but the 

 stimulus may produce the response at a distant point. 



Stimulation, Conduction, and Response. — It will 

 be observed that these are really three separate and dis- 

 tinct processes. The first is the sensitiveness of the part 

 of the protoplasm which first receives the stimulus; the 

 second part is the conduction of the stimulus through the 

 protoplasm, that is to say, the propagation of some sort 

 of change ; and third, there is the response in some other 

 part of the protoplasm. The response differs from sensi- 

 tiveness and conduction, ordinarily, in that it is accom- 

 panied by much greater energy changes. If the response 

 is to be one of movement it is obvious that energy must 

 be liberated by the decomposition of some energy- 

 containing substance in order to accomplish it. We have 

 an exhibition of this in our own bodies in a very special- 

 ized form, where the result is brought about, not within 

 a single cell, but by the cooperation of many cells. If 

 one touches his finger to a hot stove the finger will be 

 automatically withdrawn. In order that this may 

 happen, all of the three things which we have noted 

 in the single cell must occur in the human body. That 

 is to say, there must first be sensitiveness to this heat 

 energy; then some sort of stimulus must be conducted 

 along the*nerve trunk to the nerve center and back again 

 to the muscle; and finally, a liberation of energy must 

 be brought about which will enable the muscle to con- 

 tract and draw the finsier away from the hot stove. 



Cell Division and Reproduction. — When cells grow 

 they tend to reach a maximum size which is determined 

 by the kind of plant or animal in which they occur. 

 When they have reached this size division automatically 

 occurs. This is a somewhat complicated process and 

 will be described in another place. It is intended here 



