ACTION OF THE NERVOUS SYSTEM. 61 



They cause chemical and molecular changes in the 

 nervous system. This is most easily observed where 

 there is a nervous system with end-organs capable 

 of being very sensibly affected by the slightest 

 variation in the forces of the environment, as, for 

 example, the eyes are affected by variations in the 

 intensity of light, and the skin by variations of 

 temperature. But even in cases where there is 

 no nervous system, all organisms are affected by 

 light and heat, and there is reason to believe that 

 could we test the other forces equally well, we 

 would find that they produce an equally great effect 

 upon living matter, whether a nervous system were 

 present or absent. 



When the forces of the environment enter the 

 mass of living matter, it cannot be that they are de- 

 stroyed. If they be no longer recognisable in their 

 original form as light, heat, etc., they must be trans- 

 muted to some other of the various forms of energy. 

 The only other form of energy which can be discov- 

 ered as these forces vanish at the periphery of the 

 living mass, is that form of energy which appears as 

 nervous activity. Therefore, we may safely conclude 

 that the energy of the forces acting from without, 

 persists within the living matter as nervous activity 

 and change of nervous condition. 



On the other hand, when we look for the imme- 



