CHAPTER XIV. 



PHYSIOLOGICAL CHANGES IN THE CENTRAL SYSTEM. 



Nerve cell a source of energy The nerve impulse Direction of 

 impulse Nerve impulse aroused at the end of branches 

 Strength, form, and rate of impulse Arousal Pathway 

 Continuity of stimulation Diffusion Reinforcement Dif- 

 fusion illustrated Knee-kick Background of sensation. 



THE nerve element is a source of energy. Composing 

 the nerve cell are substances in chemical equilibrium so 

 unstable that stimuli easily excite them to further 

 change. As a consequence they assume a more stable 

 chemical form, and at the same time part with some of 

 their energy. Various similes have been used to illus- 

 trate this event. The irritable cell is often pictured as 

 a powder magazine to which a small spark, the stimulus, 

 may be applied, with the result of causing an explosion 

 that liberates a quantity of force many times greater 

 than that represented by the spark. The powder 

 magazine, however, explodes with the same force 

 whether it be ignited by a match or by a torch, and in 

 such a case, therefore, no proportion obtains between 

 the magnitude of the stimulus and the magnitude of the 

 explosion. For this reason the simile does not fully 

 express the facts, since in the nerve cell the larger 

 stimulus is followed by the larger explosion. A truer 

 picture may be obtained by comparing these events 

 with the result of rolling a ball against a group of pins. 



