THE ELEMENTS OF NERVE PHYSIOLOGY 41 



by a display of energy utterly out of proportion to that 

 which was brought to bear to excite them. We say that 

 the force they exert had its source in the chemical changes 

 that took place in their protoplasm, changes that must 

 have left the tissue in an altered condition and with a 

 lessened potency for further performance. 



A mechanical analogy suggests itself. Consider the 

 operation of the lock of a gun. The trigger is pulled and 

 the hammer falls, but not with the slight force applied 

 to the trigger. It comes down with the power represented 

 by the previous tension of the strong coiled spring. The 

 "stimulus" released an internal store of energy. This is 

 representative of the conditions exhibited by muscles, 

 and it is probable that with the neurons there is only a 

 difference in degree, not in kind. When we think of the 

 trifling amount of those forces which often prove suffi- 

 cient to produce extensive reactions of the organism we 

 can hardly doubt that the currents which are started 

 from the receptors receive a very marked reinforcement at 

 the cost of the gray matter. 



We have some definite evidence bearing upon this 

 point. If the perikarya make contributions from their 

 own stores to forward the nerve-impulses we may expect 

 them to show fatigue and structural change in connec- 

 tion with long-sustained action. Changes associated with 

 activity have been frequently described. Some years ago 

 Hodge announced that there is a clear difference between 

 the outline and internal appearance of the perikarya in the 

 gray matter of animals killed after a night's sleep and the 

 corresponding features as seen in specimens from animals 

 killed at the close of the day. He obtained his material 

 from birds whose intense life-processes might be expected 

 to make them most favorable subjects. The statements 

 of Hodge have been widely confirmed. We must discuss 

 this important matter again in connection with the topic 

 of fatigue. 



We shall assume provisionally that nerve-impulses may 

 be set going in the conducting pathways either by the 



