DEATH OF THE NERVE. 123 



the excitability may vary in degree, and it is therefore 

 simpler to assume that the difference in the results of 

 irritating the nerve at various points depends directly 

 on differences in the excitability at those points, instead 

 of being in the first place dependent on changes caused 

 by transmission ; it can even be shown to be probable 

 on various grounds, as indicated above, that the excite- 

 ment in propagating itself through the nerve meets 

 with resistance, and is therefore rather weakened than 

 strengthened. Why the excitability differs in different 

 parts of the same nerve we cannot explain. As long 

 as we are ignorant of the inner mechanism of nerve- 

 excitement, we must be satisfied to collect facts and to 

 draw attention as far as may be to the connection of 

 details, but we must decline to offer a full explanation 

 of these. 1 



7. The phenomena of exhaustion and recovery may 

 be exhibited in nerves as in muscles. If a single 

 point in a nerve is frequently irritated, the actions 

 become weaker after a time, and finally cease entirely. 

 If the nerve is then allowed to rest for a time, new 

 pulsations may again be elicited from the same point. 

 It is not known whether this exhaustion and recovery 

 corresponds with chemical changes in the nerve. We 

 are almost entirely ignorant of the whole subject of 

 chemical changes within the nerve. Some observers 

 maintain that in the nerve, as in the muscle, an acid 

 is set free during the active condition, but this is 

 denied by others. The generation of warmth in the 

 nerve during its activity has also been asserted, but 

 this is also doubtful. If any chemical changes do take 

 place within the nerve, they are extremely weak and 

 1 See Notes and Additions, No. 4. 



