660 PHYSIOLOGY. 



nerve is extremely variable and is in a constant state of instability. 

 Intervals of repose alternating with activity are the most favor- 

 able conditions for the maintenance of irritability. When a nerve 

 remains at rest for a long time the irritability diminishes and may 

 even be abrogated, conducing to degeneration of the nerve. Ex- 

 cessive stimulation has a similar effect to destroy the nerve. 



For a proper appreciation of so delicate a structure as the 

 nervous tissue and the changes of a fundamental order occurring 

 within it, the student should picture to himself the physical condi- 

 tion of the nerve ; how it is composed of molecules in a state of stable 

 equilibrium. With this conception he will readily see how any ex- 

 ternal stimulus may produce molecular movement in one direction 

 and hold them in said position for any variable time. 



With cessation of the exciting cause the molecules will be re- 

 leased from their rigid condition and immediately return to their 

 previous normal state. This "return" is the occasion of changes 

 in the opposite direction. Thus, any power that is capable of pro- 

 ducing movement in any one direction is sure to be succeeded by 

 movement in the opposite direction as the molecules of the nerve 

 resume their normal, stable equilibrium. 



This fundamental principle must constantly be kept before the 

 student's mind, since many of the physiological phenomena of the 

 nervous system are dependent upon it, or their conception is mate- 

 rially aided by remembering it. 



IRRITABILITY OF DIFFERENT POINTS OF THE SAME NERVE. The 

 farther from the muscle the nerve is stimulated, the higher will be the 

 original irritability. It was upon this fact that Pfliiger predicated his 

 erroneous avalanche hypothesis: that a nerve-wave gathers force as 

 it passes along the nerve-fiber. The true theory about the fact is that 

 the irritability of the nerve is elevated in the neighborhood of the 

 cross-section by the passage of the demarcation current through that 

 portion. It has been shown by mechanical stimuli that the uninjured 

 nerve has an equal irritability throughout its whole length. 



Effect of Heat on Nerves. Any sudden change of temperature 

 acts as an excitant of a nerve. A temperature below 24.8 F. or 

 above 95 F. applied to a motor nerve of a frog calls out a contrac- 

 tion of the muscle. 



If, however, a nerve be gradually frozen it will regain its excita- 

 bility upon thawing. When a nerve is cooled, in the case of the frog 

 the irritability persists for a long time. If a nerve of a frog is 

 heated to 113 F. its excitability is increased and then diminished. 



