THE NERVOUS IMPULSE 255 



of atrophy and depression of excitability. The muscle 

 ceases to respond to the alternating current. At make 

 and break of the constant current it responds with a 

 sluggish contraction. Further, while a normal muscle 

 responds better to a closing (make) current when stimulated 

 by the kathode than when stimulated by the anode 

 (KCC > ACC), in the degenerated muscle it responds to 

 kathode and anode indifferently. 



THE NERVOUS IMPULSE 



We shall now consider the excitability and conductivity 

 of the nerve-fibre with a view to understanding the nature 

 of a nervous impulse. 



Velocity 



The rate at which a nervous impulse travels is estimated 

 in the nerve-muscle preparation by stimulating the nerve 

 first at one point, then at another point along its course, 

 and measuring the difference in the latent period. For 

 the frog the velocity is twenty-eight metres per second. 

 For warm-blooded animals it is probably at least five times 

 as great, the rate of conduction increasing considerably 

 with rise of temperature. 



Reversibility of the Impulse 



A disturbance arising at any point in a nerve-fibre is 

 transmitted throughout the fibre in both directions. This 

 was proved by the classical experiment of Kiihne. The 

 frog's gracilis muscle consists of two halves, separated by 

 a fibrous band ; each axon as it enters the muscle divides, 

 one branch going to each half of the muscle. Stimulation 

 of one half of the muscle where it contains nerve-end- 

 ings causes contraction of the whole muscle, the impulse 

 having travelled up one set of branches of the axon to the 

 point of division and down the other set. 



An analogous phenomenon is found in connection with 



