MUSCLE-NERVE. 



you have done. Revolve drum a sufficient distance and repeat, 

 stimulating, this time, in the kathodic region. Record data as 

 before. 



2. Now, reverse the constant current so that the anode is away 

 from the muscle and the kathode is near the muscle. Repeat the 

 stimulation with the induced current in the anodic and kathodic 

 regions, as before. Record the muscle response, or the lack of it, 

 on the drum and make careful note of all the data. 



From these experiments, what conclusions can you draw con- 

 cerning the effect of the passage of a constant current upon the 

 irritability of a nerve ? 



3. Repeat the above experiments with ascending and descend- 

 ing currents, stimulating in the anodic and kathodic regions im- 

 mediately after the cessation of the constant current. What is the 

 after-effect of the passage of the constant current upon the irrita- 

 bility of the nerve ? 



XXII. THE CONSTANT CURRENT AS A STIMULUS. PFLU 

 GER'S LAWS. 



As we have already seen, a sudden increase in intensity of a stim- 

 ulus which is being applied to a nerve or a muscle is effective in 

 producing an impulse in the same. A gradual increase, on the 

 other hand, is not effective. 



In the same way a sudden increase in irritability will, of itself, 

 act as a stimulus. Thus, when a constant current of sufficient 

 strength is passed through a nerve, there is a sudden increase in 

 irritability in the region of the kathode and a sudden decrease in 

 irritability at the anode. There will then be a stimulation of the 

 nerve in the kathodic region which will cause an impulse to be 

 transmitted toward the muscle. If the current is an ascending one, 

 however, and the conductivity is sufficiently diminished at the 

 anode, the impulse will be blocked and the muscle will not respond 

 with a contraction. 



When the constant current ceases to flow, the irritability sud- 

 denly falls at the kathode and rises at the anode. The conductivity 



[41] 



