GENERAL PHYSIOLOGY OF MUSCLE AND NERVE TISSUE 63 



To close the constant current "polarizes" the nerve, or, better, 

 induces electrotonus. 



That segment of the nerve between the anode and cathode is 

 called intrapolar region. 



Those segments centrally and distally located are called extra- 

 polar. 



The induced current is called stimulating current. 



4. Observations. (1) Adjust for descending, polarizing current. 

 Stimulate in the region of anode. Note extent of muscle contraction. 

 Induce electrotonus; stimulate again in region of anode. If the 

 experiment is successful the contraction will be found to be decreased 

 or absent. 



The nerve is at this point in a condition of anelectrotonus. 



(2) Stimulate at M, or in the region of the cathode. Withdraw the 

 polarizing current. After a few minutes stimulate again at M. If 

 the experiment is successful the wave is higher in the former than 

 in the latter case. 



The stimulation was made in the region of the cathode and the 

 nerve in a condition of catelectrotonus. 



(3) Adjust for ascending, polarizing current. 



Stimulate at M i. e., in the region of the anode. The contrac- 

 tion is weaker than in the normal nerve, or it may be quite absent. 

 This region is now in a condition of anelectrotonus. 



(4) Stimulate in the region of the cathode. The response is 

 probably weak. Withdraw the polarizing current. Stimulate again 

 in the region of the cathode. The response is normal i. e., it is 

 greater than during the electrotonic condition. 



But in descending extrapolar catelectrotonus the response was 

 greater than normal. In the experiment just performed we stimulate 

 in the region of ascending extrapolar catelectrotonus. Note that the 

 polarizing current is relatively strong. 



. (5) Remove one cell from the battery and repeat (4). If the 

 response to stimulation is still weaker with than without the polar- 

 izing current, reduce the strength of the polarizing current still 

 farther by the use of the simple rheocord. Finally, with a weak 

 polarizing current the stimulus in the region of extrapolar catelectro- 

 tonus causes a stronger response than normal. 



The response which the muscle makes must be accepted as a 

 measure of the excitation which it receives from the nerve. But 

 the excitation delivered by the nerve depends upon two factors 

 its irritability and its conductivity. When the nerve is stimulated 

 in the region of ascending extrapolar or intrapolar catelectrotonus, its 

 increased irritability is of no avail if there is interposed between 

 that region and the muscle a region of decreased conductivity. 

 With strong polarizing current the region of the anode is not only 

 decreased in irritability, but in conductivity. 



