PFLUGER'S LAW OF CONTRACTION. 201 



(d.) Turn the commutator so that the positive pole is next 

 the muscle, at once the straw falls i.e, the excitability of the 

 nerve in the region of the positive pole is so diminished as 

 to "block " the impulse passing to the muscle, showing that 

 the positive pole lowers the excitability. 



(e.) Reverse the commutator so that the negative pole is 

 next the muscle. At once the limb becomes tetanic, the 

 negative pole (kathelectrotonic area) increases the excita- 

 bility. 



LESSON XLII. 



ELECTROTONIC VARIATION OF THE ELEC- 



TROMOTIVITY PFLUGER'S LAW OF 



CONTRACTION RITTER'S TETANUS. 



1. The Electrotonic Variation of the Electromotivity. 



(a.) Arrange a long nerve on the N.P. electrodes, as for 

 determining its demarcation current. Place the free end of 

 the nerve on a pair of N.P. electrodes the polarising 

 current arranged as in Lesson XLL, 2, A., so that the 

 current can be made ascending or descending. 



(b.) Take the deflection of the galvanometer*, needle or 

 demarcation current when the polarising current is shut off. 

 Throw in a descending polarising current, and observe that 

 the spot of light travels towards zero. Reverse the commu- 

 tator, and throw in an ascending current, the spot of light 

 shows a greater positive variation than before, From this 

 we conclude that kathelectrotonus diminishes the electromotivity, 

 while anelectrotonus increases it. 



2. Pflttger's Law of Contraction Apparatus. Several small 

 Grove's cells, commutator with cross-bars, du Bois and Morse 

 key, rheochord, N.P. electrodes, moist chamber, wires, frog, 

 recording apparatus, and usual instruments. 



