76 LABORATORY GUIDE IN PHYSIOLOGY. 



that end of the plate farthest from the muscle. Lay 

 the nerve over the electrodes and along the glass 

 plate. The segment of nerve which lies upon the 

 glass plate between the electrodes and the muscle 

 may be subjected to various stimuli, mechanical and 

 chemical. Sterling (Prac. Phys., p. 244) uses salt. 

 At a point about 1 cm. from the electrodes, marked x 

 in the figure, place upon the nerve trunk as many fine 

 crystals of common salt as would be taken up on the 

 point of a penknife. Moisten these salt crystals with 

 a drop of water. While the salt solution is per- 

 meating the sheath of the nerve trunk, adjust the com- 

 mutator for a descending current. When the muscle 

 begins to twitch, note the effect upon the signal. The 

 contractions become more and more tetanic in 

 character. 



(4) Close the commutator circuit, open the short-cir- 

 cuiting key, i. e., make the "polarizing" current. If 

 the experiment is successful the tetanus is more 

 marked. Which pole is nearer the point stimulated? 



(5) Close the short circuiting key, i. e., break the 

 "polarizing" current. Reverse the commutator; 

 make the current. The muscle is put completely or 

 almost completely at rest. Which pole is nearer the 

 stimulus? 



(6) Repeat (4) and (5) several times. It is evident 

 that the irritability of the nerve to the salt stimulus is 

 increased in the region of the kathode, and decreased 

 in the region of the anode pole. This changed con- 

 dition of the nerve due to the passage of a constant 

 current is called electrotonus. The state of increased 

 irritability in the region of the kathode is called 

 katelectrotonus. The decreased irritability in the region 

 ol the anode is called anelec trot onus. 



