62 EXPERIMENTAL GENERAL PHYSIOLOGY 



solution is permeating the sheath of the nerve trunk, adjust the 

 commutator 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. 



(3) Close the commutator circuit, open the short-circuiting key 

 i. e., make the "polarizing" current. If the experiment is successful 

 the tetanus is more marked. Which pole is near the point stimulated ? 



(4) 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? 



(5) Repeat (3) and (4) several times. It is evident that the irri- 

 tability of the nerve to the salt stimulus is increased in the region 

 of the cathode pole and decreased in the region of the anode pole. 

 This changed condition of the nerve due to the passage of a constant 

 current is called electrotonus. The state of increased irritability in 

 the region of the cathode is called catelectrotonus. The decreased 

 irritability in the region of the anode is called anelectrotonus. 



(6) Myographic Record of Anelectrotonus and of Catelectrotonus. 



1. Appliances. Three or four Daniell cells; three Du Bois- 

 Reymond keys; contact key; two commutators; inductorium; two 

 N. P. electrodes; eighteen wires; kymograph; myograph with moist 

 chamber; two pairs of platinum- wire electrodes to use with induc- 

 tion current. 



2. Preparation. Arrange apparatus according to plan as shown 

 in Fig. 38. 



FIG. 38 



3. Operation. Make and mount a gastrocnemius preparation 

 in moist-chamber myograph, or frog-board myograph. Adjust 

 electrodes as shown in diagram. 



Test apparatus and preparation by sending single make (or break) 

 induction shocks through nerve at M . Let there be a typical response 

 to these stimuli. The secondary coil should be removed to a distance- 

 that gives the stimulus of liminal intensity. 



