b. The gracilis muscle of the frog is divided by a fascia into an 

 upper shorter part and a lower longer part. Remove carefully the 

 muscle with its attached nerve and note that the nerve and blood 

 vessels divide so as to go on each side of the tendon. Cut the muscle 

 in half at the tendon without injuring the nerves. Then stimulate 

 one half. Does the other half contract? 



211. Elcctrotonits. a. Effect of constant current on irritability. 

 Two currents are to be sent through the nerves, a galvanic polarizing 

 current \vhose effect on irritability is to be studied and a stimulating 

 current of single induced shocks for testing the irritability. Set up 

 the nerve muscle preparation in the moist chamber for recording 

 en a drum, placing the nerve over non-polarizable electrodes about 

 1-2 cm. apart, connected through the rheocord and a reversing key 

 with the desk binding posts which will give a strong galvanic current. 

 Place the stimulating needle electrodes (see that they are clean ) on 

 the muscle side of the boot electrode nearest the muscle and connect 

 with the secondary of an inductorium. Stimulate the nerve with a 

 minimal break induced shock and record the contraction on a 

 stationary drum. Xow send a weak constant current through the 

 nerve and, disregarding the contraction on making the galvanic cur- 

 rent, stimulate again. Is the height of contraction increased or 

 diminished? Indicating what? Break the galvanic current and in a 

 minute or so stimulate again. Result ? The record should show the 

 result of stimulation in all phases of the experiment. At which 

 pole is irritability increased? 



Repeat the above but reverse the direction of the constant current 

 through the nerve. Result and conclusions? 



The best results are obtained with a certain strength of polariz- 

 ing current which must be determined by experiment. Do you see 

 now why the make galvanic stimulus is greater than the break ? 



b. Effect of constant current on conductivity. Apparatus the 

 same as in the preceding experiment except that the stimulating 

 electrodes are placed midway between the boot electrodes and a 

 piece of muscle is introduced in the stimulating circuit to increase 

 resistance. Do you see why? Xo record need be taken. Cse 

 minimal stimuli as before. Determine if a weak galvanic current 

 can block the passage of a nerve impulse and the pole at which the 

 block occurs. Increase the strength of the polarizing current and 

 determine its effect on the blocking of the impulse. 



212. Speed of the nerve impulse. Adjust the drum for turning 

 by hand. Place two pairs of needle electrodes in the moist chamber 



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