SUGGESTIONS FOR LABORATORY WORK 599 



SUMMATION AND INHIBITION OF REFLEXES 



Summation. Suspend a frog, whose brain has recently been 

 destroyed, by a hook through the lower jaw. Tie fine copper wires 

 34 inch (6 mm.) apart, about the right toes. Connect the wires 

 with the secondary coil of an inductorium, taking care to avoid 

 short-circuits. The primary circuit should be arranged to give 

 single shocks. 



Set the secondary coil so that a small twitch follows each 

 stimulus. Make and break the primary circuit repeatedly and 

 rapidly. 



Note that a series of stimuli produce an effect that a single 

 stimulus could not. This is summation. 



Inhibition. Bring dilute acid in a beaker in contact with the 

 left toes of the frog used in the preceding experiment. 



Determine in seconds the time required for the foot to be with- 

 drawn. 



Immediately wash thoroughly with water the acidified foot. 



Now bring the beaker of acid again in contact with the left toes, 

 at the same time stimulating the right toes with an interrupted 

 current of moderate strength. 



Determine the time for withdrawal of the foot from the acid. 

 A delay is due to inhibition. To prove that the acid has not injured 

 the foot repeat the immersion without simultaneous stimulation. 

 Prompt withdrawal should occur. 



NEURO-MUSCULAR FATIGUE 



Dissect out a gastrocnemius-sciatic preparation (p. 597). Ar- 

 range to secure a record of the contraction of the muscle. Moisten 

 the nerve frequently with salt solution to prevent drying and 

 consequent loss of irritability. With the drum moving at a slow 

 speed stimulate the nerve of the preparation with fairly strong 

 stimuli until the muscle no longer responds. Now quickly bring 

 the electrodes in contact with the muscle itself. A contraction 

 shows that the muscle is not fatigued. 



Since nerve trunks are indefatigable the fatigue must have oc- 

 curred in the neuro-muscular junctions (p. 198). 



