only partly separable. ( ireat care must be taken not to injure the 

 nerve in removing it from the frog. Never pinch a nerve with 

 forceps. 



206. Effect of alcohol and CO.^ on nerve, a. Carbon dioxide. 

 Arrange the inductorium for single induced currents. Connect 

 the secondary coil with the main posts of the pole changer (used in 

 this experiment as a double key ). Connect the two other pairs of posts 

 with the usual stimulating electrodes and the electrodes of the small 

 gas chamber. Join the inflow tube of the gas chamber with the 

 outflow tube of the O )., bottle. The gas chamber should be clamped 

 in position on a glass plate. Make a nerve-muscle preparation, prc- 

 scrviinj the full length of the sciatic nerve up to the vertebral column. 

 Pass the nerve through the holes of the gas chamber so that it lies on 

 the electrodes. The nerve should be drawn through until the muscle 

 is close to the gas chamber. Stop the holes through which the nerve 

 passes with normal saline clay. Bring the outer pair of electrodes 

 against the central (i.e., towards spinal cord) end of the nerve near 

 its exit from the gas chamber. Determine which position of the 

 double key corresponds to each pair of electrodes. Stimulate the 

 nerve first within the chamber, and then on the central end of the 

 nerve, using a current just sufficient to cause tetanus. What is the 

 result? Xow pour io c /r HC1 on the marble in the generator 

 and pass the gas through water and then through the chamber. 

 After a few minutes stimulate as before. Result? What is the 

 explanation ? 



b. Alcohol. Disconnect the rubber tube from the gas generator, 

 and blow through the gas chamber until the CO., is driven out. 

 Does the nerve recover its irritability ? Determine this by stimulating 

 from time to time. When the nerve has recovered, drop a little 

 alcohol through the long glass tube of the gas chamber, hein</ very 

 careful that only the vapor of the alcohol comes into contact t^'itli 

 the nerve. Stimulate both within and without the chamber. What 

 results do you now obtain ? Which property of nerve does the 

 alcohol affect? To obtain good results, the electrodes within the 

 gas chamber should not be too far from the opening through which 

 the nerve passes to the muscle. 



207. Threshold value of Stimulation. Prepare a gastrocnemius 

 muscle with the sciatic nerve from its point of origin in the spinal 

 cord down, and place in the moist chamber. Hang the nerve over 

 the needle electrodes. Determine the single break induced stimulus 

 which just causes contraction. This is the threshold value. Xow 



59 



