7° 



328. Mechanical. Pinch the free end of the nerve sharply 

 with a pair of forceps ; the muscles contract and the foot is 

 raised suddenly. Cut off the pinched portion. Contraction 

 again occurs. 



329. Thermal. To the same preparation apply at the 

 free end of the nerve a wire or needle heated to a dull heat. 

 Contraction again occurs. Cut off the dead part of the 

 nerve. 



330. Chemical. Place some saturated solution of sodium 

 chloride in a watch glass and let the free end of the nerve 

 dip in it. It requires a few moments for the salt to diffuse 

 into the nerve on account of the difference in the specific 

 gravity. Soon the joints of the toes twitch and by-and-by 

 the whole limb is thrown into irregular, flickering spasms, 

 which terminate in a more or less continuous contraction, 

 constituting tetanus. Cut off the part of the nerve affected 

 by the salt ; the spasms cease. 



(a) Finish the experiment by exposing the nerve to the 

 vapor of strong ammonia in a test tube. The ammonia 

 must not act directly upon the muscle, the tube should be 

 raised slightly above the level of the muscle and the end of 

 the nerve elevated to the mouth of the tube. There should 

 be no contraction if the vapor has not come in contact with 

 the muscle. The ammonia kills the nerve. Apply ammo- 

 nia to the muscle. It contracts. 



331. Electrical. For the following experiments use the 

 other leg of the frog taking the same precautions in the dis- 

 section and application of the normal salt solution. 



(a) Arrange the nerve-muscle preparation with the femur 

 in a clamp and the foot of the frog connected with a record- 

 ing lever. Arrange the Daniell Cell and DuBois Reymond 

 Key with the induction coil. Connect the battery wires 

 with the primary coil and remove the secondary coil to the 

 lower end of the scale. Arrange the electrodes and a record- 

 ing drum with smoked paper conveniently to the preparation. 



