87 

 XXII 



319. Each student is to have a frog, which is to be pithed 

 and have its brain and myel destroyed by passing- a tracer or 

 seeker throug-h the Spinal canal. The leg-s are to be used for 

 nerve muscle preparations. Dissect one leg- for the first series 

 of experiments, and reserve the other leg for the second series. 

 Begin the dissection upon the dorsal aspect of the leg, remov- 

 ing the skin and muscles very carefully until the sciatic nerve 

 is exposed. Dissect out the nerve as long as possible, and 

 moisten frequently with the normal salt solution. Remove 

 all of the muscles as far as the knee, leaving the femur and 

 nerve entirely isolated. Avoid all injury to the nerve during 

 dissection, and apply the normal salt solution every few min- 

 utes with a camel's hair brush. Arrange the nerve muscle 

 preparation by placing the femur in a clamp and allowing the 

 nerve to hang freely, A small lever may be pinned to the 

 foot to emphasize any movements that may occur. Apply 

 the following stimuli : 



320. 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. 



321. Thermal. To the same preparation apply at the free 

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

 lighted match. Contraction again occurs. Cut off the dead 

 part of the nerve. 



322. 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 or bottle. The am- 

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

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



