558 MUSCLE-NERVE PHYSIOLOGY 



b. Repeat using the break induction currents. 



2. The Muscle-nerve Preparation. The classical muscle-nerve prep- 

 aration is the gastrocnemius muscle and the sciatic nerve. Prepare it 

 as follows : a. Kill the frog by pithing. This is done by grasping the frog 

 firmly in one hand and with the other making a cut with a blunt scalpel 

 through the cranium just over the medulla, turning the scalpel so as com- 

 pletely to destroy the medulla. Now take a slender knitting needle, 

 quickly run it up into the cranial cavity to destroy the brain, and down the 

 spinal canal to destroy the cord. After a brief spasmodic contraction of the 

 muscles of practically the entire body, the frog remains limp and motion- 

 less, b. In making the muscle-nerve preparation it is better to isolate 

 the tendon Achilles and insert its hook first, then expose the nerve, and 

 finally the femur. The nerve should be prepared of as long a length as 

 possible and should not be allowed to come in contact with the skin. If 

 the preparation is to be used in a moist chamber, the skin should be entirely 

 removed; if it is to be used in the open air, the skin should be left on. Use 

 care not to stretch the nerve, and protect it from evaporation. Cut the 

 femur long. 



3. The Irritability of Nerve. Prepare a muscle-nerve with its skin 

 on and do not cut away the foot. Mount it by inserting the femur in a 

 muscle clamp, letting the leg extend vertically upward, and the foot hang 

 over. The nerve should lie along the exposed moist femur, one end being 

 slightly free. Stimulate the nerve in the following ways: 



a. Electrical Stimuli. Apply the electrodes of the secondary coil of an 

 induction apparatus to the tip of the nerve. When an induction current of 

 sufficient strength is produced, the muscle to which the nerve is attached 

 will give contractions, thus moving the foot. Notice that contractions 

 occur with both make and break inductions. Apply the electrodes from 

 the two poles of a dry battery. When the current of the battery is 

 established a contraction will occur, but does not continue during the time 

 of the flow of the current. When the current is stopped a second contrac- 

 tion occurs. The nerve is irritable to both galvanic and faradic currents. 



