16 ELEMENTARY PHYSIOLOGY. II. 



3. Dissect out similarly the nerves of the opposite 

 side, but instead of tying them in order to hold them up, 

 cut across the vertebral column a little above its end, 

 and hold them up by the piece of bone cut off. Place 

 the leg with its attached nerves on a board covered with 

 cork, and fix with pins the tissue just above the leg. 

 Keep the nerve wet with physiological salt solution 

 (Ringer's fluid) 1 . 



Cut longitudinally through the skin of the body of the 

 frog and expose the muscles over the sternum. 



4. Galvanic current. Connect the electrodes with 

 the cell interposing an open in-circuit key as in 3, p. 7. 

 Hold the electrodes steadily on the muscles on one side 

 of the sternum. Make the current and in a couple of 

 seconds break it. At the make the muscles will con- 

 tract and draw the arm up. At the break the contrac- 

 tion will be much less. The making of the current is a 

 more effective stimulus than the break. If the muscle is 

 in good condition the arm will be more or less raised 

 during the whole period of the passage of the current. 



Place the electrodes under the nerve ( 3), and make 

 and break the current. There will be a contraction at 

 make and at break but no effect will be seen during the 

 passage of the current. The relative strength of the two 

 contractions depends on the condition of the nerve, the 

 strength of the current, and its direction in the nerve. 

 Test the latter by reversing the position of the electrodes. 



5. a. Single induction currents. Connect the elec- 

 trodes with a short-circuit key, and this with the second- 



1 Ringer's fluid is a mixture of salts in approximately the same per- 

 centage as in serum. That used for the frog may consist of NaCl -65 p.c. , 

 KC1 -02 p.c., CaCl 2 -025 p.c., NaHC0 3 -015 p.c. 



