76 EXPERIMENTAL PHYSIOLOGY 



CHAPTEE VII 



INDEPENDENT MUSCULAR EXCITABILITY. EXCITATION OF MUSCLE 

 BY THE CONSTANT CURRENT. POLARISATION OF ELECTRODES 



INDEPENDENT MUSCULAR EXCITABILITY 



By this is meant the property a living muscle fibre possesses of 

 responding by a contraction to a stimulus applied directly to it. To 

 prove that the muscle substance is itself excitable, it is necessary to 

 devise an experiment in which the stimulation of the muscle-fibre via 

 its nerve is altogether avoided. The experiments commonly adopted 

 for this purpose are the following : — 



Experiment 1— BERNARD'S CURARE EXPERIMENT.— Destroy 

 the brain of a frog. Cut through the skin at the back of the left 

 thigh for about an inch, separate out a short length of the sciatic 

 nerve, and pass a stout ligature under it. Bring the ends of the 

 ligature to the front of the thigh and tie tightly, thus including the 

 whole of the structures of the limb with the exception of the sciatic 

 nerve. Now inject a few drops of a one per cent, solution of curare 

 (Indian arrow-poison) into the dorsal lymph sac and allow the frog to 

 remain for about half an hour. At the end of that time the animal 

 has become quite paralysed with the exception of the left leg. Pinch- 

 ing the skin of the right leg produces no movement in that limb, but 

 pinching the left leg leads to movements of the limb. Now dissect out 

 both sciatic nerves right up to the vertebral column. Arrange a 

 battery and coil for giving tetanising shocks and place the electrodes 

 under the right sciatic nerve. Stimulation of the nerve in any part 

 of its course has no effect upon the muscles which it supplies, but 

 can produce reflex contraction of the leg muscles on the left side. 

 Next place the electrodes under the left sciatic. Stimulation of this 

 nerve causes contractions of the leg and foot muscles of the same side 

 wherever the electrodes are placed, whether close to the knee where 

 it has not been exposed to the action of the poison in the blood, 

 or at that part near the vertebral column where the poison has been able 

 to reach it. Finally apply the electrodes directly to any of the muscles 

 of the right leg. They contract on stimulation. 



