STIMULATION OF MUSCLE AND NERVE. 113 



Cautiously remove the breast bone, doing no 

 harm to deeper parts. Open the delicate mem- 

 brane (pericardium) which surrounds the heart. 

 Tie a ligature about the sinus-auricular junction, 

 to stop the ventricular contractions. Place the 

 indifferent electrode over the larynx and the non- 

 polarizable electrode on the ventricle. Turn the 

 pole-changer so that the electrode on the heart 

 becomes the anode. Close and then open the 

 key. 



Contraction will take place on- .opening only, 

 if at all. Reverse the pole-changer so that the 

 cardiac electrode becomes the cathode. Close 

 and then open the key. 



Contraction takes place at closure only. 



POLAR STIMULATION r OF NERVE 



Law of Contraction. 1. Whether contraction 

 will follow the galvanic stimulation of a motor 

 nerve depends on the irritability of the nerve 

 and the direction and intensity of the current. 

 The current may pass through the intrapolar 

 portion of the nerve towards the muscle (de- 

 scending current) or away from it (ascending 

 current). The intensity may be weak, medium, 

 or strong ; intensity in this case is evidently 

 merely a relative term, depending on the irrita- 



8 



