6* VETERINARY PHYSIOLOGY 



stimulating first in the region of the cathode and then in the 

 region of the anode. A much stronger stimukis will be 

 found necessary to cause contraction of the muscle at the 

 second point, in the region of the anode. 



It is the sudden production of increased excitability at 

 the cathode on closing which causes an explosion, a stimu- 

 lation. The sudden removal of the increased stabihty round 

 the anode when the current is broken is sufficient to cause 

 an explosion if the current is strong enough. 



The study of electrotonus thus explains (1) why any 

 sudden change in the flow of electricity through a muscle 

 stimulates it ; (2) why the stimulation starts from 

 the cathode on closing and from the anode on opening ; 

 and (3) wby the closing contraction is stronger than the 

 opening. 



This law of Polar Excitation, while it applies to normal 



Fig. 24. — To show the passage of a galvanic current along a nerve so that 

 distinct polar effects are produced. 



muscle and nerve, does not apply to all protoplasm. Thus, 

 amceba shows contraction at the anode and expansion at the 

 cathode when a galvanic current is passed through it. 



B. Nerve under the Skin. 



In practice the galvanic current may be used to 

 stimulate nerves and muscles in situ under the skin. To 

 use the current for this purpose an electrode is placed over 

 the nerve or muscle to be investigated and the other over 

 some indifferent part of the body. The very considerable 

 electrical resistance of the skin has to be overcome by using 

 rather large electrodes usually covered with chamois leather 

 well soaked in saturated salt solution. 



Applied in this way, the current passes from pole to 



