46 -VETERINARY PHYSIOLOGY 



Eleetrotonus 



A study of the influence of the current on the muscle while 

 it is passing through it throws important light on this point. 

 (Practical Physiology.} 



While the current simply flows through the muscle no con- 

 traction is produced, but the excitability is profoundly modified. 



Round the kathode it becomes more easily stimulated, while 

 round the anode or positive pole it becomes less easily stimulated. 

 This may be expressed by saying that the part of the muscle 

 under the influence of the kathode is in a state of kateleclrotonus, 

 of increased excitability or of more unstable equilibrium, while 

 the part of the muscle under the influence of the anode is in a 

 state of aneleclrotonus, of decreased excitability or of more stable 

 equilibrium. Now it is well known that any sudden disturbance 

 of the equilibrium or balance of a series of bodies is apt to 

 cause them to fall asunder. For instance, if from a house of 

 cards one card is suddenly drawn out, the whole structure 

 passes into a condition of unstable equilibrium and is apt to 

 fall to pieces. So with a muscle, if it be suddenly made unstable, 

 as at the kathode on closing, a breaking down occurs and a 

 contraction results. On the other hand, suppose a house of 

 cards is built and made extra stable by introducing some 

 additional cards at the foundation, then if these cards are 

 suddenly withdrawn the chances are that the house falls to bits. 

 So with a muscle. When the current is opened the removal of 

 the state of increased stability at the positive pole may cause 

 a breaking down and produce the anodal opening contraction. 



The study of electrotorms thus explains why any sudden 

 change in the flow of electricity through a muscle stimulates it. 

 It further explains why the stimulation and contraction start 

 from the kathode on closing and from the anode on opening ; 

 and why the closing contraction is stronger than the opening, 

 since the sudden production of a condition of actual instability 

 must act more powerfully than the simple sudden removal of a 

 condition of increased stability. 



This law of Polar Excitation, while it applies to muscle and 

 nerve, does not apply to all protoplasm. Thus amoeba shows 

 contraction at the anode and expansion at the kathode when a 

 galvanic current is passed through it. 



