48 HUMAN PHYSIOLOGY 



(2) If a current be made weaker and weaker, breaking 

 ceases to cause a contraction, while making still produces it. 

 That is, it requires a stronger current to cause a contraction 

 on breaking than on making. 



(3) The two poles do not produce the same effect. The 

 negative pole or cathode that coming from the zinc plate 

 of the battery causes contraction of the muscle on closing ; 

 while the positive pole or anode causes contraction at opening. 

 This may be summarised as follows : 



1. Contraction on closing ; contraction on opening. 



2. Closing contraction stronger than opening contraction. 



3. Contraction at cathode on closing, at anode on opening. 

 (Practical Physiology, Chap. VII.) 



1. C.C C.O 



2. CC > CO 



3. CCC CAO 

 How can these facts be explained ? 



Electrotonus. 



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, Chap. VII.) 



While the current simply flows through the muscle no 

 contraction is produced, but the excitability is profoundly 

 modified. 



Round the cathode 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 cathode is 

 in a state of cathelectrotonus, 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 anelectro- 

 tonus, 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. If, for instance, 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 is suddenly made unstable 

 as at the cathode on closing, disintegration or katabolic 



