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HANDBOOK OF PHYSIOLOGY. 



coil in order that a minimum contraction be obtained by the induction 

 ;shock, and the secondary coil be removed slightly from the primary, the 

 induction current cannot now produce a contraction; but if the polariz- 

 ing current be sent in a descending direction, that is to say, with the 

 kathode nearest the other electrodes, the induction current, which was 

 before insufficient, will prove sufficient to cause a contraction; whereby 

 indicating that with a descending current the irritability of the nerve is 

 increased. By means of a somewhat similar experiment it may be shown 

 that an ascending current will diminish the irritability of a nerve. Sim- 

 ilarly, if instead of applying the induction electrodes below the other 

 electrodes they are applied between them, like effects are demonstrated, 

 indicating that in the neighborhood of the kathode the irritability of the 

 nerve is increased by a constant current, and in the neighborhood of the 

 anode diminished. This increase in irritability is called katlielectrotonus, 

 .and similarly the decrease is called anelectrotonus. As there is between 

 the electrodes both an increase and a decrease of irritability on the pas- 

 sage of a polarizing current, it must be evident that the increase must 

 shade off into the decrease, and that there must be a neutral point where 

 there is neither increase nor decrease of irritability. The position of the 

 neutral point is found to vary with the intensity of the polarizing cur- 

 rent when the current is weak the point is nearer the anode, when 

 strong nearer the kathode (Fig. 301); when a constant current passes 

 into a nerve, therefore, if a contraction result, it may be assumed that 

 it is due to the increased irritability produced in the neighborhood of 

 the kathode, but the breaking contraction must be produced by a rise in 

 irritability from a lowered state to the normal in the neighborhood of 

 the anode. The contractions produced in the muscle of a nerve-muscle 

 preparation by a constant current have been arranged in a table which 

 is known as Pfliiger's Law of Contractions. It is really only a state- 

 ment as to when a contraction may be expected: 



The difficulty in this table is chiefly in the effect of a weak current, 

 but the following statement will explain it. The increase of irritability 

 at the kathode is more potent to produce a contraction than the rise of 

 irritability at the anode, and so with weak currents the only effect is a 

 contraction at the make of both currents, and the descending current is 



