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HAND-BOOK OF PHYSIOLOGY. 



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

 neither increase nor decrease of irritability. The position of this neutral 

 point is found to vary with the intensity of the polarizing current; when 

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

 kathode (Fig. 293). When a constant current passes into a nerve, there- 

 fore, if a making contraction result, it may be assumed that it is due to 



FIG. 293. Diagram illustrating the effects of various intensities of the polarizing currents, n, n f 

 nerve; a, anode; fc, kathode; the curves above indicate increase, and those below decrease of irrita- 

 bility, and when the current is small the increase^nd decrease are both small, with the neutral 

 point near a, and so on as the current is increased hTstrength. 



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 statement 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 from a lower to a normal condition at the anode. With weak 

 currents the only effect is a contraction at the make of both ascending 

 and descending currents, the descending current being more potent than 

 the ascending (and with still weaker currents is the only one which pro- 

 duces any effect), since the kathode is near the muscle, whereas in the 

 case of the ascending current the stimulus has to pass through a district 

 of diminished irritability, which may either act as an entire block, or 

 may diminish slightly the contraction which follows. As the polarizing 



