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MUSCLE-NERVE PHYSIOLOGY 



is between the electrodes both an increase and a decrease of irritability on the 

 passage of a polarizing current, it is evident 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 



FIG. 335- Diagram Illustrating the Effects of Various Intensities of the Polarizing Currents. 

 n, n', Nerve; a, anode; k, cathode; the curves above indicate increase, and those below decrease 

 of irritability, and when the current is small the increase and decrease are both small, with the 

 neutral point near a, and so on as the current is increased in strength. 



cathode, figure 335. 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 cathode, 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 muscle-nerve preparation 

 by a constant current have been arranged in a table which is known as Pfluger's 

 Law of Contractions. It is really only a statement as to when a contrac- 

 tion may be expected: 



During the passage of a constant current through a nerve and immediately 

 after its cessation, there is a change in the conductivity as well as of the irri- 

 tability of the nerve at the anode and cathode, respectively. During the pas- 

 sage of the current, the conductivity is increased at the cathode and decreased 

 at the anode. After the passage of the current, the effect is reversed. With 

 strong currents the area of decreased conductivity may be sufficient to act as a 

 block, preventing the passage of impulses over it. 



