5 02 



MUSCLE-NERVE PHYSIOLOGY 



the nerve is found to be decreased. If the polarizing current is reversed, i.e., 

 made ascending, then the condition of irritability of the nerve is reversed. 

 Both methods show that the polarization consists in an increase in irritability 

 at the cathode, called catelectrotonus, and a decrease at the anode called 

 anelectrotonus. The total change is called by the term electrotonus. As there 

 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 irri- 

 tability produced in the neighborhood of the cathode, but the breaking con- 

 traction 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 

 contraction may be expected: 



Descending current Ascending current 



Strength of current used 



Make 



Break 



Make 



Break 



