780 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



disappear. This negative variation travels toward both ends of the 

 nerve, and, like the production of the negative variation of muscle- 

 current, is due to the rapid succession of interruptions of the origin of 

 the current. 



The statement as regards the negative variation of the nerve- 

 current when the nerve is stimulated by an electrical current requires 

 some modification. The statement holds when the induced current, 

 either in single or rapid shocks, is employed. If the constant current 

 be applied to a nerve which is in connection with the galvanometer the 

 effect on the nerve-current will depend upon the direction of the stimu- 

 lating current. 



If the constant current be passed through a nerve outside of the 

 part in connection with the electrodes of the galvanometer, so that its 

 current coincides in direction with that of the nerve-current (descending 

 current), the deflection of the galvanometer needle will be increased 

 instead of decreased ; such a state of affairs is spoken of as the positive 

 phase of electrotonus, and is directly proportional in its intensity to the 

 length of nerve, the strength of the galvanic current, and the nearness 

 of application of the stimulus to the section of the nerve in connection 

 with the galvanometer. If, now, the direction of the constant current 

 be reversed, so as to cause the constant current to pass in the opposite 

 direction to the nerve-current, the latter will be diminished ; such a 

 condition is spoken of as the negative phase of electrotonus. By the 

 production of electrotonus by means of such a constant polarizing 

 current the excitability of the nerve is greatly modified, not only in the 

 part through which the current is passing, but throughout the entire 

 extent of the nerve. It has been found that at the positive pole the 

 excitability is diminished; this condition is spoken of as anelectro- 

 tonus ; at the negative pole the excitability is increased and forms the 

 region of kathelectrotonus, the variation in irritability being most 

 marked in the neighborhood of the poles and decreasing in proportion 

 to the distance from the poles. 



Between the poles of the polarizing current a point exists where the 

 region of over-stimulation and under-stimulation meet, and where, con- 

 sequently, the excitability of the nerve is unchanged ; such a point is 

 spoken of as the neutral point, and with a weak current lies nearer the 

 anode and with a strong current nearer the kathode. 



The production of this condition serves to explain the character of con- 

 tractions produced on making and breaking a constant current in a motor 

 nerve. When a constant current is allowed to pass through a motor 

 nerve, or, in other words, when a current is closed, the point of greatest 

 stimulation is located at the negative pole and spreads from this point 

 throughout the remainder of the nerve. As a consequence, when the 



