GENERAL PHYSIOLOGY OF NERVE-TISSUE. 123 



current." The "natural nerve currents," the currents of injury or 

 demarcation currents, as they are variously termed, are at the same 

 time increased and decreased at opposite extremities of the nerve 

 according to the direction of the polarizing current. 



To this changed condition of the electromotive forces in a nerve 

 the term electrotonus was given (du Bois-Reymond). The currents 

 themselves are known as electrotonic currents; from their relation 

 to the anode and cathode, they are termed anelectrotonic and cat- 

 electrotonic currents. The condition of the nerve around the poles 

 both in the intra-polar and extra-polar regions is known as an- 

 electrotonus and catelectrotonus. 



The electrotonic currents vary considerably in strength and ex- 

 tent, according to the intensity of the polarizing current, increasing 

 steadily with the intensity of the latter up to the point at which the 

 polarizing current begins to destroy the physical and chemic integrity 

 of the nerve. The electrotonic currents are strongest in the imme- 

 diate neighborhood of the electrodes, but gradually diminish in strength 



rf GALVANOMETER 



; POLARIZING 3 

 o* CURRENT 



ANELECTROTONIC KATELECTROTON 1C 



CURRENTS CURRENTS 



FIG. 51. ELECTROTONIC CURRENTS. 



as the distance between the polarized and led- off portions is increased. 

 The distance to which the electrotonic currents extend along the nerve 

 will depend very largely upon the strength of the polarizing current, 

 though it is conditioned by the physical state of the nerve ; for if it be 

 ligated or injured beyond the polarized portion, the electrotonic cur- 

 rents are abolished. The electrotonic currents have no necessary 

 connection with the natural nerve currents, nor are they to be regarded 

 as branchings of the galvanic current. They are in all probability of 

 artificial origin, due to an inner positive and negative polarization of 

 the nerve which extends for a variable distance on each side of the 

 poles, and due to the action of the polarizing or the galvanic current. 

 3. An alteration in the excitability and conductivity of the nerve 

 in the neighborhood of the poles, whereby the results of nerve stimu- 

 lation that is, muscle contraction, sensation, and inhibition are 

 increased or decreased according to the strength and direction of the 

 current. To this condition the term electrotonus was also given 

 (Pfliiger). This word has thus been employed to express two distinct 



