GENERAL PHYSIOLOGY OF NERVE-TISSUE. 129 



studied the electrotonic effects of the galvanic current from this point 

 of view, sums up his conclusions in the following words: "We must 

 apply one electrode only to the nerve and attend to its effects alone, 

 completing the circuit through a second electrode, which is applied 

 according to convenience to some other part of the body. 



"Confining our attention to the first electrode, let us see what 

 will happen according as it is anode or cathode of a galvanic current 

 (Figs. 55 and 56). If this electrode be the anode of a current, the 

 latter enters the nerve by a series of points and leaves it by a second 

 series of points; the former, or proximal series of points, collectively 

 constitutes the polar zone or region; the latter, or distal series of 

 points, collectively constitutes the peripolar zone or region. In such 

 case the polar region is the seat of entrance of current into the nerve 

 i. e., is anodic; the peripolar region is the seat of exit of current from 

 the nerve i. e., is cathodic. If, on the contrary, the electrode under 

 observation be the cathode of a current, the latter enters the nerve 



FIG. 55. ANODE OF BATTERY. Polar FIG. 56. CATHODE OF BATTERY. Polar 



region of nerve is anodic. Peri- region of nerve is cathodic. Peri- 



polar region of nerve is cathodic. polar region of nerve is anodic. 



by a series of points which collectively constitute a 'peripolar' region, 

 and it leaves the nerve by a series of points which collectively con- 

 stitute a 'polar' region. The current, at its entrance, .into, the.body, 

 diffuses widely p and at its^'t it Cf)P renirflt ^gj 1>ts 'density' is greatest 

 close to tne electrode, and, the greater the distance of any point from 

 the electrode, the less the current density at that point; hence it is 

 obvious that the current density is greater in the polar than in the 

 peripolar region. These conditions having been recognized, we may 

 apply to them the principles learned by study of frogs' nerves under 

 simpler conditions. Seeing that, with either pole of the battery, 

 whether anode or cathode, the nerve has in each case points of en- 

 trance (constituting a collective anode) and points of exit to the cur- 

 rent (constituting a collective cathode), and admitting as proved that 

 make excitation is cathodic, break excitation jinodic, we may, with a 

 sufficiently strong^current, expecTTooBtain a contraction at make 

 and at break with either anode or cathode applied to the nerve; and 

 9 



