GENERAL PHYSIOLOGY OF NERVE-TISSUE 113 



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 kathode of a galvanic current (Figs. 

 56 and 57). 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 kathodic. If, on the contrary, the 

 electrode under observation be the kathode of a current, the latter enters 

 the nerve 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, and at its exit it concentrates; its 'density' is greatest close to the 

 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 kath- 

 ode, the nerve has in each case points of entrance (constituting a collective 

 anode) and points of exit to the current (constituting a collective kathode), 

 and admitting as proved that make excitation is kathodic, break excitation 

 anodic, we may, with a sufficiently strong current, expect to obtain a con- 

 traction at make and at break with either anode or kathode applied to the 

 nerve; and we do so, in fact. When the kathode is applied, and the current 

 is made and broken, we obtain a kathodic make contraction and a kathodic 

 break contraction; when the anode is applied, and the current is made and 

 broken, we obtain an anodic make contraction and an anodic break con- 

 traction. These four contractions are, however, of very different strengths; 

 the kathodic make contraction is by far the strongest; the kathodic break 

 contraction is by far the weakest; the kathodic make contraction is strongei 

 than the anodic make contraction; the anodic break contraction is strongei 

 than the kathodic break contraction. Or, otherwise regarded, if, mst 

 of comparing the contractions obtained with a sufficiently strong current, 

 we observe the order of their appearance with currents gradually increased 

 from weak to strong, we shall find that the kathodic make contraction appears 

 first, that the kathodic break contraction appears last, and the : 

 of contraction for man reads as follows: 



"Weak current K. C. C. 



Medium current. . . . . . .K. C. C. A. C. C. A. O. C. 



Strong current K. C. C. A. C. C. A. O. C. K. O. C. 



The constant or the galvanic current is frequently used for therapeutic 

 and diagnostic purposes. In accordance with the statements above ^quoted 

 one electrode should be applied to the part to be investigated, the 

 to some indifferent region. The electrode conveying the curr to o 

 from this part should be of a size sufficient to localize the current and 



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