ii2 TEXT-BOOK OF PHYSIOLOGY 



obtained with the isolated nerve. This want of accordance in the results 

 of the two classes of experiments arises from a failure to recognize the fact 

 that the physiologic anode and kathode do not coincide with the physical 

 anode and kathode. 



It has been experimentally demonstrated that owing to the large amount 

 of readily conducting tissue by which the nerve is surrounded, the current 

 density, though great immediately under the electrode, quickly decreases 

 at a short distance from it, so that for the nerve it becomes almost nil. The 

 current, therefore, shortly after entering, again leaves the nerve at various 

 points which become physiologic kathodes. Stimulation of this physio- 

 logic kathode with the induced current gives rise, therefore, to the phenome- 

 non of increased excitability in the region of the anode. If, however, the 

 galvanic and stimulating current be combined in one circuit and both be 

 applied to the same tract of nerve, results will be obtained which harmonize 

 with those obtained with the frog's nerve. 



The changes in the excitability of a nerve of a living man and the con- 

 tractions which follow the closing and opening of the constant current 

 have been thoroughly studied by Waller and de Watteville. These observers 

 employed a method similar to that of Erb, conjoining in one circuit the 

 testing and polarizing currents. By the graphic method they recorded 

 first the contraction produced by an induction shock alone; and, secondly 



FIG. 55. ANODE OF BATTERY. FIG. 55 CATHODE OF BATTERY. 



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

 polar region of nerve is cathodic. polar region of nerve is anodic. (Waller.) 



the contraction produced by the same stimulus under the influence of the 

 polarizing current. As a result of many experiments, they also demonstrated 

 an increase of the excitability in the polar region when it is cathodic, and 

 a decrease when it is anodic. Following the suggestion of Helmholtz, that the 

 current density quickly decreases with the distance from the electrodes, 

 they recognize, at the point of entrance and exit of the current from the nerve, 

 two regions a polar, having the same sign as the electrode, and a peripolar, 

 having the opposite sign (Figs. 55 and 56). The peripolar regions also 

 experience similar alterations of excitability, though less in degree, accord- 

 ing as they are kathodic or anodic. 



As it is impossible to confine the current to the trunk of the nerve when 

 surrounded by living tissues, as is easily the case when experimenting with 

 the frog's nerves, it is incorrect to speak of either ascending or descending 

 currents. Waller, 1 who has thoroughly studied the electrotonic effects of 

 the galvanic current from this point of view, sums up his conclusions in 

 the following words: u We must apply one electrode only to the nerve and 



1 "Human Physiology," p. 363, 1891, 



