128 TEXT-BOOK OF PHYSIOLOGY. 



lying nerve, e. g., the brachial, it will be found that there occurs on 

 the closure of the circuit an increase in the excitability in the extra- 

 polar anelectrotonic region and a decrease in the excitability in the 

 extra-polar catelectrotonic region, as shown by stimulating the nerve 

 in the extra-polar regions with the induced current results which are 

 in apparent contradiction to those obtained with the isolated nerve. 

 \8This want of accordance in the results of the two classes of experi- 

 ments arises from a failure to recognize the fact that the physiologic 

 anode and cathode do not coincide with the physical anode and 

 cathode. \\ 



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 

 cathodes. Stimulation of this physiologic cathode with the induced 

 current gives rise, therefore, to the phenomenon of increased excita- 

 bility 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 

 contractions which follow the closing and opening of the constant 

 current have been thoroughly studied by Waller and de Watte ville. 

 These observers employed a method similar to that of Erb, conjoin- 

 ing in one circuit the testing and polarizing currents. By the graphic 

 method they recorded first the contraction produced by an induc- 

 tion shock alone; and, secondly, 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 elec- 

 trodes,>they recognize, atthejaomt of entrance and exit of the Current 

 from me 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 excita- 

 bility, though less in degree, according as they are cathodic 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 experi- 

 menting with the frog's nerves, it Js incorrect to speakof either 

 ascending or descending currents. Waller,* who has thoroughly 



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



