144 THE PHYSIOLOGY OF NERVE 



they persist during the entire period during which the galvanic current 

 is passed through the nerve and that their direction may be altered 

 repeatedly by simply reversing the primary current. Action currents, 

 on the other hand, always retain the same direction and are of momen- 

 tary duration. They may also be produced by mechanical, thermal 

 and chemical stimuli, while the electrotonic currents cannot be gene- 

 rated by these means. Another means of differentiation is furnished 

 by the fact that the polarization currents are strongest in the extra- 

 polar regions and that their intensity diminishes with their distance 

 from the poles. These statements imply that the passage of a galvanic 

 current through nerve (polarizing current) gives rise first of all to 

 electrotonic currents (polarization current) which in turn lead to the 

 production of a nerve impulse. The latter, therefore, is the result of 

 the first two conditions and is by no means a part of them. 



Electrotonic Differences on the Making and Breaking of the Gal- 

 vanic Current. — If the nerve of a nerve-muscle preparation is stimu- 

 lated at definite intervals with a constant current of moderate strength, 

 it will be found that the muscle reacts only on the making and on the 

 breaking of this current, but not during the interim, in spite of the fact 

 that the current continues to traverse the nerve. In accordance with 

 DuBois-Reymond, it may therefore be stated that the stimulating 

 agent is not the absolute strength of the current, but rather the abrupt 

 change in its intensity which it suffers when it is made or broken. In 

 other words, a stimulus invariably fails to stimulate as long as it re- 

 mains constant, but becomes effective immediately if its striking force 

 is suddenly altered. Secondly, it has been shown by Pfliiger that 

 the making of the galvanic current gives rise to electrotonic changes at 

 the two poles, and that those at the anode are very different from those 

 at the cathode. The same holds true of the breaking of the current, 

 but naturally, the changes then occurring, cannot justly be classified 

 as true anelectrotonic and catelectrotonic phenomena, because they do 

 not arise during the passage of the current, but immediately after 

 its cessation. Strictly speaking, therefore, they should be character- 

 ized as post-anelectrotonic and post-catelectrotonic. 



These differences in the functional condition of the nerve at the 

 points of entrance and exit of the constant current may be briefly 

 summarized as follows: 



(a) On the making of the current the excitability of the nerve is markedly in- 

 creased at the cathode and decreased at the anode. These changes are most 

 pronounced at the poles, but also spread with gradually decreasing intensity into 

 the intrapolar and extrapolar regions. Consequently, an indifferent zone must 

 exist somewhere between these two poles, namely at the junction between the area 

 of heightened cathodal excitability and the area of lessened anodal irritability. 



(6) On the break of the current this condition is reversed, i.e., the anodal region 

 then possesses the greater irritability while the cathodal region is depressed. As 

 has been stated above, this effect appears in reality after the breaking of the cur- 

 rent, and forms therefore an electrotonic wave in the wake of the galvanic current. 

 Thus, if the terminology of post-anelectrotonus and post-catelectrotonus is adhered 



