GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 59 



is not likely that this difference suffices to explain the lack of response to cur- 

 rents applied at right angles to the nerve-axis. 



Relative Efficacy of the above Conditions upon the Irritating Power of the 

 Electric Current. When a current is applied to an irritable part of a nerve 

 or muscle at an angle suitable to excitation, the stimulating effect of the current 

 depends upon the rate at which its intensity is changed, the strength and 

 density of the current, i. e. its intensity, and the duration of the current. 



Fick * gives the following schema (Fig. 22) for the different ways in which 

 the intensity of the electric current may be varied, and compares the effects 

 of these different methods of application of the current. It must be re- 

 membered that a decrease of intensity acts no less than an increase to produce 



10 



111 



FIG. 22. Schema of relation of the method of application of the electric current to the 



irritating effect. 



excitation. In the above schema the abscissa represents the time, and the 

 ordiuates the strength, of the current. Suppose the rise of intensity has a 

 form such as is represented in a, Figure 22 that is, that the strength of the 

 current increases to a considerable height, but very slowly. Such a rate of 

 change, even though the rise of intensity were continued until the strength of 

 current was very great, would have no exciting effect upon a nerve and might 

 fail to irritate a striated or non-striated muscle. A more rapid rise, such as 

 is shown in 6, might irritate a non-striated muscle, but fail to irritate a nerve 

 or a striated muscle. With currents which rapidly gain their full intensity 

 and then return again to zero, the following cases would be possible: A 

 rapid rise and fall of intensity (see c), such as occurs by an induction shock 

 or by the momentary closure of a battery current, might suffice to excite 

 a nerve but not be an effective irritant to a striated, much less a non-striated 

 muscle, unless the short duration of the current were compensated for by 

 a considerable increase in the intensity (see d). On the other hand a form 

 of variation such as is shown in e, where the rate of change is very rapid, 

 although the intensity is not great, might act to irritate nerves, and, because 

 of the longer duration of the current, striated muscles, though having no effect 

 on non-striated muscles ; and the slower rate of change, and considerable dura- 



1 Beitrage zur vergleichende Physiologic der irntablen Substanzen, Braunschweig, 1863. 



