56 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ance, and if two paths are opened to it, more or less can be sent through one 

 of them by decreasing or increasing the resistance in the other. 



A useful instrument for dividing the current is the rheocord. The schema given in 



Figure 21 illustrates the way in 

 which it is used. The amount 

 of current passing to the nerve 

 will vary with the relative re- 

 sistance in , b, c, d, e, /, and 

 in a, b, g, A, e, f. The bridge 

 c, d can be slid along the fine 

 German -silver wires b, i and 

 e, j, and thus the resistance 

 FIG. 21. Rheocord. a , b, c, d, e. /, and the amount 



of current passing through the nerve, can be varied at pleasure. 



With such an arrangement we should find that the irritating effect of the 

 current is largely dependent upon its strength. In the case of strong currents, 

 however, the results may be complicated by alterations in the irritability and 

 conductivity, which we will consider later. It is true also of other forms of 

 irritants, and of muscles as of nerves, that the effect of stimulation, up to a 

 certain limit, increases with the strength of the irritant. 



(c) Effect of Density of the Current. Although the strength of the current 

 is an all-important factor in its excitatory action, the effectiveness of the cur- 

 rent as an irritant depends very largely on the density of the stream. When 

 the current enters into a conductor, it spreads widely through the conducting 

 substance, and though the larger part of it takes the path of least resistance, 

 which is usually the shortest path to the point of exit, many of the threads 

 of current make a comparatively wide circuit to reach the outlet. If the con- 

 ductor is equally good at all points, but is irregularly shaped, the density of 

 the stream will be greatest where the diameter of the conductor is least. Thus 

 it happens that if a current be made to flow from end to end of a muscle, like 

 the sartorius of the frog, which is smaller at the knee end than at the pelvic 

 end; the density of the current will be greater at the lower than at the upper 

 end, and the irritating power of the current will be greater at the lower end. 1 



This question of the effect of the density of the current is important, as it 

 helps to explain the peculiar reactions to the electric stream obtained when a 

 current is applied under normal conditions to the human nerve. 



(d) Effect of the Duration of the Electric Current on its Power to Irritate 

 Nerves and Muscles. As we have seen, a constant battery current, when flow- 

 ing uninterruptedly through a motor nerve, does not ordinarily excite it ; very 

 slow variations in the strength of the current also fail to irritate ; but rapid 

 alterations in the strength, whether in the direction of increase or decrease, act 

 as vigorous stimuli. For example, medullated nerves are irritated more vigor- 

 ously by the rapid changes of intensity of induced currents than by the some- 

 what slower changes occurring at the make and break of battery currents. 

 Within certain limits, at least, the more rapidly the intensity of the current 



1 Biedermann : Elektrophysiologie, 1895, Bd. i. p. 185. 



