in ELECTRICAL EXCITATION OF MUSCLE 199 



below a certain limit, the possibility of rhythmical sustained 

 excitation disappears under all conditions ; there can be nothing 

 more than the discharge of a single twitch, or the develop- 

 ment of a seemingly sustained constant contraction. We might 

 attempt to tind in these manifestations an exception to the 

 statement that a " twitch " (or self -transmitting wave of con- 

 traction) is only excited by a more or less steep variation in 

 intensity of the electrical current. But it must not be forgotten 

 that this, in the last resort, signifies merely that the changes pro- 

 duced by current (as by any other mode of excitation) in the 

 excitable substance must increase with a certain rapidity from 

 zero, or from a finite value, if a wave of contraction is to be 

 caused by them. More or less rapid variations of the state of 

 excitability of an irritable substance are however conceivable, 

 and really occur when the cause of excitation itself is constant ; 

 e.g. the pulsations of the apex of the heart in chemical or 

 mechanical excitation. This obviously depends only upon the 

 nature and state of excitability of the substance in question. 



AVe have thus acquainted ourselves with the dependence of 

 excitation on intensity of current, as well as on its duration and 

 kind of increase in general ; in conclusion we have to consider 

 the effect of its direction. It is a priori evident that this can 

 hardly play any part in the typical, longitudinal passage of 

 current through a muscle with parallel fibres, if the muscle 

 really is constructed with geometrical regularity, and, in parti- 

 cular, is of equal diameter at both ends, so that the density of 

 the current at all points will be uniform. Such preparations, 

 however, are rarely met with, and the generally adopted frog's 

 sartorius (although comparatively regular) exhibits in this respect 

 considerable variations. Before discussing this point in detail, 

 \ve must consider the enormous influence exerted by the angle 

 of the current, i.e. the angle between the lines of the current and 

 the direction of the fibres. 



The earlier observations on this point were very contradict- 

 ory. Sachs (15) more especially maintained that muscle pos- 

 sesses equal excitability to transverse and to longitudinal passage 

 of current, but the method which he employed leaves room for 

 doubt whether an electrical current traversing the muscle in a 

 really transverse direction can produce effective excitation. Two 

 needles were used in these experiments as electrodes, and brought 



