NERVOUS CONDUCTION AND EXCITATION 137 



the direction, duration, and intensity of constant currents in 

 relation to excitation. The effect of direction was early 

 recognised through the work of Pfluger and his contemporaries. 

 The action of the constant current is characteristically polar, 

 that is definitely orientated with reference to the surface at 

 which the stimulus is applied. By placing a commutator for 

 reversing the current in the circuit for exciting a nerve (non- 

 polarisable electrodes being, of course, used), it is easily 

 demonstrated that excitation takes place at the cathode, i£, 

 the point to which positive ions move through the tissue, 

 while simultaneously excitability is diminished at the anode, i.e. 

 at the surface from which kations move away, when the current 

 is made. When conversely the current is broken, stimulation 

 again results ; but the polar relations are reversed, excitation 

 occurring at what was the anode, i.e. at the point to which the 

 kations now tend to revert. The relation of duration and 

 intensity of stimulus to the excitation process has been 

 elucidated chiefly through the work of Lapicque and Keith 

 Lucas. There exists both a time limit and a limit of intensity 

 for effective stimulation. If the stimulus is of an intensity 

 less than a certain amount, it cannot excite, however prolonged 

 its duration may be ; on the other hand, however great the 

 intensity of the stimulus may be, it cannot excite, if the 

 duration of the current falls below a certain value. Ahernating 

 currents of very high frequency may thus be quite ineffective 

 in provoking physiological responses. This critical duration 

 during which a minimal stimulus must operate to produce 

 a manifest effect varies with different nerves in the same 

 individual and with corresponding nerves in different species 

 of animals. As a measure of the time-factor Lapicque has 

 introduced the constant chronaxie, which is defined as the 

 minimal duration required for excitation with a current whose 

 intensity is twice the threshold necessary for excitation when 

 the duration of the stimulus is indefinitely prolonged. The 

 significant fact emerging from this line of inquiry is that the 

 length of the refractory period and the interval which must 

 elapse for production of summation effects in different tissues is 

 greater or less according as the chronaxie has a high or low value. 



