1901.] on the Response of Inorganic Matter to Stimulus. 



621 



nected with it are applied to A and B. But if one of the two points, 

 say B, be injured by a cut, or burn, application of strong acids, or 

 by alkalies, then, tbe conditions of A and B being different, there 

 will be a difference of electric level or potential between them, and 

 a current will flow from the injured to the uninjured, that is from B 

 to A (Fig. 3). This current remains approximately constant as long 

 as the muscle is at rest, and is for this reason known as " current of 

 rest." As it is primarily due to injury, it is also known as " current 

 of injury." If now the muscle be thrown into an excitatory state * 

 by stimulus, there will be a greater relative disturbance at the unin- 

 jured A, and the original difference of electric level will be disturbed. 

 In this case we have an analogue to c' in n, Fig. 2, where the shock 

 produced a decrease of original difference of level. There would 

 thus be a negative variation or diminution of the original current of 

 rest. This negative variation is 

 sometimes called an "action cur- 

 rent." The transitory electrical 

 variation constitutes the " response." 

 Its intensity measures the intensity 

 of the stimulus. 



But we saw in the hydraulic 

 model the possibility of a positive 

 variation or increase of current, by 

 shock. This is also found to be 

 the case in some types of living 

 response. In the retina, the stimulus 

 of light produces a positive varia- 

 tion. It will thus be seen that 

 there are two kinds of response 

 given by living substances : (1) the 

 negative, instanced by muscle, and 

 (2) the positive, shown by the 

 retina. Again, the same tissue un- 

 der different conditions may give rise 

 to responses having opposite signs. 

 Thus Dr. Waller finds that while 

 fresh nerve gives negative, the stale nerve gives positive variation.! 



We have here, then, a way of obtaining curves of response by 

 electric means. After all it is not very different essentially from the 

 mechanical method. In this case we use a magnetic lever, the needle 

 of the galvanometer, which is deflected by the electric pull of the 

 current, generated under the action of stimulus, just as the mechanical 

 lever was deflected by the mechanical pull of the muscle contracting 

 under stimulus (Fig. 3). 



M 



E 



Fig. 4. — Simultaneous records of 

 the (M) mechanical and (JE) elec- 

 trical response of gastrocnemius 

 muscle of a frog. The muscle ex- 

 hibits fatigue (Waller). 



* The excitatory reaotion is, in the case of some living substances, of a more 

 or less local character. Ic others, as nerves, it may be conducted to distant points, 

 t See Waller, ' Animal Electricity,' p. 61. 



