1 62 COMPARATIVE ELECTRO-PHYSIOLOGY 



preparation is made, by cutting across the muscle, it is found 

 that an electrical current flows from the uninjured gland to 

 the injured muscle. From this it has been supposed that 

 such a current was not the current of injury at all, but 

 something of an unknown nature, essentially different. The 

 consequent perplexity is the result of a failure to understand 

 on the one hand that there is no such thing as a current of 

 injury per se, except as the after-effect of strong stimulation, 

 and on the other, that the current induced in the tissue is 

 always from the more excited to the less excited. In the 

 present case of muscle-and-gland preparation, the excessive 

 stimulation due to section becomes diffused all over the 

 tissue, and since the glandular surface is the more excitable, 

 its excitatory galvanometric negativity is greater than that 

 of the sectioned muscle, which thus becomes relatively positive. 

 We have here a striking demonstration of the necessity for 

 regarding the electrical reaction as the sign, not of injury, but 

 of the excitation caused by injury. In the case described, 

 for instance, the physical injury is obviously incapable of 

 transmission, and it is the consequent excitation which is con- 

 ducted to the gland. 



The account of an experiment on a sensitive leaf of 

 Mimosa will serve to elucidate the foregoing argument. If 

 one contact, A, be made with the upper half of the pulvinus, 

 and the other, C, with a distant and indifferent point, then, on 

 giving a prick near A, we shall find that that contact, owing 

 to excitation by injury, becomes galvanometrically negative. 

 If, next, we make two contacts at diametrically opposite 

 points of the pulvinus, A on the upper, and B on the lower, 

 surfaces, it will then be found, on causing injury at the upper 

 point A, that that point, relatively to B, becomes galvano- 

 metrically positive. This is because the stimulus caused bv 

 the injury has become diffused throughout the pulvinus, witn 

 the effect of causing greater excitation and consequent 

 greater galvanometric negativity at the more excitable B. 



It has been seen that a mechanical or thermal section 

 acts as a strong stimulus. It has also been shown that 



