722 A MANUAL OF PHYSIOLOGY 



When one electrode is placed on an injured part, the wave 

 of action and of electrical change diminishes as it reaches 

 the injured tissue ; and if the tissue is killed at this part, it 

 diminishes to zero ; so that here the second phase may be 

 greatly weakened or may disappear altogether, and we then 

 have what is called a monophasic variation. 



In this case the current of action can be demonstrated, even for a 

 single excitation, but still better for a tetanus, with the galvano- 

 meter, which in general is not quick enough to analyze a diphasic 

 variation with equal phases, and gives, therefore, only their algebraic 

 sum that is, zero. When the muscle or nerve is tetanized, the 

 action current appears, while stimulation is kept up, as a per- 

 manent deflection representing the ' sum ' of the separate effects. 

 It is in the opposite direction to the current of rest, since the injured 

 tissue, being less affected by the excitation, and therefore undergoing 

 a smaller negative change than the uninjured, becomes relatively to 

 the latter less negative. Appearing as a diminution or reversal of 

 the current of rest it was called the negative variation. The term 

 negative is not used here in its electrical, but in its algebraic, sense, 

 and merely as indicating the direction of the current with reference 

 to that of the demarcation current. It is in this sense that ' negative 

 variation ' and the converse term, ' positive variation,' are used 

 (pp. 734, 735) in speaking of the electrical changes produced in glands 

 and in the retina by stimulation. 



When the current of rest is compensated by a branch of an external 

 current just sufficient to balance it and bring the galvanometer image 

 back to zero (Fig. 209, p. 620), the action current appears alone in un- 

 diminished strength. This shows that the latter is not due to a change 

 of electrical resistance during excitation, since such a change would 

 equally affect current of rest and compensating current, and they 

 would still balance each other. The action current is really due to 

 a change of potential, which can be measured by determining what 

 electromotive force is just required to balance it, and which may 

 actually exceed that of the current of rest. Thus, Sanderson and 

 Gotch obtained an average of o - o8 of a Daniell cell (the electromotive 

 force of the Daniell would be about a volt) as the electromotive force 

 of the action current due to a single indirect excitation of a vigorous 

 frog's gastrocnemius, and about 0-04 Daniell as that of the current 

 of rest. The electromotive force of the current of rest in the rabbit's 

 nerve was found by du Bois-Reymond to be 0*026 ; Gotch and Horsley 

 found the average for the cat o'oi, and for the monkey only O'oo5. 



That the fusion of the successive variations of a tetanized muscle, 

 as seen with the galvanometer, is only apparent has been shown 

 by means of the capillary electrometer. Even with a frequency of 

 stimulation far beyond what is necessary for complete tetanus, each 

 stimulus is answered by a movement of the meniscus (Figs. 273, 274). 

 In nerve, also, each of two successive stimuli causes its appropriate 

 electrical change when they are separated by an interval longer 

 than a certain small fraction of a second. The precise interval at 

 which the second stimulus ceases to be effective depends on the 

 temperature of the nerve, being markedly increased by cold (Gotch 

 and Burch). 



Before Burdon Sanderson introduced the capillary electrometer 

 for the study of the electrical phenomena of living tissues, and Burch 



