NEGATIVE VARIATION 827 



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. 234, p. 727), 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 electro- 

 motive 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 0-08 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 gastroc- 

 nemius, 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 0-005. 



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

 as seen with an ordinary galvanometer, is only apparent has been 

 shown by means of the capillary electrometer or the string galvanometer. 

 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. 299, 300). 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) . 



The rate of propagation of the electrical change in muscle is the 

 same as that of the mechanical change, and in nerve the same as that 

 of the nervous impulse. The velocity of propagation of the diphasic 

 variation along a fresh sartorius at 14 C. was in one experiment 

 2-8 metres, in another at 18 C., 3-5 metres (Sanderson). (See p. 759.) 

 Lucas has pointed out that in strict accuracy what is observed is merely 

 that the time interval separating contraction at one point of the muscle 

 from contraction at another is equal to the time interval separating 

 the electrical changes which occur at the same points. The facts 

 observed do not formally prove that either the contraction or the elec- 

 trical disturbance is propagated at all. So far as they go, some other 

 perfectly distinct change may be propagated, which at all points of the 

 fibre at which it arrives sets up both the contraction and the electrical 

 change. Such direct evidence, however, as we possess goes to show 

 that it is the electrical disturbance which is the propagated one, and 

 that this evokes the contractile disturbance. 



There is ample evidence that the excitatory electrical response 

 is a normal physiological phenomenon. In human skeletal muscles 

 the current of action has been demonstrated by connecting a gal- 

 vanometer with ring electrodes passing round the forearm, and 

 throwing the muscles into contraction. A diphasic variation is thus 

 obtained; and the electrical change travels with a velocity of as 

 much as twelve metres per second, which is greater than the velocity 

 in frogs' muscles. Electromotive changes are likewise associated 

 with the beat of the heart. Action currents have also been detected 

 in the phrenic nerves of living animals accompanying the respiratory 



