GENERAL PHYSIOLOGY OF MUSCLE 



83 



stimulation of the diastolic nerves, produce a positive potential in 

 the inhibited segments in relation to those at rest. 



At the International Congress of Physiology in Turin (1901) 

 Fano communicated another interesting observation on the tortoise 

 heart, which agrees well with Gaskell's theory, and also helps to 

 interpret the diphasic character of the current of action. If while 

 the photograph of the normal diphasic current or electrical beat 

 of the heart is being recorded the vagus is excited by a slight 

 stimulus, which does not arrest the heart completely but only 



FIG. 60. Electrical beat of right auricle of tortoise heart, and its reversal during excitation of 

 vagus. (Fano.) Ad, photograph of beat of right auricle ; Pe, photograph of its electrical beat 

 or diphasic variation ; Vd, line showing duration of gentle stimulation of right vagus. 



slows down the beat and diminishes the amplitude of the systole, 

 there is usually a profound alteration in the form of the photo- 

 graph, which consists in the marked diminution, sometimes the 

 almost total disappearance, of the negative phase, with a simul- 

 taneous increase in the second or positive phase. There is, in fact, 

 a complete reversal of the electrical beat of the heart (Fig. 60). 



The same sometimes occurs after, instead of during, the 

 stimulation of the vagus, when the systolic wave is gradually 

 increasing towards its normal. 



This reversal of the electrical beat during or after vagus 

 excitation depends on the inhibitory or diastolic action of this 

 nerve, since it does not occur after the application of atropine to 

 the heart. 





