CAP ILL A E Y ELECT ROM ETER 



383 



leading to an electrometer. If the edge of the sheet is now 

 stimulated at various points, it will he ohvious that the greatest 

 movement of the mercury will he produced when the point of 

 stimulation of the muscle lies on the extension of the straight line 

 joining the electrodes. That is, the wave of negativity takes a 

 longer time to pass to B when it starts radially opposite to A than 

 when it radiates from any other point on the periphery of the 

 sheet. Further, if A is placed so near to the point of stimulation 

 that it is practicalh^ on it, then no matter where B is put, A will 

 always be zincative (galvanometrically negative) to B. 



Evidence as to the origin of the cardiac contraction at the sino- 

 atrial node may be deduced in a similar way from electrometer 

 readings. If one lead is taken from an electrode placed on the 



POSITIVE 

 VARIATION 



AURICLE. 



VENTRICLE 



Fig. so. — Record of the electrical variations in the beating heart of a tortoise, taken by a 

 capillary electrometer (after Gotch). 



node and the other lead from an electrode moved about from place 

 to place on the atria, the electrode on the node will be found 

 always electro-positive (zincative). 



The figure (00) was obtained by leading one electrode from a 

 point near the atrial sinus and the other from a point near the 

 apex of the ventricle. In order to standardise such records, the 

 leads are always arranged so that any upward movement of the 

 shadow of the mercury (or of the string of the galvanometer) 

 above the line of equal potential (rest) indicates negativity (zinca- 

 tiveness). This may be done by leading the atrial electrode to the 

 mercury in the capillary of the electrometer (Fig. 43) and the 

 ventricular electrode to the mercury in the cup. On the initiation 

 of contraction the mercury runs up the capillary away from the 

 tip, indicating that the atrium was electro-positive to the ventricle. 

 This is followed immediately by a tiny downward movement of the 

 mercury, showing that the wave of negativity had passed the site 



