ELECTRO-PHYSIOLOGY. 593 



Diphasic Variation. The base and apex of the heart are con- 

 nected by unpolarizable electrodes to the capillary electrometer. 

 When the heart contracts, there will be a diphasic variation. The 

 contracted portion at first becomes negative, and then positive, to 

 the part not contracted. The first phase, base is negative to the apex; 

 second phase, apex negative to the base. The diphasic variation fol- 

 lows from the fact that action does not take place at the same time 

 throughout the whole heart, but takes time in its transmission from 

 a point of stimulation. 



Nerves. 



The nerve presents differences in electric potential similar to 

 that of the muscle, except it is much weaker. Every part of its cut 



Fig. 229. Arrangement of Parts to Show Secondary Contraction 

 in Muscle. (After ROSENTHAL.) (From Mills's "Animal Physiology," 

 copyright, 1889, by D. Appleton and Company.) 



transverse surface is negative, whilst its longitudinal surface is 

 electro-positive. You have muscle-currents; also nerve-currents. 

 Negative Variation of the Nerve-current or Action-current. 



you place upon the electrodes connected with the galvanometer 

 iece of nerve, the deviation of the needle shows the existence of 

 nerve-current already described so long as the nerve is at rest, 

 f you tetanize the nerve the needle is seen to run back toward 

 zero, and sometimes even beyond it. This takes place in every kind 

 of nerve and in the whole length of the nerve. It can be produced 



* mechanical or chemical stimuli as readily as with electricity, 

 greater the stimulus, the greater the negative variation, but 

 ] 'e is not a definite proportion between them. Hermann has shown 



it neither in the nerve nor in the muscle do any of these currents 

 st so long as the structures are uninjured. To generate a nerve- 

 rent in repose it is necessary to make a transverse section. This 



