THE HEART CURRENTS 153 



A c is uniform the fall of potential is regular, and therefore the E.M.F. 

 between a and b is — E, where E is the E.M.F. of the Daniell. 



AC 



In any particular experiment the position of b will be found to lie 



quite close to A, so that the ratio of — is somewhere about "03. 

 ^ AC 



At times instead of using a simple wire, a c, for this purpose, two 

 variable resistances are employed : one a low resistance representing 

 A b, the other a higher resistance representing B c : but the principle 

 is just the same as for the wire. 



For another method of measuring the E.M.F. see p. 155. 



EXAMINATION" OF THE HEAET CURRENTS 



(1) Pith a frog, expose its heart, and apply the Stannius liga- 

 ture. Eoll out the clay pads of the electrodes to sharp points and 

 apply one to the base of the heart, the other to the apex. On opening 

 the key of the shunt no deflection occurs. The resting heart is there- 

 fore iso-electric. Eemove the apex electrode, and injure the apex by 

 touching it with a hot wire, then replace the electrode. On opening 

 the key there is now a deflection which shows the injured spot nega- 

 tive to the base or any other non-injured part. Stimulate the heart 

 mechanically near the base. With each contraction thus caused, 

 there is a diminution of the deflection of the galvanometer. There- 

 fore the base becomes negative to the apex. This is the current of 

 action of the heart. 



(2) A freshly excised heart is taken and most of the blood 

 removed by soaking it up with filter paper. Tt is then laid across the 

 electrodes, one touching the base, the other the apex. With each 

 spontaneous beat of the heart there is a deflection of the galvanometer. 

 This is found to be in such a direction as to show base becoming 

 negative to apex. 



On a beating heart exposed in situ it has been shown by the 

 capillary electrometer that the electromotive changes with each beat 

 are diphasic in character. It is found that first the base becomes 

 negative to the apex and then the apex to the base. If the apex be 

 injured it then becomes monophasic, base negative to apex with each 

 beat. The injured part does not contact normally, and therefore does 

 not become as negative as the non-injured during its contraction. 



If the base be injured then its potential remains unaltered and 

 apex becomes negative to base with each beat. It was in order to 

 render the change monophasic that the apex was injured in the 

 preceding experiment (1). 



