One of the most striking demonstrations of these currents is the one I shall now show you. The frog's heart beats for a long time after the death of the animal. Here is a little heart still beating. I cut off with the scissors a small portion of the apex, and I place the heart on the pads, so that the surface touches one pad and the cut apex the other. I now close the key, and you observe a swing of the galvanometer with each beat of the heart. You observe the spot of light is unsteady; it swings to the right with the current of the heart at rest, and to the left with the current of the heart when it contracts. The latter is the negative variation current. Sometimes we even get a double swing of the galvanometer. These beautiful results are shown in another way by my friend Dr. Augustus Waller. He uses an instrument called a capillary electrometer, invented by Lippmann, and he can demonstrate the electrical variations of the human heart.
The current of the cut muscle at rest is not of so much importance as the current of the same muscle in action. There are strong reasons for holding that the resting current
