With the natural enthusiasm to employ to the full every newly invented instrument, there often occurs a tendency to put it to uses for which it is unfit. The electrocardiograph is one of the inventions that have been put to uses beyond their capacity by some physicians. For this reason it has been deemed advisable to discuss briefly, with the aid of a few diagrams, the elements of the theory of electrocardiography before discussing the place of the instrument in clinical medicine. THE GENESIS OF TISSUE CURRENTS There is now a large body of evidence to show that resting muscle cells, including those of the heart, are in a state of electrical polarization at their surfaces. The negative pole is on the inside and the positive pole is on the outside of the cell surface, the area of this bipole being the total cell surface. The Injured Cell. —When two