It is known to-day that what nature reveals so strikingly in the case of the electric fish, is nothing but the manifestation of a principle at work in the bodies of all beings endowed with sensation and volition – in corporeal terms, with the duality of a nervous and a muscular system – and therefore at work also in the human body. Observation has shown that the activities of these two systems in man and animal are accompanied by the occurrence of different electric potentials in different parts of the body. Plate A, Fig. iii, shows the distribution of the two polar electric forces in the human body. The bent lines in the diagram stand for curves of equal electric potential. The straight line between them is the neutral zone. As might be expected, this line runs through the heart. What seems less obvious is its slanting position. Here the asymmetry, characteristic of the human body, comes to expression.