All subjects had gained power in the unpracticed left arm, three of them largely and three slightly. All but one had gained in the size of the unpracticed left biceps. Strangely enough, those who had gained most in power had gained least in size. The case was quite similar in regard to the girth of the forearm. The gains in power were unquestionably mostly central—that is, in the nerve centers—and not in the muscles. Yet there was also a strange but unquestionable gain in the size of the muscles at the same time.
We have arrived at the second step of the ladder, which is: The gain by practice which shows itself in cross-education consists in a development of higher nerve centers connected with the two sides
of the body. We must next ask: Is this effect of practice confined to the symmetrical organ, or does it extend to other organs? This question was answered by a peculiar experiment.
The experiment consisted in testing the effect of educating one of the feet to tap as rapidly as possible on a telegraph key. The apparatus is shown in Fig. 1. The clocklike instrument is really a piece of clockwork actuated by a magnet, so that it counts up one point every time the electric circuit is closed. The electric circuit is comprised of a battery and two keys. Any form of battery will do; the one in the figure is a "lamp battery"—that is, an arrangement of lamps in series and in shunt, such that the ordinary high-voltage city current is conveniently transformed into a low voltage current. The key to the left is the experimenter's key, and that to the right the subject's key. When the subject is set to tapping on the latter key the counter will register whenever the experimenter keeps his key closed.
For the actual experiments by Professor Davis the subject's