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POPULAR SCIi^NCE MONTHLY 



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 



Fig. 1. 



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 cir- 

 cuit 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 



