In the first place, the fact of cross-education is established. Let us ask in what this education consists. On this point some curious observations have been made by Prof. W. W. Davis,[11] now of Iowa College. The subject of the experiment began by raising a five-pound dumb-bell by flexing the arm at the elbow; this called into play chiefly the biceps muscle for lifting and the forearm muscles for grasping. This was done as many times as possible with the right arm, and then, after a rest, with the left arm. The subject then entered upon a practice extending from two to four weeks; this consisted in lifting the weight with the right arm only. At the end both arms were tested as at the start. The results were strange enough. The unpracticed left arm gained in power as we expected, but it also gained in size. Careful measurements were made by Dr. J. W. Seaver, of the Yale Gymnasium, on the girths of both upper arm and forearm. Let us compare the gains in girth with the gains in power: ----------+------------------------------+------------------------------ | GAINS IN GIRTH. | GAINS IN POWER. SUBJECT. +------+--------------+--------------+--------------- | Right biceps. | Left biceps. | Right arm. | Left arm. ----------+---------------+--------------+--------------+--------------- G | 5 mm. | -5 mm. | 820 flexions.| 200 flexions. J | 2 " | 0 " | 400 " | 225 " K | 4 " | 2 " | 724 " | 514 " H | 13 " | 6 " | 950 " | 30 " B | 6 " | 11 " | 900 " | 75 " I | 8 " | 3 " | 750 " | 75 " ----------+---------------+--------------+--------------+--------------- 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. [Illustration: FIG. 1.] 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.