MOTOR COORDINATIONS. 169 



(1) A succession of taps is physiologically a succession of interrupted 

 reciprocal innervations. Whether the interruption occurs early or late 

 in the process, whether much or little force is exerted in the tap itself, 

 will be an experimental accident which will be likely to suffer more or 

 less irregular changes as the subject's experience suggests possible 

 improvements. Wells 1 found that one effect of practice was to shorten 

 the periods of contact with the key. Langfeld 2 found that practice 

 tended to lessen the extent of the movement. 



(2) A second disadvantage of the finger-taps as recorded by the 

 telegraph-key or stylus, is the difficulty of isolating the finger-move- 

 ments from other movements of the arm and hand. Probably the 

 interchange of finger, wrist, and arm movements is less apt to occur in 

 short periods than it is in long periods of experiment under the incentive 

 of conscious fatigue. But practice may change the type of movement, 

 and may bring different groups of muscles into use in the succeeding 

 experimental periods. It seems certain that the tapping time of the 

 different limbs is not uniform. In an unpublished experimental study 

 of the finger-movements by Dodge, it proved possible to get a tapping- 

 rate of the arm when all the muscles of the arm were in voluntary 

 tetanus that could not be duplicated with the finger alone. In less 

 degree the same holds true of the wrist-movements. This seems to cor- 

 respond with the finding of Griffiths, 3 that " loaded muscles in tetanus 

 show a higher number of responses per second than unloaded." In the 

 above case, the load was the contraction of the antagonistic muscle. 



(3) Moreover, all arrangements for recording the rapidity of the 

 finger-movements by stylus or telegraph-key demand a more or less 

 consciously controlled position of the subject's arm, with a more or less 

 conscious control of the aim of the finger or arm movements. By 

 reducing the tapping test to its lowest physiological term, i. e., the true 

 reciprocal innervation of the finger, we have preserved the freedom of 

 movement of the original experiments by Von Kries, 4 and of the myo- 

 graphic experiments by Binet and Vaschide, 5 without introducing the 

 questionable ergographic complication of the latter work. 



The reciprocal innervation of the finger, like the tapping test, seems 

 to satisfy our demands for relatively slow practice improvements. As 

 Wells states, "This would seem to indicate that such unsystematic 

 practice in this function as we receive in normal life eliminated the 

 marked gains so frequently seen at the beginning of practice curves." 



APPARATUS. 



In our experiments records of the finger-movements were never taken 

 separately, but always in conjunction with corresponding pulse-records. 

 The pulse-records are electro-cardiograms. The finger-movements 

 were recorded on the same photographic record by the following device: 



s, Am. Journ. Psychol. 1908, 19, p. 445. 4 Von Kries, Archiv f. Physiol., 1886, Supplbd., p. 1. 

 2 Langfeld, Psychol. Review, 1915, 22,p. 453. 5 Binet and Vaschide, L'Annee Psychol., 1897, p. 267. 

 'Griffiths, Journ. Physiol., 1888,9, p. 39. 



