252 Mr. Graham Brown and Prof. C. S. Sherrington. [Mar. 16, 



movement of that muscle recorded by registering the movements of the lower 

 end of the fore-arm. 



To fix the origins of the recording muscles, the humerus was transfixed by 

 steel holders at the head and through the condyles. This fixation was of 

 sufficient firmness to prevent the movements of other muscles from 

 complicating the records. 



Cortical stimulation was applied by the unipolar method. Two electrodes 

 were so arranged that either could be used separately at will. The secondary 

 circuit was completed by the simultaneous closure of two keys, one of which 

 was in the wire of the unipolar electrode, and the other of which was in the 

 path of the indifferent one. A third key was closed mechanically at the 

 same time. This made a signal circuit. A similar arrangement of keys 

 was used in the case of the second electrode. The advantage of this 

 arrangement, when both electrodes were on the cortex, was that there was 

 no danger of a short circuit through one electrode, as might be the case if a 

 system of short-circuit keys was used. 



The stimulating arrangements were such that the circuits might also be 

 applied either to the contralateral or to the ipsilateral ulnar nerve by the 

 bi-polar method, so that peripheral stimulation might be applied at will. 



The left arm muscles were used for registration in all the observations. 

 In the following descriptions, the words contralateral and ipsilateral always 

 refer to the locus of the stimulus in reference to the observed arm. 



In regard to stimulation of the cortex, our general procedure has been 

 to seek out in the arm area two points, one yielding primary flexion at the 

 elbow, and the other primary extension. In our experience a flexion-point 

 can usually be found somewhat more easily than can be an extension-point ; 

 and the field of cortex from which the former can be selected is the wider. 

 Points yielding extension lie for the most part higher up upon the convo- 

 lution {gyrus precentralis) and nearer to the dorsal limit of the arm area. 

 But the points which best yielded extension have varied unmistakably in 

 their exact position from experiment to experiment — although sometimes 

 closely corresponding in situation. In some experiments, the area whence 

 extension-points could be chosen has been distinctly larger than in others. 

 Our impression from the observations has been that this variability signifies 

 less a difference in the permanent arrangement than a difference in the 

 condition of the nervous system from time to time. In one case the same point 

 which yielded primary extension with much regularity, on re-examination 

 28 hours later in the same animal, yielded at first primary flexion instead of 

 the usual primary extension. The cause of this difference between experi- 

 ments carried out under regularly similar conditions we are unable to assign. 



