180 Ley ton and Sherrington 



between these, so as to associate them together in extremely varied com- 

 binations. The acquirement of skilled movements, though certainly a 

 process involving far wider areas (cf. v. Monakow) of the cortex than the 

 excitable zone itself, may be presumed to find in the motor cortex an 

 organ whose synthetic properties are part of the physiological basis which 

 renders that acquirement possible. What has been termed above the 

 " functional instability " of cortical motor points seems but one aspect, 

 revealed to experiment, of the many-sided motor synthesising which this 

 cortex can effect. Such " instability " may be a means used in those cortical 

 readjustments which the experiments of Osborne and Kilvington (35) and 

 of Robert Kennedy (23) prove to take place where, after the experimental 

 crossing of nerve-trunks, willed movements of normal effect are practically 

 restored. As Franz (17) and Bayliss (1) point out, the "instability" may 

 serve as part of the basis on which is founded the educability of the cortex. 



III. Experiments by Ablation. 



Ablation-Experiment 1. Ablation in Arm Area of Left 

 Hemisphere, Chimpanzee (figs. 19, 20). 



Troglodytes niger, ^ , strong, adult ; tame. Not infrequently walks 

 erect. Accepts fruit, etc., with either hand, no apparent preference of right. 

 After food picks teeth with extended index of either hand. Generally 

 walks quadrupedally ; a common posture is the semi-erect, the support 

 from front-limbs being given by knuckles touching floor. Shakes hands 

 with either hand, and occasionally the grip of the hand is then felt to be 

 very powerful. Picks up nuts with deftness, but the thumb is less used 

 in such movement than might be expected ; it seems too short to help the 

 fingers for things requiring the finger tips. Sleeps with arm under head 

 for pillow, generally with body not on side, but fully supine. When 

 wanting to attract notice it has a habit of stamping one foot or both feet 

 on the floor. 



March 26, forenoon. — Under deep chloroform anaesthesia the left 

 hemisphere exposed through enlarged trephine hole over lower part of 

 centralis region. The lower part of arm area of cortex and the upper part 

 of face area explored by unipolar faradisation. The stimulation results 

 mapped (fig. 19) and recorded. Then the whole of the area yielding primary 

 movements of thumb, index, fingers, wrist, and elbow excised to a depth 

 of about 7 mm. The excised field included the anterior wall of centralis 

 fissure but not the posterior wall, because no motor responses were elicited 

 from it or from the free face of post-centralis gyrus. Wound closed 

 aseptically. 



Afternoon. — The animal since recovering from the operative narcosis 

 has eaten two bananas, and is lively. The right arm, which it seems sur- 

 prised to find disability in using, shows marked wrist-drop. It moves right 

 elbow and shoulder imperfectly, but with less imperfection at shoulder 



