480 PRINCIPLES OF GENERAL PHYSIOLOGY 



not form synapses directly with the motor neurones of the cord, as was supposed at 

 one time, but with intermediate neurones more on the afferent side, appears to be 

 favoured by most neurologists at the present time. It should also be mentioned 

 that the motor area has a different histological structure from that of other, 

 non-excitable regions. 



The first definite proof that movements are evoked by electrical stimulation of 

 particular regions of the cortex was given by Fritsch and Hitzig (1870) and was 

 a very important advance in knowledge. It was supposed by many that " mental " 

 functions were independent of the material constitution of the nervous system and 

 insufficient credit is given to Gall for having propounded a more scientific view. 

 It is true that his system was based on very superficial considerations, but it was 

 Auguste Comte (1877, 3, 565-570) who first drew attention to the philosophical 

 importance of his work. 



When we call the area of the cortex from which movements can be excited, 

 the motor area, it is not to be supposed that we use the words in the same sense as 

 when applied to spinal motor neurones in the ventral horn of the grey matter. 

 What we stimulate in the former case appears to be some part of a certain complex 

 system of neurones, the activity of which implies a particular movement. So that, 

 if we regard all that part of a complex arc up to the final motor neurone as 

 belonging to the afferent side, we may speak of these cortical areas as " sensori- 

 motor " or " kincesthetic " in accordance with the view of Bastian (see his book of 

 1880, pp. 584-588). 



The work of Graham Brown and Sherrington (1913) shows that destruction of 

 the motor cortex does not produce permanent paralysis of even delicate voluntary 

 movements of the part whose " area " has been removed, even in an animal as high 

 as the chimpanzee. The arm area on the left side was removed, with the usual 

 result of paralysis of the right arm. In the course of four and a half months, 

 recovery was so complete that no difference could be detected in the behaviour of 

 the two arms. Now there are three explanations that might be suggested for the 

 recovery. 



1. Regeneration of the area destroyed. This is excluded by the fact that, 

 six and a half months after the first operation, another operation was performed 

 and the area in question was. found to be completely inexcitable. 



2. Taking over of the movements of both arms by the corresponding area 

 on the normal side of the cortex. To test this, four and a half months after the 

 first operation, the arm area was destroyed on the right side. Although the 

 immediate result of this was paralysis of the left arm, there was no change in 

 the movements of the right arm, which had recovered from the previous 

 paralysis. In two months more, complete recovery of both arms had taken place. 



3. The post-central convolution, itself not motor, that is, not excitable by 

 electrical stimulation, might have taken over the function of the arm area 

 immediately in front of it. Two months after the second operation, this 

 convolution was removed. At the operation it was found to be, as usual, 

 inexcitable and its removal did not cause paralysis of voluntary movement, 

 although for two or three weeks after the removal there was weakness in some 

 movements, but this completely disappeared later. 



The reactions to be obtained by stimulation of the motor cortex are, compared 

 with those of spinal reflexes, much more modified by slight variations in the 

 condition of the animal, blood supply, narcosis, etc. A systematic investigation 

 of the reaction to be obtained by electrical stimulation of cortical points was 

 made by Graham Brown and Sherrington (1912). They took two points, one 

 giving primary flexion at the elbow, the other primary extension. The two 

 antagonistic muscles, supinator longus and the humeral head of the triceps, were 

 connected to levers for tracing. The effects obtained were very complex. 

 Variable latency, various after-actions, such as rebound, tonic and clonic, mutual 

 relations of great diversity as regards the pair of antagonists, show the high 

 complexity of cortical reactions. Inhibition appears more prominent than excita- 

 tion and seems to be independent of simultaneous excitation of the antagonist 

 muscle, thus differing from typical reciprocal innervation of spinal reflexes to 



