954 THE CENTRAL NERVOUS SYSTEM 



investigation the next day, may give the opposite result of extension oi 

 the elbow. Even within shopt intervals reversal of the reaction elicited 

 from one and the same point may be seen. They do not question at all 

 the general regularity of the results which such cortical points give 

 when investigated by suitable methods after sufficient intervals of rest, 

 and on which the current statements as to the reactions elicited from 

 the various ' motor ' areas are based. But they see in the influence of 

 transient excitation either of the point itseW or of other more distant 

 points in modifying or reversing the reaction an indication that one 

 of the functions of the cortex may be the carrying out of such phe- 

 nomena of reversal, a function which may play some part in the 

 co-ordination of voluntary movements. In this connection it may be 

 remarked that a phenomenon analogous to the ' facilitation ' already 

 described for the reflex arc is also exhibited by the motor cortex. 

 If a motor reaction e.g., the movement of flexion at the elbow, as 

 studied in a pair of isolated antagonistic muscles in monkeys, is elicited 

 by stimulation of the appropriate cortical area, and a second cortical 

 stimulation is made to follow the first, the reaction evoked by the 

 second stimulus is greater than that following the first, provided that 

 the interval of time between the two is not too long. If the first stimu- 

 lus is so chosen that it is just below the strength necessary to evoke a 

 movement, a second or third stimulus, which also by itself would fail 

 to cause a reaction, may do so. Not only so, but stimulation of one 

 cortical point, say in the area which gives flexion of the elbow, may 

 raise the excitability of neighbouring points in that area (so-called 

 ' secondary facilitation '), so that a stimulus which previously did not 

 elicit the flexion movement from these points may now do so. 



Inhibition from the Cortex. Contraction is not the only effect on 

 the muscles which can be elicited by stimulating the cortex. Cor- 

 tical inhibition of tonus and of active contraction is just as char- 

 acteristic, though not so obvious a result. There is abundant evi- 

 dence of reciprocal innervation of volitional movements from the 

 cortex. When, e.g., the part of the arm area which presides over 

 extension of the elbow is stimulated (in the monkey) , it can be shown 

 that the biceps relaxes as the triceps contracts. In like manner, 

 stimulation of the appropriate part of the leg area will cause along 

 with contraction of the extensors of the hip relaxation of such flexors 

 as the psoasiliacus and the tensor fascia femoris. Such observations 

 are most easily made when, in a certain stage of narcosis, the limbs, 

 instead of hanging limp, assume a position of tonic flexion, especially 

 at the elbow and hip. Under other conditions the position of tonic 

 extension of a joint may be assumed, and then it can be shown that 

 excitation of the appropriate focus for flexion of that joint will 

 cause simultaneous contraction of the flexors and relaxation of the 

 extensors. 



The observer cannot fail to be struck with the general resem- 

 blance between these cortical reactions and their co-ordination and 

 the co-ordinated bulbo-spinal reflex movements previously studied. 

 There are, however, certain differences which place the cortical 

 reactions upon a higher level. One of the most important is the 

 part played by visual, auditory, and pure ' touch ' stimuli in eliciting 



