FUNCTIONS OF THE BRAIN 921 



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

 the elbow. Even within short "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 itself 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. 



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 evidence 

 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 contrac- 

 tion of the extensors of the hip relaxation of such flexors as the psoas- 

 iliacus 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 exten- 

 sion of a joint may be assumed, and then it can be shown that excita- 

 tion 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 

 cortical motor responses e.g., ' the closure of the hand, pricking 

 of the ear, opening of the eyes, and turning of the head in the 

 direction of the gaze ' (Sherrington). The facility of response to 

 stimuli acting from a distance through the distance-receptors, such 

 as those of the retina and labyrinth, is one of the great characteristics 

 of the cerebrum as an organ concerned in movements, and helps 

 to place the ' motor ' cortex at the helm, since these distance- 

 receptors control more than others the skeletal musculature as a 

 whole. Spinal reflex movements are mainly such as are elicited 

 by harmful (nocuous) stimuli (protective reflexes), or through the 

 sexual skin nerves, or from the visceral afferent fibres, or such as 

 are concerned in the chief movements of locomotion. 



