THE CENTRAL NERVOUS SYSTEM 847 



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 psoas-iliacus and the tensor 

 fascia lemons. 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 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 the 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 (pro- 

 tective reflexes), or through the sexual skin nerves, or from the 

 visceral afferent fibres, or such as are concerned in the chief 

 movements of locomotion. 



Decerebrate Rigidity is a phenomenon closely related to the 

 inhibitory function of the cerebral cortex. It is a condition of 

 prolonged spasm of certain groups of skeletal muscles (especially 

 the retractor muscles of the head and neck, the elevators of the 

 jaw and tail, and the extensors of the elbow, knee, shoulder, 

 and hip), supervening on removal of the cerebral hemispheres 

 by transection anywhere in the mid-brain or in the posterior 

 part of the thalamus, and favoured by suspending the animal 

 in the vertical posture. If the afferent roots belonging to one of 

 the rigid limbs are severed, it at once becomes flaccid, while the 

 other limbs remain rigid. The tonus is therefore reflex through 

 the local afferent nerves, and, to be more precise, through those 

 that supply the deep structures (joints, muscles, etc.). The 



