FUNCTIONS OF THE BRAIN 955 



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. 



Decerebrate Rigidity is a phenomenon closely related to the in- 

 hibitory function of the cerebral cortex. It is a condition of pro- 

 longed 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, a.nd, to 

 be more precise, through those that supply the deep structures 

 (joints, muscles, etc.). The centre must be situated somewhere 

 between cerebrum and spinal bulb, since section of the bulb 

 abolishes the rigidity. It is not apparently in the cerebellum. It 

 is noteworthy that the muscles mainly involved in decerebrate 

 rigidity are those which are much more easily inhibited than excited 

 from the ' motor ' cortex, and also in the local spinal reflexes. After 

 removal of the cerebrum, the mechanism which maintains their 

 tonic contraction has free play. Sherrington points out that this 

 mechanism sustains the steady muscular tension necessary to pre- 

 serve against the force of gravity the attitude or posture of the body. 

 According to him, decerebrate rigidity is simply ' reflex standing.' 

 When the transient spinal reflex or the transient cortical effect 

 breaks in upon this tonic contraction e.g., in locomotion inhibi- 

 tion of the contracted extensors accompanies contraction of the 

 flexors (see also p. 942). 



Removal of a single ' motor ' region leads to paralysis of the 

 corresponding limb, or part of a limb, on the opposite side. For 

 example, after extirpation of the hand area the hand is for a few 

 days practically useless and apparently powerless. In a few weeks, 

 however, it recovers remarkably, so that it is once more used in 

 climbing or in conveying food to the mouth. It is an important 

 question in what way this recovery is brought about. If the whole 



