922 THE CENTRAL NERVOUS SYSTEM 



Decerebrate Rigidity is a phenomenon closely related to the in- 

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

 onged 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 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. 

 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. 911). 



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 

 of the corresponding area in the opposite hemisphere is now removed, 

 a similar paralysis occurs in the other hand, but the hand whose 

 ' motor ' area was first extirpated remains entirely unaffected by the 

 second lesion. On the contrary, the first hand is used more freely 

 and more adroitly than before the second operation, probably be- 

 cause the animal needs to use it more. The second hand recovers 

 eventually, like the first. If when this has taken place the remain- 

 ing part of the arm area from which the hand area was first excised 

 be removed, neither hand is apparently affected, although there is 

 severe paralysis of the shoulder and slighter paralysis of the elbow 

 on the side opposite to the lesion, which is again largely recovered 

 from. The recovery of the hand movement cannot therefore be 

 attributed to the taking on of the function of the corresponding 



