848 A MANUAL OF PHYSIOLOGY 



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 contrac- 

 tion has free play. Sherrington points out that this mechanism 

 sustains the steady muscular tension necessary to preserve 

 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 

 inhibition of the contracted extensors accompanies contraction 

 of the flexors (see also p. 836). 



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 con- 

 trary, the first hand is used more freely and more adroitly than 

 before the second operation, probably because the animal needs 

 to use it more. The second hand recovers eventually, like the 

 first. If when this has taken place the remaining 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 re- 

 covered from. The recovery of the hand movement cannot 

 therefore be attributed to the taking on of the function of the 

 corresponding motor area either by the opposite hand area or 

 by the adjacent ' motor ' cortex of the same hemisphere. Accord- 

 ing to some authorities, the recovery is due to the representa- 

 tion of the upper limb in the post-central gyrus (ascending 

 parietal convolution in man) acting through fibres that descend 

 from this gyrus to the optic thalamus, and thence through the 

 rubro-spinal tract, which runs to the spinal cord (p. 765). 



Removal of the whole of the ' motor ' cortex of one hemi- 

 sphere, in such animals as this operation has been performed on, 

 causes paralysis of movement on the opposite side of the body. 

 The paralysis is less marked in the case of bilateral muscles that 



