214 CENTRAL NERVOUS SYSTEM 



contracts. Related to this phenomenon is one known as decere- 

 brate rigidity. It is a condition of prolonged spasm of certain 

 groups of skeletal muscles which follows transection anywhere 

 in the midbrain or in the posterior part of the thalamus. If 

 .the afferent nerves belonging to one of the rigid limbs are sev- 

 ered, it relaxes while the other limbs remain rigid. The spasm, 

 therefore, is a reflex through nerves that supply joints, muscles, 

 etc., and this reflex must be controlled by a centre situated 

 somewhere between the cerebrum and the bulb, since section 

 of the latter abolishes the rigidity. It is said that the centre 

 is not in the cerebellum. The muscles involved in decerebrate 

 rigidity are those, particularly, which are most easily inhibited 

 from the cerebrum. Removal of the latter organ unmasks an 

 action of a mechanism which tends to keep the muscles in tonic 

 contraction. 



Pathological paralyses are not always permanent, and it is 

 an important question in what way recovery is brought about. 

 Experimentally, extirpation of the hand area in monkeys 

 renders the hand on the opposite side useless, but in a few weeks 

 recovery has taken place to such an extent that it is freely but 

 clumsily used. If, now, the corresponding area on the other 

 side of the cortex is removed, a corresponding paralysis takes 

 place, but the hand first paralyzed is not affected. On the con- 

 trary, it may be used more freely. In time recovery of hand 

 number two takes place. If, then, the entire arm area of one 

 side is extirpated, neither hand is affected, although paralysis 

 of shoulder and elbow on the side opposite the lesion are ob- 

 vious. The recovery of function of the hand, therefore, is 

 not due to the acquisition of this duty by the hand area of the 

 other side, nor does it seem to be due to the cortex immediately 

 surrounding the hand area. According to some authorities it 

 is due to a representation of the hand area in the postcentral 

 gyrus acting through fibers that descend from that point to 

 the optic thalamus, and thence through the rubrospinal tract. 



Entire extirpation of the motor area of one cerebral hemi- 

 sphere leads to extensive paralysis on the opposite side of the 

 body, but it does not include all of the muscles, and, furthermore, 

 it leads to some muscular weakness on the same side. Those 

 muscles which ordinarily act in unison, like the diaphragm, 

 intercostal muscles, and muscles of the larynx, are but little 



