CENTRAL CONTROL OF POSTURAL REACTIONS 925 



for complete transections of the cord at levels up to the medulla. It 

 appears that the higher centers exert a reinforcing effect on the local 

 tonic reflex, which may be compensated for when these influences are 

 removed, but never with complete success. The reinforcement of the 

 tone of skeletal muscles by the higher centers of the nervous system 

 represents the resultant of two opposing tendencies, one augmenting the 

 tonic contraction, the other inhibiting it. 



The augmentation of tone is seen in the condition known as decere- 

 brate rigidity, to which we have already referred, in which the tone of 

 the extensor muscles is greatly exaggerated. Experiments on animals 

 show that the rigidity is not the result of separating the cerebrum from 

 the central nervous system, as the name would imply, for if successive 

 slices of the brain be removed from above downward the condition does 

 not develop until the section separates the optic thalamus from the mid- 

 brain. The researches of Sherrington, 31 Thiele, 33 and particularly of 

 Weed, 32 indicate that the mechanism supporting the rigidity is probably 

 somewhat as follows. The afferent impulses on which the " reflex stand- 

 ing" depends arise in the muscles themselves, since cutting the posterior 

 roots of the spinal nerves supplying the muscles in question abolishes the 

 rigidity. They pass up the cord in the ventrolateral column of the same 

 side and probably enter the cerebellum through the superior cerebellar 

 peduncles and pass to the cerebellar cortex. From thence the path leads 

 back through the same peduncles to the midbrain in which lies the main 

 center for maintaining this condition, probably the nucleus ruber. From 

 there the paths descend through the cord in extrapyramidal tracts, prob- 

 ably the rubrospinal. Afferent paths also probably lead directly to the 

 midbrain center without passing through the cerebellum, for in some 

 animals removal of the cerebellum is not followed by loss of rigidity or 

 its loss is not immediate. 



The Inhibition of Tone. The rigidity may also be inhibited, once it 

 has developed, by stimuli applied to various parts of the nervous system. 

 If the "decerebration" is limited to one side of the body rigidity may 

 develop on that side only and it becomes possible to study the effect of 

 stimulating the cortex of the other half of the cerebrum. Stimulation of 

 the sensory-motor area inhibits the existing tone in the extensor mus- 

 cles. Excitation of the cerebral peduncles also inhibits the rigidity. 

 Weed 32 considers that the cerebellum forms an essential link in the path 

 over which these inhibitory impulses pass from the cerebrum to the 

 centers which maintain the rigidity, because severence of the middle 

 cerebellar peduncles eliminates the inhibition which is produced by 

 stimulation of the cerebral peduncles. In support of this observation is 

 the fact that excitation of the anterior part of the superior vermis or of 



