964 CENTRAL NERVOUS SYSTEM 



When a lower level in the organization of the nervous system is 

 freed from control by a superior level we observe, not only the inde- 

 pendent activities which this level carries on in the normal animal, 

 but certain additional activities commonly held in check by the higher 

 centers. Injury to the nervous system consequently results in certain 

 positive as well as negative symptoms, and these are attributed to the 

 removal of an inhibition from the lower levels which is exerted normally 

 by influences from higher levels now cut off by the lesion. Thus we have 

 seen in the decerebrate animal that the removal of the control of the high- 

 est level allows the postural tone of the extensor muscles to become 

 greatly exaggerated. The spastic paralysis which accompanies many 



Cerebral Cortex 



Cerebellum and Midbrain 



SpinaJ Reflex 



Nerve Net 



i 



Skeletal Muscle Smooth Muscle and 



Glands 



cerebral and spinal lesions is probably a positive symptom due to a 

 similar cause (Walshe 34 ). In the decerebrate animal many reflexes may 

 be elicited with a certainty and to a degree unobtained in the unmutilated 

 organism because of the removal of cerebral inhibition. The symptom! 

 of cerebellar injury give a picture of the activity of the nervous system 

 deprived of the normal influence of the mesencephalic regulation of tone. 

 Voluntary movements can still be executed, but no longer with the cus- 

 tomary smoothness or precision, because the mechanism which governs 

 the coordination of muscular movement and tone is no longer effective. 

 In the isolated visceral organs, and particularly in the bladder, indepen- 

 dent activity of the peripheral nerve net is demonstrated. We have 

 seen that these organs may function in an adequate way, but their ac- 



