THE NERVOUS SYSTEM 25 



of one of the most impoitant aspects of per- 

 sonality. 



Experimental demonstrations of this truth 

 have been abundantly carried out. By a care- 

 ful surgical operation the cerebral cortex of a 

 bird or even of a mammal, such as a dog, can 

 be removed and the animal can be kept alive 

 for as much as a year or more after the oper- 

 ation. Such animals yield most important 

 evidence as to the function of the cerebral 

 cortex. After recovery from the operation, 

 they exhibit no permanent motor disturbances, 

 such as paralyses and so forth, and they re- 

 spond to practically all forms of stimulation, 

 but their responses are not what would be 

 called intelligent. A pigeon from which the 

 cortex has been removed, when placed on a 

 hot plate will raise first one foot, then the 

 other, and finally squat rather than fly away. 

 A dog without a cortex will respond to a 

 painful stimulation of the skin by a growl or 

 a bark and will turn its head toward the spot 

 stimulated, but will not attempt to bite. Ca- 

 ressing calls forth no sign of pleasure, threat- 

 ening no sign of fear. In sleep there is no 

 evidence of dreaming-. Such animals thus re- 

 spond to the immediate stimulus, but as 



