316 THE NERVOUS SYSTEM 



artificial stimulation of the cortex it is not always possible 

 to say whether the part stimulated is sensory or motor. 

 The latent period of the reaction gives us some clue, a 

 long latent period indicating that the part stimulated is 

 sensory, the stimulus calling up a sensation which evokes 

 a response. In other words, the cortex forms, as it were, 

 a rounded summit to the arc. 



When the cerebrum is disconnected from the remainder 

 of the nervous system by section through the mid-brain 

 the whole skeletal system goes into decerebrate rigidity, a 

 condition characterised by increased tonus involving all 

 , the muscles, but. some to a greater degree than others. 

 This tonus is produced reflexly by the proprioceptive 

 system, soon to be described. The cerebrum must there- 

 fore send down a constant stream of unconscious impulses, 

 inhibitory in character. 



The functions of the cerebrum may be summarised in 

 this way. The cerebrum contains the receiving centres for 

 those impulses aroused by external stimuh and for the con- 

 scious sensations of the relative positions of parts of the 

 body. Ally impression arriving at the cerebrum causes, on 

 account of the free interconnection of different parts of the 

 cortex, a more varied and more diffuse, and therefore more 

 complete, response than can occur in the spinal animal. In 

 addition, impressions which reach it tend to stamp upon 

 it a more or less permanent record in the form of memory, 

 and the animal is able to modify his reaction to an 

 external stimulus according to his past experience — that 

 is, according to the accumulation and association of his 

 previous impressions. 



Stimuli of a painful character tend to be more indehbly 

 stamped upon the brain than those which are indifferent, 

 owing to the fact that the former are accompanied by a 

 subjective state known as an emotion. The human 

 cerebrum is the seat of emotional feehngs, of intellectual 

 processes and of consciousness itself. The cerebrum, in 



