616 THE CEREBRAL RELATIONS OP 



may have been gathered by the reader as probabilities from 

 what has already been said (see also p. 637, footnote). 

 These conclusions will now, however, be confirmed by 

 illustrations drawn from the histories of certain carefully 

 selected examples of Brain-disease. 



It must continually be borne in mind by those who 

 study these examples, that each Perceptive Centre is 

 capable of being called into activity in three modes : — 

 (1) By means of external Impressions ; (2) by ' Associa- 

 tion ' — that is, by impulses communicated from another 

 Centre, during some act of Perception or during some 

 Thought-process ; and (3) by ' Voluntary ' recall of past 

 impressions, as in an act of Recollection.* 



The excitability of the Centres — that is, the molecular 

 mobility of their constituent tissue-elements, may vary 

 much with age, state of health, or different morbid condi- 

 tions. Their mobility may be so much loioered, that they 

 are only capable of responding to powerful stimuli ; so that 

 whilst ' Volitional ' recall or Recollection may be impos- 

 sible or difficult within their province, they may still be 

 capable of acting in * Association ' with other centres 

 (that is in an automatic manner during an ordinary pro- 

 cess of Thought), and still more easily under the 'sensory* 

 stimulus or external impression which is the forerunner of 

 a Perceptive-process. At other times, the excitability of 

 Perceptive Centres may be unduly exalted, so as to lead 

 to hallucinations, illusions, and a wholly different class of 

 defects often met with among Insane persons, but which 

 will not here be considered. 



Again, the Auditory Word-Centres, the Visual Word- 

 Centres, and the double Kinesthetic Word-Centres (viz., 



* These second and third modes of activity are probably closely 

 related to one another, though we have no definite knowledge con- 

 cerning the processes involved in the latter. 



