THE EVOLUTION OF MIND 



and irregularity; and the increase is most marked 

 in those persons who do the most brain work. 

 In the brains of five very eminent men exam- 

 ined by Wagner, the heterogeneity of surface is 

 described as quite astonishing. Such facts prove 

 that the operations of thought work strongly 

 marked structural changes in individual brains, 

 in the course of a few years. And as these 

 strongly marked changes are but the summing 

 up of countless little changes in the arrange- 

 ments of cells and fibres, the inference is inevi- 

 table that such little changes must be going on 

 all the time. This is the testimony of observa- 

 tion, and deduction might have taught us to 

 expect as much ; since the molecules of nerve 

 tissue are chemically by far the most unstable 

 molecules known to science, ever ready to un- 

 dergo metamorphosis and arrange themselves 

 in new groups. Waste and repair go on more 

 rapidly in the brain than in any other part of the 

 body ; the cerebrum, weighing between three 

 and four pounds, receives at each pulsation 

 one fifth of all the blood sent from the heart, 

 and if the supply is stopped for an instant, con- 

 sciousness ceases. Where nutritive change is 

 so excessively rapid, such structural changes as 

 are involved in the continual setting-up of new 

 transit lines must be readily effected. And 

 quite in harmony with this course of inference 

 is the fact that when cerebral nutrition is nota- 

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