THE EVOLUTION OF MIND 



'The convolutions into which the human 

 brain is divided by these furrows consist for 

 the most part of " eight distinct and concentric 

 layers, formed chiefly of closely packed fibres, 

 and of crowds of cells of very different shapes, 

 the layers differing in the relative proportion 

 of cells and fibres, and in the manner of their 

 arrangement." ^ Each cell sends forth processes 

 with which the tissue of certain fibres becomes 

 continuous. The office of the fibres is to es- 



of the outside of the head, but it also depends largely, as Mr. 

 Lewes well reminds us, upon the very important element of 

 vascular irrigation. ** Many individual variations in mental 

 character depend on the variations in the calibre of the cere- 

 bral and carotid trunks — and many variations in the intellec- 

 tual, emotive, and active tendencies depend on the relative 

 importance of the cerebral and carotid trunks. The energy 

 of the brain depends mainly on the calibre of its arteries ; the 

 special directions of that energy depend on the territorial dis- 

 tribution,'*^ Problems of Life and Mind, vol. i. p. 151. 

 Again, the quantity of available mental energy which can be 

 evolved in a given period of time depends, to a very great 

 extent, upon the efficiency with which the blood is supplied 

 with oxygen and freed from carbonic acid ; so that mental 

 capacity not only depends upon capacity of brain, but also 

 upon capacity of lungs and Hver. In short, a thorough exam- 

 ination shows that while Mind is most directly correlated with 

 Brain, it is indirectly but closely correlated with the entire 

 organism. So that the attempt to estimate individual differ- 

 ences in mental capacity by referring to brain-size alone is an 

 utter absurdity. 



^ Maudsley, Physiology and Pathology of Mind, p. 55. 



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