SOURCES OF TERRESTRIAL ENERGY 



materials ; while there are yet other materials 

 upon the presence of which the rate and com- 

 plication of conscious changes largely depend. 

 The familiar fact that consciousness cannot for 

 an instant continue unless oxygen is in contact 

 with the gray tissue of the cerebrum is alone 

 sufficient to prove that no conscious change is 

 possible, save as the accompaniment of a chemi- 

 cal change. On the other hand, the presence of 

 carbonic acid or of urea in considerable quanti- 

 ties retards the rate and prevents the elaboration 

 of thinking ; and in still larger quantities it puts 

 an end to consciousness. And in similar wise 

 the effects of alcohol, opium, and hemp, as well 

 as of that Siberian fungus whose inhaled va- 

 pour makes a straw in the pathway look too 

 large to be jumped over, show us most vividly 

 how immediate is the dependence of complex 

 mental operations upon chemical changes. 



Fourthly, the fact that the vigour and com- 

 plexity of mental manifestations bear a marked 

 ratio to the weight of the brain, to the amount 

 of phosphorus contained in its tissue, and to 

 the number and intricacy of the fine sinuous 

 creases in the gray surface of the hemispheres, 

 shows plainly that changes in consciousness are 

 conditioned both by the amount and by the 

 arrangement of nerve-material. 



Fifthly, we may see a like significance in the 

 facts that the amount of alkaline phosphates 

 vou n 337 



