36 A CHEMICAL SIGN OF LIFE 



determine positively whether or not nervous functions 

 involve metabolic change. 



As far as the brain the master nervous tissue of the 

 body is concerned, it is perfectly obvious that its action 

 involves a very intense chemical activity. This is 

 shown by various circumstances. It is made evident 

 in the first place by the fact that the brain has an 

 extremely abundant blood supply and that the blood 

 returning from the brain has lost a considerable part 

 of its oxygen. Direct measurement of the amount of 

 oxygen actually consumed by the brain shows that it is 

 greater than that of any other tissue in the body relative 

 to its weight. The carbon dioxide production is also 

 greater. Everyone knows, also, that keen intellectual 

 work depends on a plentiful blood supply to the head. 

 When one works hard intellectually the face flushes; 

 often the hands and feet become cold, owing to con- 

 centration of the blood in the head. If this increase of 

 blood does not occur, keen intellectual effort is impos- 

 sible. If the circulation stops, or even if the blood 

 pressure becomes low, we become unconscious or faint. 

 These facts are sufficient to prove that the functions of 

 the brain, at any rate, involve oxygen and are expressed 

 in its respiration. The attempt to measure the amount 

 of heat produced in the brain during intellectual effort 

 has thus far been unsuccessful, owing in part to the fact 

 that it is impossible ever to get the brain in a state of 

 rest. It is always in partial activity. In the second 

 place, the brain makes only a small portion of the total 

 weight of the body, so that its heat makes but a small 

 fraction of that of the whole body, and it is this which 

 we have to measure. It has been observed, also, that 



