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 
