INCOME AND OUTGO 341 



found that at this age of active development the metabo- 

 lism is definitely above the standard of maturity even when 

 referred to a unit of surface. There has been a strong 

 protest of late against the excessive importance attached to 

 surface as the chief factor in determining the rate of heat- 

 loss and so of heat-production. 



Students are likely to question whether mental exer- 

 tion noticeably increases the metabolism. Careful ob- 

 servations have been made upon this point and it may 

 be said that any such increase is trifling. It can prob- 

 ably be said further that an increase noted in a man 

 who is using his cerebrum is not due to extra oxidation 

 in the gray matter but to a tension of the muscles un- 

 consciously developed. We must bear in mind that 

 the brain makes only about 2.5 per cent, of the total 

 mass of the body, and that only a small fraction of it 

 can be supposed to be the seat of the special activity 

 attending intellectual work. Emotional excitement 

 means a great increase in metabolism, but here again 

 it is not cerebral oxidation which is registered, but 

 the secondary activity evoked in muscles and glands. 



