20 APPLIED PHYSIOLOGY 



From a therapeutic point of view, it is interesting to 

 note that massage has no appreciable influence upon 

 metabolism, and is therefore in no real sense a sub- 

 stitute for exercise. 



As regards the best source from which to obtain the 

 increased amount of energy that hard work entails, 

 physiologists are now almost unanimous in recommend- 

 ing the carbohydrates and fats, for protein is only used 

 by the body when these are not available. Whether 

 fat or carbohydrate should be selected is chiefly a matter 

 of the digestion, but it is possible that when a rapid 

 output of energy is desired carbohydrate is preferable, 

 whereas for a more gradual expenditure over a long time 

 fat may have some advantages. Whatever the empirical 

 results yielded by a large meat diet in training may be, 

 therefore, there is certainly no scientific grounds for its 

 adoption. 



Of the influence of mental work upon metabolism we 

 know very little, but there is some reason to believe that 

 it is accompanied by a diminution of general, and an 

 increase of cerebral metabolism, as manifested especially 

 by an increased elimination of phosphoric acid.* There 

 is thus some justification for the old belief that phos- 

 phoric acid is a useful * brain-food.' 



4. Of the influence of such external conditions as 

 season, period of the day, and weather upon meta- 

 bolism we know but little. It would seem, however, as 

 if, contrary to general opinion, the metabolic processes 



* See Mairet and Florence, ' Le Travail Intellectuel et les 

 Fonctions de 1'Organisme,' Montpellier, 1907 (rev. in Brit. Med. 

 Journ., 1907, ii. 539). 



