280 ELEMENTARY PHYSIOLOGY less. 



develop heat ; and, on the other, that the carbohydrates 

 and fats in being oxidised, suliserve only the production 

 of heat. 



Undoubtedly proteins are tissKe-furmers, inasmuch as 

 no tissue can be produced without them ; for all the 

 tissues are nitrogenous, some containing a large and 

 others a small quantity of nitrogen, and proteins aie the 

 only nitrogenous food stuffs ; they alone can supply the 

 nitrogenous elements of the tissues. But there is reason 

 to think that the fats and carbohydrates taken as food 

 may also be directly built up into the tissues. 



Moreover if the proteins alone are the tissue-formers 

 then the energy set free during the contracting activity of 

 the pre-eminently nitrogenous muscles ought to come 

 from the metabolism of their proteins. But this is not 

 found to be the case, for even the most violent muscular 

 exercise does not lead immediately to any increased output 

 of nitrogenous waste which is in the least proportionate 

 to the work being done. On the other hand exercise at 

 once, and largely, increases the excretion of carbonic acid 

 to an extent which may be five times as great as during 

 rest ; that is to sjiy, the non-nitrogen(jus part of the 

 tissue seems to be used up more quickly than the nitro- 

 genous part ; and the consumption of this particular 

 constituent of the muscular substance may be made good 

 by non-nitrogenous food, by fats or carbohydrates. 



On the other hand, proteins must be regarded as heat- 

 producers also. For if oxidation be, as has Ijeen sug- 

 gested, largely confined to the tissues, though in some 

 tissues, as in muscles, the non-nitrogenous part seems to 

 be most rapidly changed, yet the nitrogenous part, sup- 

 plied by the proteins, is sooner or later oxidised, and in 

 being oxidised must give rise to heat. 



As soon as the eleuients of the food, in fact, get into 

 the tissues, the distinction between the two classes is 

 lost ; both form tissues, and both supply heat. 



If it is worth while to make a special classification of 



