SOURCE OF MUSCULAR WORK. 427 



The Source of the Energy Expended in Muscular 

 Work. This question, which was postponed in the chap- 

 ters dealing with the muscular tissues, on account of its 

 importance demands here a discussion. It may be put thus: 

 Does a muscle-fibre work by the oxidation of its proteids, 

 i.e. by breaking them down into compounds which are 

 then removed from it and conveyed out of the Body? or 

 does it work by the energy liberated by the oxidation of 

 carbon and hydrogen compounds only? The problem may 

 be attacked in two ways; first, by examining the excretions 

 of a man, or other animal, during work and rest; second, 

 by examining directly the chemical changes produced in a 

 muscle when it contracts. Both methods point to the same 

 conclusion, viz. that proteid oxidation is not the source of 

 the mechanical energy expended by the Body. 



One gram (15.5 grains) of pure albumen when completely 

 burnt liberates, as heat, an amount of energy equal to 2117 

 kilogrammeters (15,270 foot-pounds). But in the Body 

 proteids are not fully oxidized; part of their carbon is, to 

 form carbon dioxide, and part of the hydrogen, to form 

 water; but some carbon and hydrogen pass out, combined 

 with nitrogen and oxygen, in the incompletely oxidized 

 state of urea. Therefore all of the energy theoretically ob- 

 tainable is not derived from proteids in the Body: from 

 the above full amount for each gram of proteid we must 

 take the quantity carried off in the urea, which will be the 

 amount liberated when that urea is completely oxidized. 

 Each gram (15.5 grains) of proteid oxidized in the Body gives 

 % of a gram (5.14 grains) of urea; and since one gram of 

 urea liberates, on oxidation, energy amounting to 934 kilo- 

 grammeters (6740 foot-pounds) each gram of proteid 

 oxidized, so far as is possible in the Body, yields during the 

 proccess 2117- -^ 1805.7 kilogrammeters (13 r 037 foot- 

 pounds) of energy. Knowing that urea carries off practi- 

 cally all the nitrogen of proteids broken up in the Body, and 

 contains 46.6 per cent of nitrogen, while proteids contain 

 1.6 per cent, it is easy to find that each gram of urea repre- 

 sents the decomposition of about 2.80 grams of proteid and, 

 therefore, the liberation of 5060.00 kilogrammeters (36,533.0 



