72 HUMAN PHYSIOLOGY 



ately to the work done, but may cause only a very slight 

 increase in the excretion of nitrogen (Fig. 32, 3). 



From the increased excretion of nitrogen and carbon the 

 consumption of proteids may be calculated, since proteids 

 contain 16 per cent, of nitrogen and 52 per cent, of carbon 

 i.e. 3*4 times more carbon than nitrogen. Each gram, of 

 nitrogen excreted thus represents the breaking down of 6-25 

 grams, of proteid, and it is accompanied by 3*4 grams, of 

 carbon. If more carbon is excreted, it must come from 

 carbohydrates or fat. 



Proceeding in this way, it is found that in the fasting 

 animal and in the animal fed on proteids, the muscles get 

 their energy from proteids, but that in an animal on an 



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FIG. 32. To illustrate the influence of Muscular Work upon the 

 Excretion of Carbon and of Nitrogen (1) in a fasting or 

 underfed animal ; (2) in an animal fed on proteids ; (3) in an 

 animal on a normal diet. 



ordinary diet the muscles get it chiefly from the carbo- 

 hydrates and fats of the food. 



An example of such an investigation may be given. 

 Suppose that a man during a period of rest excretes daily 

 10 grams, of nitrogen, and that he then does 100,000 Kgms. 

 of work, and during the next three days the excretion of 

 nitrogen is raised 2 grams, above the 10 per diem. This 

 means that 2 x 6*25 = 12-5 grams, of proteid has been decom- 

 posed. Now the amount of energy which can be liberated 

 from 1 gram, of proteid has been found to be equivalent to 

 1738 Kgms. (kilogrammetres), and therefore the 12*5 grams, 

 decomposed in the experiment is sufficient to yield 21,635 

 Kgms. of energy, about 20 per cent, of the total energy 

 expended in the work. The rest of the energy must be 

 derived from the fats and carbohydrates. 



5. A study of the ordinary diet of men doing muscular 



