Torinr.To . c. 



THE TISSUES 69 



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

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

 their energy chiefly from proteins, but that in an animal on an 

 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 an animal during a period of rest excretes 

 daily 10 grams of nitrogen, and that it 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 2x6'25 = 12'5 grams of protein has been decom- 

 posed. Now the amount of energy which can be liberated 

 from 1 grain of protein has been found to be equivalent to 

 1738 kgins. (kilogramrnetres), 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 fats and carbohydrates. 



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

 work corroborates the conclusions arrived at by an examina- 

 tion of the excreta. The diet of a horse on hard muscular work 

 consists of something like the following proportions of food 

 constituents per 1000 kilos, of body weight : 



Amount. Yielding Calories. 



Proteins . 2,300 9,500 



Fats. . 800 7,500 



Carbohydrates . . . 12,500 50,000 



67,000 



The energy is here expressed in heat units, Calories the 

 amount of heat required to raise 1 kilogram of water 

 through 1 degree centigrade. Of the total 67,000 Calories of 

 energy daily taken in the food, only 14 per cent, is derived from 

 proteins, the rest comes from the carbohydrates and fats. 



Thus during muscular work the three great constituents 

 of the body and of the food proteins, fats, and carbohydrates 

 are broken down to liberate their energy, and apparently 

 the muscle tends to use the non-nitrogenous fats and carbo- 

 hydrates in preference to the proteins. Only when forced 



