THE TISSUES 73 



work corroborates the conclusions arrived at by an examina- 

 tion of the excreta. In this country the diet of a labourer 

 consists of something like the following proportions of food 

 constituent s : 



Amount. Yielding Calories. 



Proteids ... 130 533 



Fats . . . . 100 930 



Carbohydrates . . 500 2050 



The energy is here expressed in heat units, Calories the 

 amount of heat required to raise 1 kilogramme of water 

 through 1 degree Centigrade. Of the total 3513 Calories of 

 energy daily taken in the food, only 15 per cent, is derived 

 from proteids, the rest comes from the carbohydrates and fats. 

 The same is found to be the case in the diet of many other 

 animals, such as the horse. 



Thus during muscular work the three great constituents 

 of the body and of the food proteids, fats, and carbo- 

 hydrates are broken down to liberate their energy, and 

 apparently the muscle tends to use the non-nitrogenous fats 

 and carbohydrates in preference to the proteids. Only when 

 forced to do so does it take a large proportion of its energy 

 from these substances. 



It may be urged that in athletic training proteids must 

 be a source of energy, since experience has taught that 

 they are of such value. But their great value is as material 

 from which the energy-liberating machine, the muscles, can 

 be built up and increased, so that it can dispose of larger and 

 larger quantities of food. 



Muscle then is a machine which has the power of libe- 

 rating energy from proteids, fats, and carbohydrates, but it 

 uses proteids more especially in construction and repair. 



The muscles liberate energy from these substances by 

 breaking them down into simpler molecules just as a blow 

 causes the disintegration of nitre-glycerine and liberates its 

 stored energy. There is not such a direct oxidation as occurs 

 in the coals in the furnace of an engine, for if this were so, 

 the consumption of oxygen would always be equivalent to 

 the elimination of carbon dioxide and the other products of 

 disintegration. It has, however, been shown that a frog, 



