CHAPTER VI 



THE BALANCE OF NUTRITION 



i. GENERAL CONCEPTION 



274. The animal as a prime motor. The living animal 

 constitutes what is known as a prime motor ; that is, it gener- 

 ates power for its own operation and is able to produce a surplus 

 which may be applied to do external work. In particular, a 

 fairly close analogy may be drawn between the animal body and 

 what are known as internal combustion motors. In such motors, 

 a fuel (gas, gasoline, alcohol, etc.) is burned in the cylinder of 

 the engine itself and its available chemical energy is transformed 

 in part into motion and in part into heat. In a somewhat 

 similar manner the compounds supplied to the cells of the body 

 by the processes of digestion, resorption and circulation are 

 katabolized, combine with the oxygen introduced through the 

 lungs, and yield energy for the various activities of the organism. 

 It should be noted that these activities include not merely ex- 

 ternal work done by the animal but likewise a variety of internal 

 work, such as that of circulation, respiration, digestion, resorp- 

 tion, secretion, etc. In other words, the animal machine is 

 always in operation, even when performing no external work. 



275. Expenditure by the body. When in operation, a me- 

 chanical prime motor (a gasoline engine, for example) consumes 

 two things. First, the material of which the working parts are 

 composed is gradually worn away so that ultimately repairs 

 are necessary, and second, fuel is consumed in amount depend- 

 ing upon the work done. Substantially the same thing is true 

 of the animal body. 



The working machinery of the body may be regarded as 

 composed essentially of water, ash and protein. This ma- 

 chinery, like that of the engine, is continually wearing out ; that 

 is, the protein in particular is being continually katabolized and 



192 



