70 VETERINARY PHYSIOLOGY 



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

 these substances. 



It may be urged that in athletic training proteins 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 liberating 

 energy from proteins, fats, and carbohydrates, but it uses 

 proteins 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 nitro -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, 

 deprived of all free oxygen by placing it in the receiver of 

 an air pump and then transferred through mercury to an 

 atmosphere of nitrogen, still continues to produce carbon 

 dioxide. This means that its oxygen must be intramolecular, 

 must be in the muscle molecule, like the oxygen of nitro- 

 glycerine. Probably the presence of this oxygen is one of 

 the causes of the instability of the molecule. 



The muscle then takes these substances into itself makes 

 them part of its molecule assimilates them before breaking 

 them down. It is not necessary to suppose that all the 

 substances are equally intimately associated with the muscle 

 protoplasm. In all probability the protein becomes much 

 more truly a part of the muscle than the carbohydrates and 

 fats, but with each one of them it is essential that it should 

 come into the domain of the muscle and not simply remain 

 in the blood and lymph, in which it cannot be used. 



B. Visceral Muscles 



In several important respects the visceral muscles differ in 

 their mode of action from the skeletal muscles. 



