THE MtTSCULAR SYSTEM 63 



cannot manufacture or destroy energy; neither can a steam engine. 

 Muscular energy is derived from the oxidation in the muscle of 

 organic substances derived from the food. The necessary oxygen 

 comes from the lungs and is carried to the muscles by the red 

 blood corpuscles. The great law of conservation of energy in 

 nature which states that energy can neither be created nor 

 destroyed, is followed in the animal body. The nutrients in the 

 hay and grain which the animal consumes are oxidized and the 

 stored or potential energy in them is liberated as free or kinetic 

 energy. This oxidation process results in the formation of new 

 but simpler chemical compounds, chief of which, as has been 

 stated, are carbon dioxide gas and sarcolactic acid. 



Heat production in the body is largely the result of muscular 

 activity. It follows that the more active the muscles are, the 

 more heat will be produced. Some authorities consider the heat 

 produced by muscles to be a by-product, for the most part, as 

 less than half of that formed is required for the maintenance of 

 the normal temperature of the body. The balance cannot be 

 utilized so is liberated through radiation and conduction. Heat 

 energy is sometimes called internal work, but this term also 

 applies to the actual work of the visceral organs, so is now less 

 frequently used in this connection. 



Work performance of muscle is that force that is exhibited 

 when muscle contracts. Only that produced by the voluntary 

 muscles will be considered, i.e., external or direct work like loco- 

 motion or pulling a load. Work is of great economic importance 

 in beasts of burden. Various devices for measuring it have shown 

 that it varies with the load. When a muscle contracts against 

 too heavy a load, no actual work is produced, but much heat is 

 generated. 



Of practical importance is the question how much of the energy 

 liberated by a muscle is available for external work, and how much 

 is lost as heat? In attempting to answer this question it should 

 be realized that the generation of heat and production of work in 

 the body follow the same laws that govern these forces in artificial 

 motors like steam and gasoline engines, but combustion in the 

 living organism is not such a simple process. The food, before 

 it reaches its final end products, undergoes a series of chemical 

 changes of varying energetic significance. In the engine the 

 heat derived from the chemical energy of the fuel is the motive 



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