CHAPTER XIV 

 WORK PRODUCTION 



627. Prime purpose of excess feed. Aside from reproduc- 

 tion, the prime purpose for which a mature animal consumes 

 feed in excess of its maintenance requirement is the production 

 of the external mechanical work required for its diverse ac- 

 tivities, either natural in the wild animal or enforced in the 

 domesticated work animal. It is true that more feed may be 

 consumed than is required for this purpose and that a fattening 

 of the animal may result. The latter, however, is simply a 

 laying aside of reserve material which may be utilized later 

 and, however important economically, may be regarded as 

 physiologically incidental. Any considerable fattening of the 

 work animal is not only a diversion of energy from the main 

 purpose of the feeding but constitutes an extra weight to be 

 carried by the animal, while if too extensive it may interfere 

 with heart action and respiration. 



Since horses or mules are substantially the working animals 

 of the United States, the following discussion will have refer- 

 ence chiefly to these animals. 



i. THE PHYSIOLOGY OF WORK PRODUCTION 



Nature of muscular work 



628. The muscles. Mechanical work is performed by an 

 animal by means of its muscles (84, 85), of which there are two 

 kinds called, respectively, striped, or striated, and smooth, or 

 non-striated, muscles from the appearance of the microscopic 

 fibers of which they are composed. The skeletal muscles, by 

 means of which external work is performed, are striated muscles. 

 They are also called voluntary muscles because they are inner- 



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