CHAPTER VI 



PRODUCTION OF WORK, MILK, AND WOOL 



I. Production of Work 



It has long been known that muscular exertion or external body 

 work greatly increases the amount of food material broken down in 

 the body, but scientists have disagreed as to whether one or all of the 

 nutrients — protein, carbohydrates, or fat — furnishes the energy. 



The source of muscular energy. — By painstaking trials it was fi- 

 nally proved that all the organic nutrients, not only the carbohy- 

 drates and fats, but also the proteins and apparently the pentosans, 

 may serve as sources of muscular work. Under normal conditions, 

 the carbohydrates and fats of the food are first drawn upon, no more 

 protein being broken down than during rest. Should. these not fur- 

 nish enough energy for the work, the body fat is next used. If this 

 is still insufficient or is much diminished by continued work, then, 

 as the last resort, the muscles or other protein tissues will be called 

 upon for the needed energy. 



Production of muscular energy. — AVe know that in doing work the 

 muscles of the body contract, that is, become shorter and thicker. 

 However, we do not yet understand in just what manner the energy 

 of the nutrients is converted into the energy of muscular action, tho 

 we do know some of the processes which take place in the working 

 muscles. 



In some marvelous manner the nutrients are suddenly broken down 

 in the cells of the muscles, and part of the energy they contain is 

 changed into mechanical work, while another part is set free as heat. 

 In this change oxygen is taken up from the blood and carbon dioxid 

 is given off, just as occurs when fuel is burned. The production of 

 mechanical energy in the muscles is in a way similar to the production 

 of power by a gasoline engine, where the fuel undergoes such a rapid 

 breaking down that it becomes an explosion. As in the muscle, part 

 of the energy of the fuel is turned into useful work and part is set 

 free as heat. This latter part yields no useful work. 



During rest, glycogen, or animal starch, is stored in the muscles, 

 forming from 0.5 to 0.9 per ct. of the weight of well-nourished muscle 

 when resting. A smaller amount of glucose is also stored in the 

 muscles. ]\Iost of the energy produced in work is doubtless formed 



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