88 Feeds and Feeding. 



are put out of use and other less used muscles are called upon to a 

 constantly increasing degree. These muscles cannot perform the 

 work so efficiently or economically. 



The part of the expended energy appearing in useful work varies 

 in accordance with the build of the animal, the development of its 

 muscular apparatus, and the structure of its extremities which 

 bring about the work. Zuntz found great variations in the energy 

 expended by different horses of the same weight in traveling upon 

 a level track, a lame horse expending 99 per ct. more energy than a 

 sound one. In the work of climbing a grade he found a variation 

 with different horses of as much as 52 per ct. in the proportion of 

 the total energy expended which appeared as useful work. An ani- 

 mal which is able to accomplish one form of work most economically 

 may have to expend an unusual amount of energy at other kinds of 

 work. Horses bred for generations to the saddle can carry the 

 rider with smaller expenditure of energy than those whose breed- 

 ing, form, and qualities specially fit them for draft purposes. 



Certain forms of labor are performed with greater economy of 

 energy than others. Katzenstein 1 found in experiments with men 

 that about 65 per ct. more energy was used in turning a wheel a 

 given number of times with the arms than was required when the 

 same work was done with the legs. 



112. The animal as a machine. The extensive investigations by 

 Zuntz and associates with men, dogs, and horses show that aside 

 from small variations, due to the nature of the work and other fac- 

 tors, the part of the energy expended which is actually transformed 

 into external work is quite constant for each class. With animals 

 at moderate work the part of the energy which appeared in exter- 

 nal work varied from 28.8 to 36.6 per ct. of the total energy ex- 

 pended. On the average it is reasonable to hold that with men and 

 animals about one-third of the energy consumed in muscular ex- 

 ertion is recovered as external work. The rest takes the form of 

 heat within the body, and is lost so far as the production of work is 

 concerned. This does not take into account the energy lost in the 

 excreta, nor that expended for digestion, assimilation, and the 

 maintenance of the body during rest. Atwater 2 found that a man 

 returned 19.6 per ct. of the fuel value of his food as external work. 

 The best steam engines have about the same efficiency, while the 

 average engine falls below 10 per ct. Gasoline engines range in 

 efficiency from 18 to 25 per ct. Thus as a mere machine the animal 



1 Wolff, Farm Foods, p. 84. 2 U. S. Dept. Agr., Office Expt. Sta., Bui. 136. 



