546 NUTRITION OF FARM ANIMALS 



lations, work at a less mechanical advantage than others, while 

 the extent to which a group of muscles is called into action will 

 vary with the nature of the work. Moreover, the performance 

 of labor by an animal sets up various secondary activities, 

 notably of the circulatory and respiratory organs (633, 634), 

 which consume their share of energy and yet do not contribute di- 

 rectly to the performance of the work, and the extent of these 

 secondary activities varies with the nature and the severity of 

 the work. Some of these sources of loss of energy are anal- 

 ogous to the radiation losses from the cylinder of a heat engine, 

 while others are comparable with the internal resistances of the 

 engine itself. 



Determinations of the efficiency of the isolated muscle, there- 

 fore, afford no adequate means of estimating the efficiency of 

 the body as a whole and the latter must be determined by di- 

 rect experiment. Such a determination is made by causing 

 the animal to perform a measured amount of work under con- 

 ditions which also permit the measurement, either directly as 

 heat or by the methods of indirect calorimetry described in 

 Chapter VI, of the total body energy metabolized. 



Thus in experiments by Benedict and Cathcart l upon a man 

 riding a bicycle ergometer, the subject breathed through the 

 mouthpiece of a Benedict universal respiration apparatus 

 (298), by means of which the oxygen consumption and the car- 

 bon dioxid elimination could be determined. From these 

 data the amount of energy metabolized in the body was com- 

 puted and compared with the amount of mechanical work done 

 as measured by the ergometer. For example, in one of these 

 tests the energy output per minute as computed from the res- 

 piratory exchange was 6.32 Cals., while the mechanical work 

 done per minute was equivalent to 1.02 Cals. In other words, 

 i. 02 -f- 6.32 = 16.1 per cent of the total energy output was 

 recovered as useful work, the remainder taking the form of 

 heat. 



648. Gross and net efficiency. Comparisons like that of 

 the preceding paragraph give what is called the gross efficiency 

 of the body, i.e., they show what proportion of the total energy 

 metabolized during work is recovered in the useful work done. 

 It is analogous to the efficiency of an engine as computed from 



1 Muscular Work; Carnegie Inst. of Washington, Publication No. 187 (1913). 



