464 Journal of Agriciihural Research voi. iii, no. 6 



In considering, however, the weight to be attached to the influence of 

 variations in the muscular activity of the animal, and the degree of 

 refinement necessary in experimental methods, it is important not to 

 forget the main purpose of the experiments. This purpose was, as 

 already explained, substantially an economic one — viz, to determine how 

 much of the energy supplied in metabolizable form by the feed is, under 

 ordinary conditions, dissipated through the heat production caused 

 directly or indirectly by the ingestion of the feed and what proportion of 

 it remains available for the physiological uses of the body. From this 

 point of view it is immaterial whether the increased heat production is 

 caused by "work of digestion" in the narrower sense, by the stimulating 

 effect of the resorbed products of digestion upon the cell metabolism 

 shown by the investigations of L,usk (31, 32, 33, 34, 35, 36, 49), or indi- 

 rectly by giving rise to increased acti\'ity of the voluntary muscles. 

 While it is of interest and value to learn as much as possible of the rela- 

 tive importance of these factors, nevertheless they are "all in the day's 

 work," and from the economic \'iewpoint their aggregate constitutes the 

 increased energy expenditure consequent upon feed consumption. Even 

 if it were practicable to base comparisons upon periods of minimum 

 activity the results, however interesting physiologically, would include 

 only a part of the effects which the feed actually exerts upon the metab- 

 olism. If the feed causes greater restlessness in the animal while stand- 

 ing or causes it to get up and to lie down more frequently, this gives rise, 

 under the conditions of practice, to just as real losses of energy as does the 

 increase of the general cell metabolism when in the lying position and 

 from the economic point of view must be taken into account. What is 

 needed is a comparison of periods of average rather than of minimum 

 muscular actixaty and the correction to 12 hours of standing aims to 

 reduce conditions to such an average (assumed) as regards this very 

 important factor. 



Of course, however, the possibility of variations in other forms of 

 muscular activity, arising from differences in external conditions other 

 than the feed, has to be reckoned with. Naturally the endeavor has 

 been to make those conditions as nearly uniform as possible. The feed- 

 ing was identical from day to day during the three weeks of each period 

 and was given at the same hours. The surroundings during the days 

 spent outside the calorimeter were uniform, and the animals were handled 

 by the same attendants. 



During the days in the calorimeter even greater uniformity of condi- 

 tions existed. The temperature varied only a few hundredths of a 

 degree, the triple walls of the apparatus practically shut off all external 

 sounds except the slight monotonous click of the meter pump, while in 

 the comparatively dim interior the change outside from daylight to arti- 

 ficial light could not have been very noticeable. Visitors were not ad- 

 mitted during the runs. As already stated, all the animals were docile 



