FACTORS AFFECTING THE ENERGY REQUIREMENT. 



65 



less variable. For example, in the case of cattle, for which the most 

 extensive and accurate data are available, the range of the energy 

 requirement per day and 1,000 pounds live weight for thin animals 

 in those experiments which are apparently the most accurate is 4.9 

 to 7.3 therms available energy or 8.5 to 12.8 therms metabolizable 

 energy. Several causes may be responsible for these variations. 



MUSCULAR ACTIVITY. 



In considering the factors of the fasting katabolism (p. 9), atten- 

 tion was called to the large share which the muscles, and especially 

 the voluntary muscles, have in the heat production of the animal. 

 Even in a state of the most complete rest possible, a very considerable 

 share of the total katabolism takes place in these tissues, due, pre- 

 sumably, to the state of constant slight tension or " tonus " of the 

 living muscle. 



MINOK MUSCULAR MOTIONS. 



It is rarely the case, however, that an animal, even when at rest 

 in the ordinary sense, does not execute more or less motions of various 

 parts of the body, all of which involve an expenditure of energy, and 

 even apparently insignificant movements may materially increase the 

 amount of metabolism. 



Zuntz and Hagemann, 1 for example, report a respiration experi- 

 ment upon a horse in Avhich the uneasiness caused by the presence of 

 a few flies in the chamber of the apparatus caused an increase of 10 

 per cent in the metabolism. Johansson 2 compared the excretion 

 of carbon dioxid by a fasting man when simply lying in bed (awake) 

 with that occurring when all the muscles were as perfectly relaxed 

 as possible. The results per hour were : 



Excretion of C0 2 oy fasting man. 



Grams. 



Lying in bed 24 .94 



Complete muscular relaxation 20 .72 



Benedict and Carpenter 3 have compared the metabolism of men 

 during sleep with that of the same subjects lying quietly in bed im- 

 mediately after waking. In the three cases which they regard as 

 strictly comparable the increase in the heat production during the 

 waking period ranged from 5.8 to 15.2 per cent, averaging 11.4 

 per cent. 



If, then, these comparatively insignificant movements have such a 

 striking effect upon the metabolism, it is evident that the amount of 

 muscular activity must be an important factor in determining the 



1 Landwirtschaftliche Jahrbiicher, vol. 23, p. 161. 



2 Skandinavisches Archiv fur Physiologic*, vol. 8, p. 85. 



3 Carnegie Institution of Washington, Publication 126, p. 241. 



8489°— Bull. 143—12 5 



