342 PRINCIPLES OF ANIMAL NUTRITION. 
muscle is in a constant state of slight tension or tonus, and is con- 
stantly the seat of metabolic activities which we may presume 
serve, in part at least, to maintain that tonus. This is, of course, 
equivalent to saying that there is a continual liberation of kinetic 
energy in the resting muscle, which temporarily takes the form of 
muscular elasticity but ultimately appears as heat. As to the 
amount of energy thus liberated exact information seems to be lack- 
ing, but in view of the relatively large mass of the muscles as com- 
pared with that of the other active tissues we may assume that it 
is not inconsiderable. The same thing would seem to be indicated 
also, as noted in Chapter VI (p. 191), by the great decrease in the 
metabolism and heat production ordinarily observed as the result 
of paralysis of the motor nerves by curari. 
Incidental Muscular Work.—It is rare 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 a conver- 
sion of potential energy into the kinetic form. Even apparently 
insignificant movements may materially increase the amount of 
metabolism. Zuntz & Hagemann,* for example, report a respira- 
tion experiment upon a horse in which the uneasiness caused by the 
presence of a few flies in the chamber of the apparatus caused an 
increase of over 10 per cent. in the metabolism. Johanson, Lan- 
dergren, Sonden & Tigerstedt,t in two-hour periods, found the fol- 
lowing average and minimum values per day and kilogram weight 
for the excretion of carbon dioxide by a fasting man during sleep, 
the results plainly showing the increased metabolism due to rest- 
lessness: 
| Average, | Minimum, 
Grms. Grms. 
Third day (first day of fasting) ............. 7.296 6.744 
OUT ges cosas heey potencies ie ust ate Susan dSae VE 7.704 6.768 
Fifth ‘ (very restless) .................. 8.136 7.524 
SIGH 8 ea chaning ems mmaee aaa Mae eas 7.488 6.684 
Seventh vc. woes ce eee dk eae sees ea wey 7.212 6.564 
Subsequently Johanson { compared the excretion of carbon 
dioxide by a fasting man when simply lying in bed (awake) with 
* Landw. Jahrb., 28, 161. 
t Skand. Arch. f. Physiol., 7, 29. 
t Ibid., 8, 85. 
