INTERNAL WORK. ‘ 341 
as muscular, glandular, and cellular. To the demand for energy 
for these purposes we have probably to add, at least in some cases, 
a direct demand for heat production. 
Muscutar Worx.—The more obvious forms of muscular work 
in the quiescent animal are circulation and respiration. To these 
are to be added as minor factors any movements of other in- 
ternal organs, and especially the general tonus of the muscular 
system, while finally, the various incidental movements made by 
‘such an animal, although not logically belonging in the category 
of internal work, practically have to be classed there in actual 
experimentation. It would be aside from the purpose of this 
volume to enter into any detailed consideration of these forms of 
internal work, but a few general statements regarding their amount 
may be of interest. 
Circulation.—The work performed by the heart is determined 
by two factors, viz., the weight of the blood moved and the mean 
arterial pressure overcome. Quite divergent results have been ob- 
tained by various investigators for the former factor, while the 
latter is more readily determinable. Zuntz & Hagemann * estimate 
the output of blood by the heart of the horse from a comparison 
between the blood gases and the respiratory exchange, and compute 
the expenditure of energy in circulation to be 5.01 per cent. of the 
total metabolism of the horse in a state of rest and 3.77 per cent. 
during moderate work. Hill ¢ estimates the average work of the 
heart in man at about 24,000 kilogram-meters in twenty-four hours. 
As the velocity of the circulation increases, the friction in the pe- 
ripheral blood-vessels, and consequently the arterial pressure, rap- 
idly augments, so that in case of severe muscular exertion, for ex- 
ample, the work of the heart may readily become excessive. 
Respiraition.—The work of respiration consists essentially of 
an expansion of the thorax against the resistance caused by the 
atmospheric pressure and the elasticity of the lungs and the rib 
cartilages. Zuntz & Hagemann { estimate its amount in the 
horse at about 4.7 per cent. of the total metabolism. 
Muscular Tonus.—As was pointed out in Chapter VI, the living 
* Landw. Jabrb., 27, Supp. ITI, 371. 
} Schiffer’s Text-book of Physiology, IT, 43. 
t Loc. cit. 
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