34° PRINCIPLES OF ANIMAL NUTRITION. 
and enter the flume comparatively soon, while another part may 
remain in the pond for a long time. Pursuing the comparison still 
further, as but a small proportion of the energy liberated in the de- 
scent of the water in the flume takes the form of mechanical energy, 
most of it being converted into heat, so in the body but a small 
proportion of the energy expended in physiological work takes 
ultimately the form of mechanical energy. Finally, if we compare 
the flow of water in the stream below the dam to the heat produc- 
tion of the body, that flow may be increased, in case of need, in two 
ways, viz., by opening the gate wider and letting more water pass 
through the flume (increase of physiological work) or by lowering 
the dam and allowing more water to flow over, corresponding to a 
heat production for its own sake, if such takes place. 
The succeeding sections of this chapter will be devoted to a con- 
sideration of the expenditure of energy in the various forms of in- 
ternal work, including that of digestion and assimilation, while the 
subjects of the production of external work and of! the storage of 
energy may be more appropriately considered in subsequent chap- 
ters. 
§ 2. The Fasting Metabolism. 
If an animal be deprived of food for a sufficient length of time 
to empty the digestive tract, and kept in a state of rest as regards 
muscular exertion, the expenditure of energy in external work and 
in the work of digestion and assimilation are both eliminated, while 
there can be, of course, no storage of energy. Under these condi- 
tions the metabolism of energy in the organism is confined to the 
maintenance of those essential vital activities which were grouped 
above under the term “internal work” in the narrower sense, to- 
gether with any direct production of heat for its own sake. The 
fasting animal, then, affords the most favorable opportunity to 
study the laws governing the expenditure of energy for the internal 
work of the body. The fasting metabolism has already been con- 
sidered in Part I from the side of the matter involved; here we are 
concerned with its energy relations. 
Nature of Demands for Energy. 
Without attempting to enter into details, it may be said that 
the internal work of the fasting organism may be roughly classified 
