548 PRINCIPLES OF ANIMAL NUTRITION. 
In any case it is obvious that while the extensive researches 
of Zuntz and his associates afford very reliable data as to the 
ratio between the energy liberated in muscular work and the 
amount of external work accomplished, or, in other words, as to 
the utilization of the net available energy of the food, we have as 
yet, notwithstanding the vast amount of work done by Wolff and 
his co-laborers and others, but very fragmentary and uncertain 
data as to the utilization of the metabolizable energy of the food for 
work production. 
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