INTERNAL WORK. 385 
on the nitrogen cleavage; in other words, that what was called 
on p. 375 the work of assimilation is an important factor. 
ReEsuLts on Fat.—The relatively small increase in the metabo- 
lism resulting from the ingestion of fat is worthy of notice as bear- 
ing upon the hypothesis, already several times referred to, that it 
undergoes a cleavage into dextrose, carbon dioxide, and water in 
the liver, and that the resulting dextrose is the material which 
serves as the source of potential energy for the general metabolism. 
As was pointed out in Chapter V (p. 153), however, the dextrose 
derived from one gram of fat according to the commonly accepted 
equation would contain about 6.1 Cals. of potential energy out of 
the 9.5 Cals. contained in the original fat. In other words, over 
one third of the energy of the fat would be liberated as heat in the 
intermediary metabolism supposed to take place in the liver. 
While the heat production was not directly measured in Magnus- 
Levy’s experiments, and while the method of computation em- 
ployed may be open to criticism in details, his results certainly fail 
to indicate any such large increase in the metabolism as this hypoth- 
esis would require. 
It should be noted, in conclusion, that the above experiments 
did not include a determination of the work of mastication and in- 
gestion of the food, and also that, according to the author, there 
was little if any production of fat in the experiments in which carbo- 
hydrates were fed. 
Experiments on the Horse.—Zuntz, Lehmann & Hagemann * 
have investigated the effect of digestive work and also of the masti- 
cation of the food on the metabolism of the horse, the respiratory 
exchange being determined by the Zuntz method and a correction 
made for the cutaneous and intestinal respiration. In addition to 
this, however, other data were secured which serve the authors as 
the basis for computations of the energy metabolism of the animal 
and of the available energy of the digested food. Since their most 
important conclusions as to digestive work are based in large part. 
on the results of these computations it is necessary to consider 
their method in some detail. 
Meruop or Computation.—At six different times between the 
* Landw. Jahrb., 27, Supp., III, pp. 271-285. 
