472 PRINCIPLES OF ANIMAL NUTRITION. 
Naturally this can be true only within limits, and exposure to 
very low temperatures, especially in a damp climate, and particu- 
larly to cold rains, causing a large expenditure of heat in the evapo- 
ration of water from the surface of the body, may very well pass 
the limit and cause an increase in the metabolism simply to main- 
tain the temperature of the body. Finally, the time element, as 
pointed out on p. 439, is one to be taken into consideration. 
Swine.—As was remarked on p. 435, the work of digestion is 
doubtless less with the swine than in ruminants, on account of the 
more concentrated nature of his food, and as was shown on p. 438, 
the maintenance requirement appears to be affected by the thermal 
environment. The same reason would tend to make fattening 
swine more susceptible to this influence than fattening -ruminants. 
This conclusion is borne out by the experiments of Shelton * at the 
Kansas Agricultural College, who found that swine kept in an open 
yard during rather severe weather required 25 per cent. more corn 
to make a given gain than those sheltered from extreme cold. 
Influence of Character of Food.—Attention was called in the 
previous chapter to the fact that the expenditure of energy in the 
digestion and assimilation of the food is largely dependent upon the 
nature of that food, but as was there pointed out, we have few 
quantitative determinations of the differences. Experiments of 
the class now under consideration show marked variations in the 
proportion of the metabolizable energy of different foods which 
is utilized, and we should naturally be inclined to ascribe these 
variations to differences in the work of digestion and assimilation 
rather than to differences in the physiological processes involved 
in tissue production. 
The data recorded in the foregoing pages constitute only a 
beginning of the study of the utilization of the energy of feeding- 
stuffs, but a brief consideration of the main results will prove at 
least suggestive. 
CONCENTRATED FEEDING-STUFFS.—As we saw in connection 
with the discussion of the metabolizable energy of feeding-stuffs in 
Chapter X, the Méckern experiments, to which we owe the larger 
share of our present knowledge regarding the metabolism of energy 
in farm animals, were made for the purpose of comparing the 
* Rep. Prof. of Agriculture, 1883. 
