34^ Journal of Agricultural Research voi. xxi, No. 5 
development of heat inside the body, and thermolysis, the loss of heat 
from the body, principally by radiation and conduction and as latent 
heat of water vapor. The external temperature tends to influence the 
outflow of heat, but the animal is able to regulate it by physical and 
chemical methods. 
There is a certain external temperature, called the critical tempera¬ 
ture, at which the outflow of heat just balances the necessary heat pro¬ 
duction of the animal as a result of internal work. Above this temperature 
the radiating capacity of the body surface is varied to meet the varying 
conditions; below it, this method of regulation is largely exhausted, 
and therefore more or less oxidation of tissue is required to maintain 
the normal temperature of the body. 
EFFECT OF FEED CONSUMPTION ON THE HEAT PRODUCTION AND ON THE 
CRITICAL, TEMPERATURE 
It is a fact demonstrated by numerous experiments that the con¬ 
sumption of feed results in increasing the heat production of an animal. 
When an animal is fasting it produces a certain amount of heat due 
to the vital functions of the internal organs. This is generally termed 
basal, or fasting katabolism. When the animal is fed, its heat produc¬ 
tion is increased over that of the fasting state. This increment of heat 
brought about by the consumption of feed has been ascribed to various 
causes, one of which is the expenditure of energy in the digestion and 
assimilation of the feed, often collectively termed “work of digestion.” 
The more heat the animal produces the more cold it naturally can 
withstand without being compelled to oxidize body tissue in order to 
maintain the normal body temperature. This, in other words, means 
that the consumption of feed lowers the critical temperature, the effect 
varying with the nature and quantity of the feed. Animals fed heavily, 
as in productive feeding, can therefore withstand more cold, or have a 
lower critical temperature, than animals kept on a simple maintenance 
ration, while the critical temperature of the latter is higher than that 
of the fasting animal. 
EFFECT OF EOW THERMAE SURROUNDINGS ON MAINTENANCE 
REQUIREMENT 
From what has just been said it appears that a ration sufficient for 
maintenance at a temperature higher than the critical may be insufficient 
for maintenance when the thermal surroundings are lower than the 
critical, because of the failure to meet the demand for heat. What the 
critical temperatures of the different farm animals are is therefore a 
question not only of physiological but also of economic significance. 
It is apparent, however, that the critical temperatures of farm animals 
do not lend themselves to accurate determination. At best they are 
estimated only approximately. A summary of the results of different 
