INTERNAL WORK. 349 
Regulation of Rate of Emission.—Heat is given off by the body 
in four principal ways: (1) by conduction; (2) by radiation; (3) 
by evaporation of water; (4) as the sensible heat of the excreta. 
By conduction; heat is transferred directly from the body to 
its surroundings, including such solid objects as it may be in con- 
tact with and particularly the air. The rate of loss in this way 
will depend upon the relative temperature and conductivity of 
the surface of the body and of the substances with which it is in 
contact, and in case of the air will be also influenced by the rate 
of motion of the latter relatively to that of the body. 
By radiation, a constant exchange of heat goes on between the 
body and objects not in immediate contact with it. Since the body 
is usually warmer than its surroundings, the net result of this ex- 
change is a loss of heat by the body, the amount of which depends 
upon the specific radiating power of the surface of the body and 
upon the difference in temperature between the latter and sur- 
rounding objects. 
By evaporation of water from the skin, and to a less degree 
from the mucous membrane of the air-passages, a large amount 
of heat may be removed as latent heat of vaporization. The 
amount of water evaporated from the skin, and consequently the 
rate at which heat is carried off, will depend in part on the 
amount transpired -by the skin, but when this is abundant, 
chiefly upon the relative humidity of the air and upon its rate of 
movement. 
Finally, the heat removed in the excreta is relatively small, and 
in the case of the fasting animal in particular is insignificant as 
compared with the losses through the other three channels. 
In general we may say that the rate of emission of heat in all 
of the first three ways named is determined by two sets of condi- 
tions, viz., those relating to the environment of the animal (tem- 
perature, relative humidity, movement of air) and those relating 
to the animal itself and particularly to its surface. 
The conditions of the first set, of course, are beyond the control 
of the organism. Their tendency is to produce the same effect upon 
the rate of emission of heat that they would upon that of a lifeless 
body, viz., to increase it as the temperature of the surroundings is 
lowered and their conducting power increased. In the case of the 
