350 PRINCIPLES OF ANIMAL NUTRITION. 
living animal this tendency is offset by the regulative mechanism 
acting upon the second set of conditions, so that, e.g., a fall in 
the temperature of its surroundings within certain limits instead 
of increasing the rate of emission, as in the case of a lifeless body, 
has no effect upon it. This regulation of the rate of emission is 
effected chiefly by means of changes in the temperature and state 
of moisture of the skin, brought about on the one hand through the 
vaso-motor mechanism and on the other through the special nerves 
of perspiration. 
Variations of external temperature acting upon the peripheral 
nerves influence by reflex action the activity of the vaso-motor 
nerves which regulate the caliber of the minute blood-vessels. 
Exposure to cold causes a contraction of the capillaries of the 
skin and a relaxation of those of the viscera. As a result more 
blood passes through the latter, while the flow through the skin 
is diminished, the latter becomes paler, and since the heat given 
off is not fully replaced by the blood current, its temperature falls. 
Exposure to heat has the contrary effect. The capillaries of the 
skin relax, more blood flows through them, the skin becomes flushed 
and its temperature rises, while the flow of blood to the viscera is 
checked. A fall in the temperature of the skin, however, tends to 
diminish the rate of emission of heat both by conduction and radia- 
tion, while a rise in its temperature has the opposite effect, thus 
counteracting the tendency of changes of external temperature. 
In other words, the “emission constant” of the skin changes to 
meet changes in external conditions. So exactly are these mech- 
anisms adjusted in health that within certain rather narrow limits 
they maintain the rate of emission of heat, and consequently the 
average temperature of the body, very nearly constant. 
Obviously, however, there must be a limit above which the 
temperature and radiating power of the skin cannot be increased 
to compensate for a rise in external temperature. The second 
method of regulation then comes more markedly into play through 
the familiar act of perspiration, or sweating. At high temperatures 
the activity of the sweat-glands is greatly stimulated, in part 
doubtless by the more abundant supply of blood to the skin, but 
chiefly by reflex stimulation of the special nerves which control: the 
secretion of sweat. The evaporation of the relatively large amount 
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