lied. Also a certain amount of heat is produced by the
changes which the food undergoes in the alimentary canal before it
really enters the body. But heat while continually being produced is
also continually being lost by the skin, lungs, urine and faeces.
And it is by the constant modification of these two factors, (1)
heat production and (2) heat loss, that the constant temperature of
a warm-blooded animal is maintained. Heat is lost to the body through
the faeces and urine, respiration, conduction and radiation from
the skin, and by evaporation of perspiration. The following are
approximately the relative amounts of heat lost through these
various channels (different authorities give somewhat different
figures):--faeces and urine about 3, respiration about 20, skin
(conduction, radiation and evaporation) about 77. Hence it is clear
the chief means of loss are the skin and the lungs. The more air that
passes in and out of the lungs in a given time, the greater the loss
of heat. And in such animals as the dog, who do not perspire easily by
the skin, respiration becomes far more important.

But for man the great heat regulator is undoubtedly the skin, which
regulates heat loss by its vasomotor mechanism, and also by the
nervous mechanism of perspiration. Dilatation of the cutaneous
vascular areas leads to a larger flow of blood through the skin, and
so tends to cool the body, and _vice versa_. Also the special nerves
of perspiration can increase or lessen heat loss by promoting or
diminishing the secretions of the skin. There are greater difficulties
in the exact determination in the amount of heat produced, but there
are certain well-known facts in connexion with it. A larger living
body naturally produces more heat than a smaller one of the same
nature, but the surface of the smaller, being greater in proportion
to its bulk than that of the larger, loses heat at a more rapid rate.
Hence to maintain the same constant bodily temperature, the smaller
animal must produce a relatively larger amount of heat. And in the
struggle for existence this has become so.

Food temporarily increases the production of heat, the rate of
production steadily rising after a meal until a maximum is reached
from about the 6th to the 9th hour. If sugar be included in the meal
the maximum is reached earlier; if mainly fat, later. Muscular work
very largely increases the production of heat, and hence the more
active the body the greater the p