My dear Sir,

The source of animal heat, its laws, and the influence it exerts
upon the functions of the animal body, constitute a curious and
highly interesting subject, to which I would now direct your
attention.

All living creatures, whose existence depends upon the absorption of
oxygen, possess within themselves a source of heat, independent of
surrounding objects.

This general truth applies to all animals, and extends to the seed
of plants in the act of germination, to flower-buds when developing,
and fruits during their maturation.

In the animal body, heat is produced only in those parts to which
arterial blood, and with it the oxygen absorbed in respiration, is
conveyed. Hair, wool, and feathers, receive no arterial blood, and,
therefore, in them no heat is developed. The combination of a
combustible substance with oxygen is, under all circumstances, the
only source of animal heat. In whatever way carbon may combine with
oxygen, the act of combination is accompanied by the disengagement
of heat. It is indifferent whether this combination takes place
rapidly or slowly, at a high or at a low temperature: the amount of
heat liberated is a constant quantity.

The carbon of the food, being converted into carbonic acid within
the body, must give out exactly as much heat as if it had been
directly burnt in oxygen gas or in common air; the only difference
is, the production of the heat is diffused over unequal times. In
oxygen gas the combustion of carbon is rapid and the heat intense;
in atmospheric air it burns slower and for a longer time, the
temperature being lower; in the animal body the combination is still
more gradual, and the heat is lower in proportion.

It is obvious that the amount of heat liberated must increase or
diminish with the quantity of oxygen introduced in equal times by
respiration. Those animals, therefore, which respire frequently, and
consequently consume much oxygen, possess a higher temperature than
others, which, with a body of equal size to be heated, take into the
system less oxygen. The temperature of a child (102 deg) is higher
than that of an adult (99 1/2 deg). That of birds (104 deg to 105.4
deg) is higher than that of quadrupeds (98 1/2 deg to 100.4 deg) or
than that of fishes or amphibia, whose proper temperature is from
2.7 to 3.6 deg higher than that of the medium in which they live.
All animals, strictly speaking, are warm-blooded; but in those only
which possess