ferent species of the same class, according to the
special organization of the animal and the general activity of its
functions. The temperature of the frog may be 85 degrees F. in June and 41
degrees F. in January. The structure of its tissues is unaltered and their
vitality unimpaired by such violent fluctuations. But in man it is
necessary not only for health, but even for life, that the temperature
should vary only within narrow limits around the mean of 98-1/2 degrees F.

We are ignorant of the precise significance of this constancy of
temperature in warm-blooded animals, which is as important and peculiar as
their average height, Man, undoubtedly, must possess a superior delicacy
of organization, hardly revealed by structure, which makes it necessary
that he should be shielded from the shocks and jars of varying
temperature, that less highly endowed organisms endure with impunity.

224. Sources of Bodily Heat. The heat of the body is generated by the
chemical changes, generally spoken of as those of oxidation, which are
constantly going on in the tissues. Indeed, whenever protoplasmic
materials are being oxidized (the process referred to in sec. 15 as
katabolism) heat is being set free. These chemical changes are of
various kinds, but the great source of heat is the katabolic process,
known as oxidation.

The vital part of the tissues, built up from the complex classes of food,
is oxidized by means of the oxygen carried by the arterial blood, and
broken down into simpler bodies which at last result in urea, carbon
dioxid, and water. Wherever there is life, this process of oxidation is
going on, but more energetically in some tissues and organs than in
others. In other words, the minutest tissue in the body is a source of
heat in proportion to the activity of its chemical changes. The more
active the changes, the greater is the heat produced, and the greater the
amount of urea, carbon dioxid, and water eliminated. The waste caused by
this oxidation must be made good by a due supply of food to be built up
into protoplasmic material. For the production of heat, therefore, food is
necessary. But the oxidation process is not as simple and direct as the
statement of it might seem to indicate. Though complicated in its various
stages, the ultimate result is as simple as in ordinary combustion outside
of the body, and the products are the same.

The continual chemical changes, then, chiefly by oxidation of combustible