, an
adult man takes into the system daily 46,000 cubic inches of oxygen,
which, if the temperature be 77 deg. F., weighs 32-1/2 oz., but when the
temperature sinks to freezing-point will weigh 35 oz. It is obvious,
also, that in an equal number of respirations we consume more oxygen at
the level of the sea than on a mountain. The quantity of oxygen inspired
and carbonic acid expired must, therefore, vary with the height of the
barometer. In our climate the difference between summer and winter in
the carbon expired, and therefore necessary for food, is as much as
one-eighth.

_II.--The Cause of Animal Heat_

Now, the mutual action between the elements of food and the oxygen of
the air is the source of animal heat.

This heat is wholly due to the combustion of the carbon and hydrogen in
the food consumed. Animal heat exists only in those parts of the body
through which arterial blood (and with it oxygen in solution)
circulates; hair, wool, or feathers, do not possess an elevated
temperature.

As animal heat depends upon respired oxygen, it will vary according to
the respiratory apparatus of the animal. Thus the temperature of a child
is 102 deg. F., while that of an adult is 99-1/2 deg. F. That of birds is higher
than that of quadrupeds or that of fishes or amphibia, whose proper
temperature is 3 deg. F higher than the medium in which they live. All
animals, strictly speaking, are warm-blooded; but in those only which
possess lungs is their temperature quite independent of the surrounding
medium. The temperature of the human body is the same in the torrid as
in the frigid zone; but the colder the surrounding medium the greater
the quantity of fuel necessary to maintain its heat.

The human body may be aptly compared to the furnace of a laboratory
destined to effect certain operations. It signifies nothing what
intermediate forms the food, or fuel, of the furnace may assume; it is
finally converted into carbonic acid and water. But in order to sustain
a fixed temperature in the furnace we must vary the quantity of fuel
according to the external temperature.

In the animal body the food is the fuel; with a proper supply of oxygen
we obtain the heat given out during its oxidation or combustion. In
winter, when we take exercise in a cold atmosphere, and when
consequently the amount of inspired oxygen increases, the necessity for
food containing carbon and hydrogen increases in the same ratio; and by
gratifying the