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564. In the capillary vessels, the oxygen of the arterial blood unites
with the carbon and hydrogen which the refuse materials contain, and
carbonic acid and water are formed. The combustion of carbon and
hydrogen in the capillaries of every part of the system, (the lungs
not excepted,) is attended with a disengagement of heat, and the
carbonic acid and water are returned to the lungs in the dark-colored
blood, and evolved from the system.

565. Sir Benjamin Brodie and some others have maintained, that the
heat of the system is generated exclusively by the influence of the
brain and nerves. This theory is discarded by most physiologists; yet
it is true that the nervous system exercises a great influence over
the action of the capillary vessels in the process of nutrition,
secretion, and absorption. When these operations are most active, the
change among the particles of matter of which the body is composed, is
then greatest, and the generation of heat is increased in a
corresponding degree.

566. The necessity of pure, red blood in the production of animal
heat, is shown when the vessels that carry blood to a limb are
ligated, or tied; the part immediately becomes colder. The necessity
of nervous influence is seen in the diminished temperature of a
paralytic limb.

567. Our next inquiry is, By what means is the uniformity of
temperature in the body maintained? As there is a constant generation
of heat in the system, there would be an undue accumulation,--so much
so as to cause disagreeable sensations,--if there were no means by
which it could be evolved from the body, or its production lessened.

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564. Where does it unite with the carbon and hydrogen contained in the
body, and how is heat generated? 565. What was the theory of Sir
Benjamin Brodie? Is this theory in general discarded? What is true of
this theory? 566. How is the necessity of pure, red blood and nervous
action shown in the production of animal heat?

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568. It has been ascertained that the principal means by which the
system is kept at a uniform temperature, is the immense evaporation
from the skin and lungs. These membranes, in an ordinary state, are
constantly giving out water, which is converted into vapor, and
carried off by the surrounding air. The quantity of heat abstracted
from the system to effect this, depends on the rapidity of the change
of air, its