oxygen disappears in the process of tissue transformation, and is
replaced, in the venous blood, by carbonic acid. The nutritive portions
of food are converted into a homogeneous fluid, which pervades every
part of the body, is the basis of every tissue, and which is termed the
_blood_. This varies in color and composition in different animals. In
the polyp the nutritive fluid is known as _chyme_, in many mollusks, as
well as articulates, it is called _chyle_, but in vertebrates, it is
more highly organized and is called blood. In all the higher animal
types it is of a red color, although redness is not one of its essential
qualities. Some tribes of animals possess true blood, which is not red;
thus the blood of the insect is colorless and transparent; that of the
reptile yellowish; in the fish the principle part is without color, but
the blood of the bird is deep red. The blood of the mammalia is of a
bright scarlet hue. The temperature of the blood varies in different
species, as well as in animals of the same species under different
physiological conditions; for this reason, some animals are called
_cold-blooded._ Disease also modifies the temperature of the blood; thus
in fevers it is generally increased, but in cholera greatly diminished.
THE blood has been aptly termed the "vital fluid," since there is a
constant flow from the heart to the tissues and organs of the body, and
a continual return after it has circulated through these parts. Its
presence in every part of the body is one of the essential conditions of
animal life, and is effected by a special set of organs, called the
_circulatory organs_.

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