PHYSICAL AND VITAL PROPERTIES OF THE BLOOD.
[Illustration: Fig. 38.
Red corpuscles of human blood, represented
at _a_, as they are seen when
rather _beyond_ the focus of the microscope;
and at _b_ as they appear when,
_within_ the focus. Magnified 400 diameters.]
[Illustration: Fig. 39.
Development of human lymph and chyle-corpuscles
into red corpuscles of blood. _A_. A lymph, or white
blood-corpuscle. _B_. The same in process of conversion
into a red corpuscle. _C_. A lymph-corpuscle with the
cell-wall raised up around it by the action of water. _D_.
A lymph-corpuscle, from which the granules have
almost disappeared. _E_. A lymph-corpuscle, acquiring
color; a single granule, like a nucleus, remains. _F_. A
red corpuscle fully developed.]
_Blood_ is the animal fluid by which the tissues of the body are
nourished. This pre-eminently vital fluid permeates every organ,
distributes nutritive material to every texture, is essentially modified
by respiration, and, finally, is the source of every secretion and
excretion. Blood has four constituents: Fibrin, Albumen, Salts (which
elements, in solution, form the _liquor sanguinis_), and the Corpuscles.
Microscopical examination shows that the corpuscles are of two kinds,
known as the _red_ and the _white_, the former being by far the more
abundant. They are circular in form and have a smooth exterior, and are
on an average 1/3200 part of an inch in diameter, and are about
one-fourth of that in thickness. Hence more than ten millions of them
may lie on a space an inch square. If spread out in thin layers and
subjected to transmitted light, they present a slightly yellowish color,
but when crowded together and viewed by refracted light, exhibit a deep
red color. These blood-corpuscles have been termed _discs_, and are not,
as some have supposed, solid material, but are very nearly fluid. The
red corpuscles although subjected to continual movement, have a tendency
to approach one another, and when their flattened surfaces come in
contact, so firmly do they adhere that they change their shape rather
than submit to a separation. If separated, however, they return to their
usual form. The colorless corpuscles are larger than the red and differ
from them in being extremely irregular in their shape, and in their
tendency to adhere to a smooth surface, while the red corpuscles float
about and tumble over one another. They are chiefly remarkable for their
continual variation in form. The shape of the red corpuscles is only
altered by external influences, but the white are constantly undergoing
alterations, the result of changes taking place within their own
substance. When diluted with water and placed under the microscope they
are found to consist of a spheroidal sac, containing a clear or granular
fluid and a spheroidal vesicle, which is termed the _nucleus_. They have
been regarded by some physiologists as identical with those of the lymph
and chyle. Dr. Carpenter believes that the function of these cells is to
convert albumen into fibrin, by the simple process of cell-growth. It is
generally believed that the red corpuscles are derived in some way from
the colorless. It is supposed that the red corpuscle is merely the
nucleus of a colorless corpuscle enlarged, flattened, colored and
liberated by the bursting of the wall of its cell. When blood is taken
from an artery and allowed to remain at rest, it separates into two
parts: a solid mass, called the clot, largely composed of fibrin; and a
fluid known as the _serum_, in which the clot is suspended. This process
is termed _coagulation_. The serum, mostly composed of _albumen_, is a
transparent, straw-colored fluid, having the odor and taste of blood.
The whole quantity of blood in the body is estimated on an average to be
about one-ninth of its entire weight. The distinctions between the
arterial and the venous blood are marked, since in the arterial system
the blood is uniformly bright red, and in the venous of a very dark red
color The blood-corpuscles contain both oxygen and carbonic acid in
solution. When carbonic acid predominates, the blood is dark red; when
oxygen, scarlet. In the lungs, the corpuscles give up carbonic acid, and
absorb a fresh supply of oxygen, while in the general circulation the
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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