cle 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 co