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