conditions are reversed, of separating from it. It forms about nine tenths
of the solid matter of the red corpuscles and to it is due the colors of
the blood. When united with the oxygen it forms a compound, called
_oxyhemoglobin_, which has a bright red color; the hemoglobin alone has a
dark red color. These colors are the same as those of the blood as it
takes on and gives off oxygen. The stroma, which forms only about one
tenth of the solid matter of the corpuscles, serves as a contrivance for
holding the hemoglobin. The conditions which cause the hemoglobin to unite
with oxygen in the lungs and to separate from it in the tissues, will be
considered later (Chapter VIII).

*Disappearance and Origin of Red Corpuscles.*--The red corpuscles, being
cells without nuclei, are necessarily short-lived. It has been estimated
that during a period of one to two months, all the red corpuscles in the
body at a given time will have disappeared and their places taken by new
ones. The origin of new corpuscles, however, and the manner of ridding the
blood of old ones are problems that are not as yet fully solved. The
removal of the products of broken down corpuscles is supposed to take
place both in the liver and in the spleen.(9)

Regarding the origin of the red corpuscles, the evidence now seems
conclusive that large numbers of them are formed in the red marrow of the
bones. The red marrow is located in what is known as the spongy substance
of the bones (Chapter XIV) and consists, to a large extent, of cells
somewhat like the red corpuscles, but differing from them in having
nuclei. These appear to be constantly in a state of reproduction. The
blood, flowing through the minute cavities containing these cells, carries
those that have been loosened out into the blood stream. Nuclei appear in
the red corpuscles at the time of their formation, but these quickly
separate and, according to some authorities, form the blood platelets.

*White Corpuscles.*--The white corpuscles, or _leucocytes_, are cells of a
general spherical shape, each containing one, two, or more nuclei. They
are much less numerous than the red, there being on the average only one
white corpuscle to about every five hundred of the red ones. On the other
hand, the white corpuscles are larger than the red, one of the former
being equal in volume to about three of the latter.

[Fig. 10]

Fig. 10--*Escape of white corpuscles from a small