blood vessel* (Hall). At
_A_ the conditions are normal, but at _B_ some excitation in the
surrounding tissue leads to a migration of corpuscles. 1, 2, and 3 show
different stages of the passage.

The white corpuscles are found, when studied under favorable conditions,
to possess the power of changing their shape and, by this means, of moving
from place to place. This property enables them to penetrate the walls of
capillaries and to pass with the lymph in between the cells of the
tissues. The white corpuscles are, therefore, not confined to the blood
vessels, as are the red corpuscles, but migrate through the intercellular
spaces (Fig. 10). If any part of the body becomes inflamed, the white
corpuscles collect there in large numbers; and, on breaking down, they
form most of the white portion of the sore, called the _pus_.

New white corpuscles are formed from old ones, by cell-division. Their
production may occur in almost any part of the body, but usually takes
place in the lymphatic glands (Chapter VI) and in the spleen, where
conditions for their development are especially favorable. In these places
they are found in great abundance and in various stages of development.

*Functions of White Corpuscles.*--The main use of the white corpuscles
appears to be that of a destroyer of disease germs. These consist of
minute organisms that find their way into the body and, by living upon the
tissues and fluids and by depositing toxins (poisons) in them, cause
different forms of disease. Besides destroying germs that may be present
in the blood, the white corpuscles also leave the blood and attack germs
that have invaded the cells. By forming a kind of wall around any foreign
substance, such as a splinter, that has penetrated the skin, they are able
to prevent the spread of germs through the body. In a similar manner they
also prevent the germs from boils, abscesses, and sore places in general
from getting to and infecting other parts of the body.(10) Another
function ascribed to the white corpuscles is that of aiding in the
coagulation of the blood (page 31); and still another, of aiding in the
healing of wounds.

*Plasma.*--The plasma is a complex liquid, being made up of water and of
substances dissolved in the water. The dissolved substances consist mainly
of foods for the cells and wastes from the cells.

1. _The foods_ represent the same classes of materials as are taken in the
daily fare