importance, for it is by means of this that wounds heal and diseases
are recovered from. This constant destruction and renewal of the body
is well known, and it is no doubt this which has given rise to the
belief, widely held, that the body renews itself in seven years and
that the changes impressed upon it by vaccination endure for this
period only. The truth is that the destruction and renewal of most
tissues in the body takes place in a much shorter interval, and, as we
shall see, this has nothing to do with the changes concerned in
vaccination. All these activities of the cells vary in different
individuals, in different parts and at different ages.

The lesions or injuries of the body which form so prominent a part of
disease vary in kind, degree and situation, depending upon the
character of the injurious agent, the duration of its action and the
character of the tissue affected. The most obvious injuries are those
produced by violence. By a cut, blood vessels are severed, the
relations of tissues disturbed, and at the gaping edges of the wound
the tissue usually protected by the skin is exposed to the air,
resulting in destruction of the cells contained in a thin layer of the
surface. The discoloration and swelling of the skin following a blow
is due to rupture of vessels and escape of blood and fluid, and
further injury may result from the interruption of the circulation.

By the application of heat the tissue may be charred and the albumen
of the blood and tissue fluids coagulated. Living cells are very
susceptible to the action of heat, a temperature of 130 degrees being
the thermal death point, and even lower temperatures are fatal when
their action is prolonged. The action of the heat may produce definite
coagulation of the fluid within the cells in the same way that the
white of an egg is coagulated. Certain of the albumens of the body