The formative activity of cells is also essential to the normal state.  Destruction of cells is constantly taking place in the body, and more rapidly in certain tissues than in others.  Dried and dead cells are constantly and in great numbers thrown off from the surface of the skin:  such epidermic appendages as the hair and nails grow and are removed, millions of cells are represented in the beard which is daily removed.  Cells are constantly being destroyed on the intestinal surface and in the glands.  There is an enormous destruction of the blood cells constantly taking place, certain essential pigments, as that of the bile, being formed from the haemoglobin which the red blood corpuscles contain and which becomes available on their destruction.  All such loss of cells must be made good by the formation of new ones and, as in the case of the nutritive and functional activity, the loss and renewal must balance.  The formative activity of cells is of great 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 coagulate at a much lower temperature than the white of the egg (as the myosin, one of the albumens of the muscle which coagulates at 115 deg.  F., egg white coagulating at 158 deg.  F.), and in addition to such coagulation or without it the ferments within the cell and to the action of which cellular activity is due may be destroyed.