All these changes which an injured part undergoes are found when closely analyzed to be purposeful; that is, they are in accord with the conditions under which the living matter acts, and they seem to facilitate the operation of these conditions.  It has been said that the life of the organism depends upon the cooerdinated activity of the living units or cells of which it is composed.  The cells receive from the blood material for the purpose of function, for cell repair and renewal, and the products of waste must be removed.  In the injury which has been produced in the tissue all the cells have suffered, some possibly displaced from their connections, others may have been completely destroyed, others have sustained varying degrees of injury.  If the injury be of an infectious character, that is, produced by bacteria, these may be present in the part and continue to exert injury by the poisonous substances which they produce, or if the injury has been produced by the action of some other sort of poison, this may be present in concentrated form, or the injury may have been the result of the presence of a foreign body in the part.  Under these conditions, since the usual activities of the cells in the injured part will not suffice to restore the integrity of the tissue, repair and cell formation must be more active than usual, any injurious substances must be removed or such changes must take place in the tissue that the cell life adapts itself to new conditions.

[Illustration:  FIG. 16.—­PHAGOCYTOSIS. a, b, c are the microphages or the bacterial phagocytes. (a) Contains a number of round bacteria, and (b) similar bacteria arranged in chains, and (c) a number of rod-shaped bacteria (d) Is a cell phagocyte or macrophage which contains five red blood corpuscles.]

All life in the tissues depends upon the circulation of the blood.  There is definite relation between the activity of cells and the blood supply; a part, for instance, which is in active function receives a greater supply of blood by means of dilatation of the arteries which supply it.  If the body be exactly balanced longitudinally on a platform, reading or any exercise of the brain causes the head end to sink owing to the relatively greater amount of blood which the brain receives when in active function.  The regulation of the blood supply is effected by means of nerves which act upon the muscular walls of the arteries causing, by the contraction or the relaxation of the muscle, diminution or dilatation of the calibre of the vessel.  After injury the dilatation of the vessels with the greater afflux of blood to the part is the effect of the greatly increased cell activity, and is a necessity for this.  In many forms of disease it has been found that by increasing the blood flow to a part and producing an active circulation in it, that recovery more readily takes place and many of the procedures which have been found useful in