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 inflammation, such as hot applications, act by increasing the blood flow. So intimate is the association between cell activity, as shown in repair and new formation of cells, and the blood flow, that new blood vessels frequently develop by means of which the capacity for nutrition is still more increased. The cornea or transparent part of the eye contains no blood vessels, the cells which it contains being nourished by the tissue fluid which comes from the outside and circulates in small communicating spaces. If the centre of the cornea be injured, the cells of the blood vessels in the tissue around the cornea multiply and form new vessels which grow into the cornea and appear as a pink fringe around the periphery; when repair has taken place the newly formed vessels disappear. The exudate from the blood vessels in various ways assists in repair. An injurious substance in the tissue may be so diluted by the fluid that its action is minimized. A small crystal of salt is irritating to the eye, but a much greater amount of the same substance in dilute solution causes no irritation. The poisonous substances produced by bacteria are diluted and washed away from the part by the exudate. Not only is there a greater amount of tissue fluid in the inflamed part, but the circulation of this is also increased, as is shown by comparing the outflow in the lymphatic vessels with the normal. The fluid exudate which has come from the blood and differs but slightly from the blood fluid exerts not only the purely physical action of removing and diluting injurious substances, but in many cases has a remarkable power, exercised particularly on bacterial poisons, of neutralizing poisons or so changing their character that they cease to be injurious.