It is a still more difficult matter for obvious reasons to estimate how much blood a man can lose and yet remain alive. This will depend partly on the power of physiological accommodation possessed by the individual in his vaso-motor system and tissue fluids and partly on the rapidity with which the bleeding takes place. Clinical observations have shown that after a moderate hæmorrhage, such as the withdrawal of 800 cc. of blood from a donor, the blood volume may be restored to normal within an hour. If, on the other hand, the hæmorrhage is excessive, a condition results in which the normal process of rapid restoration of volume fails, and the circulation remains in a dangerously depleted condition. The heart attempts to keep the blood pressure at an adequate level by an increase in its rate, but it is in effect attempting to circulate a small volume of fluid in a vascular system which has become too big for it. Imperfect oxygenation of the medullary and cerebral centres with exhaustion of the heart results, and this is accompanied by all the symptoms of anæmia which have been already described. If the initial hæmorrhage be very rapid, death may result almost at once, since the physiological processes may have no time to act. On the other hand, a rapid hæmorrhage may under certain circumstances save the patient’s life, for the immediate syncope which results produces so great a fall in the blood pressure that hæmorrhage almost ceases and a clot may form in the lumen of the divided vessel. If the hæmorrhage be more gradual, the physiological compensation may at first be adequate to maintain the blood volume, but finally a point is reached at which this process fails and the patient then passes into the condition of acute anæmia.