The actual amount of blood therefore that must be lost to be fatal will vary according to circumstances. Experience shows that hæmorrhage may take place into the peritoneal or pleural cavities to the extent of two litres or even more, and it may be stated as a rough guess that 2·5 litres, that is to say, even as much as almost half the total blood volume, may be lost without immediate death resulting. This degree of depletion could not, however, be endured for long. A series of clinical observations made by Keith by the vital-red method upon the blood volume in soldiers suffering from the combined effects of hæmorrhage and wound shock showed that in the most serious cases the volume was below 65 per cent. of the normal, frequently even between 50 and 60 per cent. Serious symptoms followed a reduction to between 65 and 75 per cent. In patients without distressing symptoms the volume was never below 75 per cent. of the normal. There is direct evidence, therefore, that those patients who are most in need of treatment, such as a transfusion of blood, will probably have lost from 25 to 50 per cent. of their blood volume, that is to say, 1·5 to 3 litres in amount, and will need from 750 cc. to 1·5 litres to restore them to, or near to, the 75 per cent. level at which the compensatory processes can begin to regain their power. It is thus possible to arrive at a theoretical basis on which an idea can be formed of the amount of blood that should be given in acute anæmia. Practical experience is in agreement with the theory, and it will now be easier to understand how it is that in treating acute anæmia no attempt need be made to replace the whole amount of blood that has been lost, or indeed anything approaching it. In an extreme case 2 to 3 litres of blood will have been lost and 1 litre or more will be needed to restore the blood volume to approximately 75 per cent. of the normal. A case of this sort, however, is fortunately not often to be met. One has already been described on page 21; this patient received altogether nearly 1,600 cc. of blood in two transfusions, and 1,000 cc. of normal saline were given in addition.