In reality, two kinds of death are to be considered. First, there is somatic death, which implies the inability of the body to continue to function as an integrated entity. Without the use of artificial measures, somatic death is inevitably followed by cellular death, the cells of different organs and tissues of the body dying at different rates in accordance with their different oxygen requirements, those of the brain being the most delicate. For almost the entire history of man, therefore, death was synonymous with lack of oxygen due to cessation of cardiorespiratory function. But now those whose respiratory muscles have been paralysed by poliomyelitis can be maintained fully alive by mechanical respirators, as can victims of renal failure by renal dialysis. The function of the heart - which is but a sort of pump - can also be replaced by mechanical means. But nothing can replace the functions of the brain. Once the fragile cells of the grey matter that forms its outer layer have been deprived of oxygen for more than a few minutes the person, if not the rest of his/her body, is irrevocably dead. S/he will never again think, feel, do, see, hear, smell, taste, touch, or be a person, although the action of the heart, lungs, and other vital organs may be maintained for years by artificial means and their cells continue to live. It is the technical possibility of using the organs of persons in this state to save another life that has made it urgent to arrive at a new and ethically acceptable definition of death.
Increasingly, the conception of 'brain death' as constituting the essential criterion has gained ground. The activity of the brain may be measured electronically by the electroencephalograph, and when the brain has ceased to function this instrument records a tracing variously described as 'flat', 'linear', or 'isoelectric'. This conception of 'brain death' as signifying the end of the life in any meaningful sense has been so widely accepted that it has been incorporated into the legislation of twelve of the States of the USA. The only controversial point is the time that should elapse before the flat tracing of the electroencephalograph should be considered irreversible.