Death of the body as a whole takes place from the cessation of the action of the central nervous system or of the respiratory system or of the circulation.  There are other organs of the body, such as the intestine, kidney, liver, whose function is essential for life, but death does not take place immediately on the cessation of their function.  The functions of the heart, the brain and the lungs are intimately associated.  Oxygen is indispensable for the life of the tissues, and its supply is dependent upon the integrity of the three organs mentioned, which have been called the tripos of life.  Respiration is brought about by the stimulation of certain nerve cells in the brain, the most effective stimulus to these cells being a diminution of oxygen in the blood supplying them.  These cells send out impulses to the muscles concerned in inspiration, the chest expands, and air is taken into the lungs.  Respiration is then a more complicated process than is the action of the heart, for its contraction, which causes the blood to circulate, is not immediately dependent upon extrinsic influences.  Death is usually more immediately due to failure of respiration than to failure of circulation, for the heart often continues beating for a time after respiration has ceased.  Thus, in cases of drowning and suffocation, by means of artificial respiration in which air is passively taken into and expelled from the lungs, giving oxygen to the blood, the heart may continue to beat and the circulation continue for hours after all evident signs of life and all sensation has ceased.

By this general death is meant the death of the organism as a whole, but all parts of the body do not die at the same time.  The muscles and nerves may react, the heart may be kept beating, and organs of the body when removed and supplied with blood will continue to function.  Certain tissues die early, and the first to succumb to the lack of oxygenated blood are the nerve cells of the brain.  If respiration and circulation have ceased for as short a time as twelve minutes, life ceases in certain of these cells and cannot be restored.  This is again an example of the greater vulnerability of the more highly differentiated structure in which all other forms of cell activity are subordinated to function.  There are, however, pretty well authenticated cases of resuscitation after immersion in water for a longer period than twelve minutes, but these cases have not been carefully timed, and time under such conditions may seem longer than it actually is; and there is, moreover, the possibility of a slight gaseous interchange between the blood and the water in the lungs, as in the case of the fish which uses the water for an oxygen supply as the mammal does the air.  There are also examples of apparent death or trances which have lasted longer, and the cases of fakirs who have been buried for prolonged periods and again restored to life.  In these conditions, however, all the activities of the body are reduced to the utmost, and respiration and circulation, so feeble as to be imperceptible to ordinary observation, suffice to keep the cells living.