Metschnikoff should have no more regard given it than the many other
hypotheses which have been presented.

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