d in which no change is present to which
death can be attributed, all organs seem to share alike in the
senescence. Death is usually due to some of the accidents of life, a
slight infection to which the less resistant body succumbs, or to the
rupture of a weakened blood vessel in the brain, or to more advanced
decay in some organ whose function is indispensable. The causes and
conditions of age have been a fertile source for speculation. Many of
the hypotheses have been interesting, that of Metschnikoff, for
example, who finds as a dominating influence in causing senescence the
absorption of toxic substances formed in the large intestine by
certain bacteria. He further finds that the cells of the body which
have phagocytic powers turn their activity against cells and tissues
which have become weakened. There may be absorption of injurious
substances from the intestines which the body in a vigorous condition
is able to destroy or to counteract their influence, and these may be
more operative in the weaker condition of the body in the old.
Phagocytes will remove cells which are dead and often cells which are
superfluous in a part, but there is no evidence that this is ever
other than a conservative process. Since it is impossible to single
out any one condition to which old age is due, the hypothesis of
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