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.