hich he made brief
allusion in "The Nature of Man," and he lays down certain
principles for the prolongation of life which have been put into
practice by a large number of people during the last two or three
years, and are steadily gaining more attention. Sour milk as an
article of diet appears to have a peculiar value in arresting the
supposed senile changes which are largely due to auto-intoxication
or self-poisoning.

_I.--Senile Debility_

When we study old age in man and the lower animals, we observe certain
features common to both. But often among vertebrates there are found
animals whose bodies withstand the ravages of time much better than that
of man. I think it a fair inference that senility, that precocious
senescence which is one of the greatest sorrows of humanity, is not so
profoundly seated in the constitution of the higher animals as has
generally been supposed. The first facts which we must accept are that
human beings who reach extreme old age may preserve their mental
qualities, notwithstanding serious physical decay, and that certain of
the higher animals can resist the influence of time much longer than is
the case with man under present conditions.

Many theories have been advanced regarding the cause of senility. It is
certain that many parts of the body continue to thrive and grow even in
old age, as, for instance, the nails and hair. But I believe that I have
proved that in many parts of the body, especially the higher elements,
such as nervous and muscular cells, there is a destruction due to the
activity of the white cells of the blood. I have shown also that the
blanching of the hair in old age is due to the activity of these white
cells, which destroy the hair pigment. Progressive muscular debility is
an accompaniment of old age; physical work is seldom given to men over
sixty years of age, as it is notorious that they are less capable of it.
Their muscular movements are feebler, and soon bring on fatigue; their
actions are slow and painful. Even old men whose mental vigour is
unimpaired admit their muscular weakness. The physical correlate of this
condition is an actual atrophy of the muscles, and has for long been
known to observers. I have found that the cause of this atrophy is the
consumption of the muscle fibres by what I call phagocytes, or eating
cells, a certain kind of white blood cells.

In the case of certain diseases we find symptoms, whi