constantly all that it can carry, there quickly comes a time when some
slight and unforeseen increase of weight brings disaster. The
conditions in the body are rather better than in the case of the
bridge, because with the increased demand for activity the heart, for
example, becomes larger and stronger, and reserve force rises with the
load to be carried, but the ratio of reserve force is diminished.

This discussion of injury and repair leads to the question of old age.
Old age, as such, should not be discussed in a book on disease, for it
is not a disease; it is just as natural to grow old and to die as it
is to be born. Disease, however, differs in many respects in the old
as compared with the young and renders some discussion of the
condition necessary. Changes are constantly taking place in the body
with the advance of years, and in the embryo with the advance of days.
In every period of life in the child, in the adult, in the middle-aged
and in the old we meet with conditions which were not present at
earlier periods. There is no definite period at which the changes
which we are accustomed to regard as those of old age begin. This is
true of both the external appearances of age and the internal changes.
One individual may be fully as old, as far as is indicated by the
changes of age, at fifty as another at eighty.

With advancing age certain organs of the body atrophy; they become
diminished in size, and the microscopic examination shows absence or
diminished numbers of the cells which are peculiar to them. The most
striking example of this is seen in the sexual glands of females, and,
to a less degree, in those of the male. There is a small mass or
glandular tissue at the root of the neck, the thymus, which gradually
grows from birth and reaches its greatest size at the age of fifteen,
when it begins slowly to atrophy and almost disappears at the age of