ition 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
forty. This is the gland which in the calf is known as the sweetbread
and is a delicious and valued article of food. The tonsils, which in
the child may be so large as to interfere with breathing and
swallowing, have almost disappeared in the adult; and there are other
such examples.

In age atrophy is a prominent change. It is seen in the loss of the
teeth, in the whitening and loss of the hair, in the thinning of the
skin so that it more easily wrinkles, in the thinning and weakening of
the muscles so that there is not only diminished force of muscular
contraction, but weakening of the muscles of support. The back curves
from the action of gravity, the strength of the support of the muscles
at the back not counteracting the pull of the weight of the abdominal
viscera in front. The bones become more porous and more brittle.

The effect of atrophy is also seen in the diminution of all functions,
and in loss of weight in individual organs. That the brain shares in
the general atrophy is evident both anatomically and in function.
Mental activity is more sluggish, impressions are received with more
difficulty, their accuracy may be impaired by accompanying changes in
the sense organs, and the concepts formed from the impressions may
differ from the usual. The slowness of mental action and the
diminut