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 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 diminution in the range of mental activity excited by impressions, and the slowness of expression, may give a false idea of the value of the judgment expressed.