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.