An infection which may but slightly affect the ordinary well-being of the individual may produce a considerable damage.

Excess or deficiency or improper food, occasional or continued use of alcohol and other poisons may lead to very definite lesions.

Repair after injury is rarely perfect, the repaired tissue is more susceptible to injury, and with advancing age there is constant diminution in the ease and perfection of repair.

The effect of the sum of all these changes becomes operative: a vicious circle is established in which injury becomes progressively easier to acquire and repair constantly less perfect.

There is some adjustment, however, in that the range of activities is diminished, the environment becomes narrower and the organism adapts its life to that environment which makes the least demands upon it.
Whether there is, entirely apart from all conditions affecting nutrition and the effect of injuries which disturb the usual cell activities, an actual senescence of the cells of the body is uncertain.