The stomach provides secretion to meet the usual demands of digestion, but can take care of an unusual amount of food.

The work of the heart may be doubled by severe exertions, and it meets this demand by increased force and rapidity of contraction; and the same is true of the muscles attached to the skeleton.

The constant exercise of this reserve force breaks down the adjustment.

If the weight of the traffic over the bridge be 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.