Loss brought about by the injuries of disease can be compensated for, even when the healing is imperfect, by increased function of similar tissue in the body. There always seems to be in the body under the usual conditions a reserve force, no tissue being worked to its full capacity. Meltzer has compared the reserve force of the body to the factor of safety in mechanical construction. A bridge is constructed to sustain the weight of the usual traffic, but is in addition given strength to meet unusual and unforeseen demands. 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. 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.