een extensive, and in a tissue of complex structure,
complete restoration does not take place and a less perfect tissue is
formed which is called a scar. Examination of the skin in almost
anyone will show some such scars which have resulted from wounds. They
are also found in the internal organs of the body as the result of
injuries which have healed. The scar represents a very imperfect
repair. In the skin, for example, the scar tissue never contains such
complicated apparatus as hair and sweat glands; the white area is
composed of an imperfectly vascularized fibrous tissue which is
covered with a modified epidermis. The scar is less resistant than the
normal tissue, injury takes place more easily in it and heals with
more difficulty.

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
cond