Repair will also take place the more readily the less complicated is the architectural structure of the part affected.  When a series of tissues variously and closely related to one another enter into the structure of an organ, there may be new formation of cells; but when the loss involves more than this, the complicated architectural structure will not be completely replaced.  A brick which has been knocked out of a building can be easily replaced, but the renewal of an area of the wall is more difficult.  In the kidney, for example, the destruction of single cells is quickly made good by new cell formation, but the loss of an area of tissue is never restored.  In the liver, on the other hand, which is of much simpler construction, large areas of tissue can be newly formed.  For the formation of new cells in a part there must be a sufficient amount of formative material; then the circulation of the blood becomes more active, more blood being brought to the part by dilatation of the vessels supplying it.

Repair after a loss can be perfect or imperfect.  The tissue lost can be restored so perfectly that no trace of an injury remains; but when the loss has been 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.