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.