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.