process varies. Simple structures, such as skin,
cartilage, bone, periosteum, and tendon, for example, have a high power
of regeneration, and in them the reparative process may result in almost
perfect restitution to the normal. More complex structures, on the other
hand, such as secreting glands, muscle, and the tissues of the central
nervous system, are but imperfectly restored, simple cicatricial
connective tissue taking the place of what has been lost or destroyed.
Any given tissue can be replaced only by tissue of a similar kind, and
in a damaged part each element takes its share in the reparative process
by producing new material which approximates more or less closely to the
normal according to the recuperative capacity of the particular tissue.
The normal process of repair may be interfered with by various
extraneous agencies, the most important of which are infection by
disease-producing micro-organisms, the presence of foreign substances,
undue movement of the affected part, and improper applications and
dressings. The effect of these agencies is to delay repair or to prevent
the individual tissues carrying the process to the furthest degree of
which they are capable.

In the management of wounds and other diseased conditions the main
object of the surgeon is to promote the natural reparative process by
preventing or eliminating any factor by which it may be disturbed.

#Healing by Primary Union.#--The most favourable conditions for the
progress of the reparative process are to be found in a clean-cut wound
of the integument, which is uncomplicated by loss of tissue, by the
presence of foreign substances, or by infection with disease-producing
micro-organisms, and its edges are in contact. Such a wound in virtue of
the absence of infection is said to be _aseptic_, and under these
conditions healing takes place by what is called "primary union"--the
"healing by first intention" of the older writers.

#Granulation Tissue.#--The essential and invariable medium of repair in
all structures is an elementary form of new tissue known as _granulation
tissue_, which is produced in the damaged area in response to the
irritation caused by injury or disease. The vital reaction induced by
such irritation results in dilatation of the vessels of the part,
emigration of leucocytes, transudation of lymph, and certain
proliferative changes in the fixed tissue cells. These changes are
common to the processes of inflammation and