uctures.

#Rate of Healing.#--While the rate at which wounds heal is remarkably
constant there are certain factors that influence it in one direction or
the other. Healing is more rapid when the edges are in contact, when
there is a minimum amount of blood-clot between them, when the patient
is in normal health and the vitality of the tissues has not been
impaired. Wounds heal slightly more quickly in the young than in the
old, although the difference is so small that it can only be
demonstrated by the most careful observations.

Certain tissues take longer to heal than others: for example, a fracture
of one of the larger long bones takes about six weeks to unite, and
divided nerve trunks take much longer--about a year.

Wounds of certain parts of the body heal more quickly than others: those
of the scalp, face, and neck, for example, heal more quickly than those
over the buttock or sacrum, probably because of their greater
vascularity.

The extent of the wound influences the rate of healing; it is only
natural that a long and deep wound should take longer to heal than a
short and superficial one, because there is so much more work to be
done in the conversion of blood-clot into granulation tissue, and this
again into scar tissue that will be strong enough to stand the strain on
the edges of the wound.

THE TRANSPLANTATION OR GRAFTING OF TISSUES

Conditions are not infrequently met with in which healing is promoted
and restoration of function made possible by the transference of a
portion of tissue from one part of the body to another; the tissue
transferred is known as the _graft_ or the _transplant_. The simplest
example of grafting is the transplantation of skin.

In order that the graft may survive and have a favourable chance of
"taking," as it is called, the transplanted tissue must retain its
vitality until it has formed an organic connection with the tissue in
which it is placed, so that it may derive the necessary nourishment from
its new bed. When these conditions are fulfilled the tissues of the
graft continue to proliferate, producing new tissue elements to replace
those that are lost and making it possible for the graft to become
incorporated with the tissue with which it is in contact.

Dead tissue, on the other hand, can do neither of these things; it is
only capable of acting as a model, or, at the most, as a scaffolding for
such mobile tissue elements as may be derived from, the parent tissue