_Secretory Glands._--The regeneration of secretory glands is usually
incomplete, cicatricial tissue taking the place of the glandular
substance which has been destroyed. In wounds of the liver, for example,
the gap is filled by fibrous tissue, but towards the periphery of the
wound the liver cells proliferate and a certain amount of regeneration
takes place. In the kidney also, repair mainly takes place by
cicatricial tissue, and although a few collecting tubules may be
reformed, no regeneration of secreting tissue takes place. After the
operation of decapsulation of the kidney a new capsule is formed, and
during the process young blood vessels permeate the superficial parts
of the kidney and temporarily increase its blood supply, but in the
consolidation of the new fibrous tissue these vessels are ultimately
obliterated. This does not prove that the operation is useless, as the
temporary improvement of the circulation in the kidney may serve to tide
the patient over a critical period of renal insufficiency.

_Stomach and Intestine._--Provided the peritoneal surfaces are
accurately apposed, wounds of the stomach and intestine heal with great
rapidity. Within a few hours the peritoneal surfaces are glued together
by a thin layer of fibrin and leucocytes, which is speedily organised
and replaced by fibrous tissue. Fibrous tissue takes the place of the
muscular elements, which are not regenerated. The mucous lining is
restored by ingrowth from the margins, and there is evidence that some
of the secreting glands may be reproduced.

Hollow viscera, like the oesophagus and urinary bladder, in so far
as they are not covered by peritoneum, heal less rapidly.

_Nerve Tissues._--There is no trustworthy evidence that regeneration of
the tissues of the brain or spinal cord in man ever takes place. Any
loss of substance is replaced by cicatricial tissue.

The repair of _Bone_, _Blood Vessels_, and _Peripheral Nerves_ is more
conveniently considered in the chapters dealing with these structures.

#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
with which the graft is in contact: a portion of sterilised marine
sponge, for example, may be observed to become permeated with
granulation tissue when it is embedded in the tissues.

A successful graft of living tissue is not only capable of regeneration,
but it acquires a system of lymph and blood vessels, so that in time it
bleeds when cut into, and is permeated by new nerve fibres spreading in
from the periphery towards the centre.

It is instructive to associate the period of survival of the different
tissues of the body after death, with their capacity of being used for
grafting purposes; the higher tissues such as those of the central
nervous system and highly specialised glandular tissues like those of
the kidney lose their vitality quickly after death and are therefore
useless for grafting; connective tissues, on the other hand, such as
fat, cartilage, and bone retain their vitality for several hours after
death, so that when they are transplanted, they readily "take" and do
all that is required of them: the same is true of the skin and its