in the tissues, _e.g._ a piece of unabsorbable chromicised catgut, the
healing process may be modified. After primary union has taken place the
scar may broaden, become raised above the surface, and assume a
bluish-brown colour; the epidermis gradually thins and gives way,
revealing the softened portion of catgut, which can be pulled out in
pieces, after which the wound rapidly heals and resumes a normal
appearance.


REPAIR IN INDIVIDUAL TISSUES

_Skin and Connective Tissue._--The mode of regeneration of these tissues
under aseptic conditions has already been described as the type of ideal
repair. In highly vascular parts, such as the face, the reparative
process goes on with great rapidity, and even extensive wounds may be
firmly united in from three to five days. Where the anastomosis is less
free the process is more prolonged. The more highly organised elements
of the skin, such as the hair follicles, the sweat and sebaceous glands,
are imperfectly reproduced; hence the scar remains smooth, dry, and
hairless.

_Epithelium._--Epithelium is only reproduced from pre-existing
epithelium, and, as a rule, from one of a similar type, although
metaplastic transformation of cells of one kind of epithelium into
another kind can take place. Thus a granulating surface may be covered
entirely by the ingrowing of the cutaneous epithelium from the margins;
or islets, originating in surviving cells of sebaceous glands or sweat
glands, or of hair follicles, may spring up in the centre of the raw
area. Such islets may also be due to the accidental transference of
loose epithelial cells from the edges. Even the fluid from a blister, in
virtue of the isolated cells of the rete Malpighii which it contains, is
capable of starting epithelial growth on a granulating surface. Hairs
and nails may be completely regenerated if a sufficient amount of the
hair follicles or of the nail matrix has escaped destruction. The
epithelium of a mucous membrane is regenerated in the same way as that
on a cutaneous surface.

Epithelial cells have the power of living for some time after being
separated from their normal surroundings, and of growing again when once
more placed in favourable circumstances. On this fact the practice of
skin grafting is based (p. 11).

_Cartilage._--When an articular cartilage is divided by incision or by
being implicated in a fracture involving the articular end of a bone, it
is repaired by ordinary cicatricial fibrous tissue derived from the
proliferating cells of the perichondrium. Cartilage being a non-vascular
tissue, the reparative process goes on slowly, and it may be many weeks
before it is complete.

It is possible for a metaplastic transformation of connective-tissue
cells into cartilage cells to take place, the characteristic hyaline
matrix being secreted by the new cells. This is sometimes observed as an
intermediary stage in the healing of fractures, especially in young
bones. It may also take place in the regeneration of lost portions of
cartilage, provided the new tissue is so situated as to constitute part
of a joint and to be subjected to pressure by an opposing cartilaginous
surface. This is illustrated by what takes place after excision of
joints where it is desired to restore the function of the articulation.
By carrying out movements between the constituent parts, the fibrous
tissue covering the ends of the bones becomes moulded into shape, its
cells take on the characters of cartilage cells, and, forming a matrix,
so develop a new cartilage.

Conversely, it is observed that when articular cartilage is no longer
subjected to pressure by an opposing cartilage, it tends to be
transformed into fibrous tissue, as may be seen in deformities attended
with displacement of articular surfaces, such as hallux valgus and
club-foot.

After fractures of costal cartilage or of the cartilages of the larynx
the cicatricial tissue may be ultimately replaced by bone.

_Tendons._--When a tendon is divided, for example by subcutaneous
tenotomy, the end nearer the muscle fibres is drawn away from the other,
leaving a gap which is speedily filled by blood-clot. In the course of a
few days this clot becomes permeated by granulation tissue, the
fibroblasts of which are derived from the sheath of the tendon, the
surrounding connective tissue, and probably also from the divided ends
of the tendon itself. These fibroblasts ultimately develop into typical
tendon cells, and the fibres which they form constitute the new tendon
fibres. Under aseptic conditions repair is complete in from two to three
weeks. In the course of the reparative process the tendon and its sheath
may become adherent, which leads to impaired movement and stiffness. If
the ends of an accidentally divided tendon are at once brought into
accurate apposition and secured by sutures, they unite directly with a
minimum amount of scar tissue, and function is perfectly restored.

_Muscle._--Unstriped muscle does not seem to be capable of being
regenerated to any but a moderate degree. If the ends of a divided
striped muscle are at once brought into apposition by stitches, primary
union takes place with a minimum of intervening fibrous tissue. The
nuclei of the muscle fibres in close proximity to this young cicatricial
tissue proliferate, and a few new muscle fibres may be developed, but
any gross loss of muscular tissue is replaced by a fibrous cicatrix. It
would appear that portions of muscle transplanted from animals to fill
up gaps in human muscle are similarly replaced by fibrous tissue. When a
muscle is paralysed from loss of its nerve supply and undergoes complete
degeneration, it is not capable of being regenerated, even should the
integrity of the nerve be restored, and so its function is permanently
lost.