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
i