In the course of operations also, portions of skin, fascia, or bone, or even a complete joint may be transplanted, and unite by primary union.

Healing under a Scab.—­When a small superficial wound is exposed to the air, the blood and serum exuded on its surface may dry and form a hard crust or scab, which serves to protect the surface from external irritation in the same way as would a dry pad of sterilised gauze.  Under this scab the formation of granulation tissue, its transformation into cicatricial tissue, and the growth of epithelium on the surface, go on until in the course of time the crust separates, leaving a scar.

Healing by Blood-clot.—­In subcutaneous wounds, for example tenotomy, in amputation wounds, and in wounds made in excising tumours or in operating upon bones, the space left between the divided tissues becomes filled with blood-clot, which acts as a temporary scaffolding in which granulation tissue is built up.  Capillary loops grow into the coagulum, and migrated leucocytes from the adjacent blood vessels destroy the red corpuscles, and are in turn disposed of by the developing fibroblasts, which by their growth and proliferation fill up the gap with young connective tissue.  It will be evident that this process only differs from healing by primary union in the amount of blood-clot that is present.

Presence of a Foreign Body.—­When an aseptic foreign body is present 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,