#Modifications of the Process of Repair.#—­The process of repair by primary union, above described, is to be looked upon as the type of all reparative processes, such modifications as are met with depending merely upon incidental differences in the conditions present, such as loss of tissue, infection by micro-organisms, etc.

Repair after Loss or Destruction of Tissue.—­When the edges of a wound cannot be approximated either because tissue has been lost, for example in excising a tumour or because a drainage tube or gauze packing has been necessary, a greater amount of granulation tissue is required to fill the gap, but the process is essentially the same as in the ideal method of repair.

The raw surface is first covered by a layer of coagulated blood and fibrin.  An extensive new formation of capillary loops and fibroblasts takes place towards the free surface, and goes on until the gap is filled by a fine velvet-like mass of granulation tissue.  This granulation tissue is gradually replaced by young cicatricial tissue, and the surface is covered by the ingrowth of epithelium from the edges.

This modification of the reparative process can be best studied clinically in a recent wound which has been packed with gauze.  When the plug is introduced, the walls of the cavity consist of raw tissue with numerous oozing blood vessels.  On removing the packing on the fifth or sixth day, the surface is found to be covered with minute, red, papillary granulations, which are beginning to fill up the cavity.  At the edges the epithelium has proliferated and is covering over the newly formed granulation tissue.  As lymph and leucocytes escape from the exposed surface there is a certain amount of serous or sero-purulent discharge.  On examining the wound at intervals of a few days, it is found that the granulation tissue gradually increases in amount till the gap is completely filled up, and that coincidently the epithelium spreads in and covers over its surface.  In course of time the epithelium thickens, and as the granulation tissue is slowly replaced by young cicatricial tissue, which has a peculiar tendency to contract and so to obliterate the blood vessels in it, the scar that is left becomes smooth, pale, and depressed.  This method of healing is sometimes spoken of as “healing by granulation”—­although, as we have seen, it is by granulation that all repair takes place.

Healing by Union of two Granulating Surfaces.—­In gaping wounds union is sometimes obtained by bringing the two surfaces into apposition after each has become covered with healthy granulations.  The exudate on the surfaces causes them to adhere, capillary loops pass from one to the other, and their final fusion takes place by the further development of granulation and cicatricial tissue.

Reunion of Parts entirely Separated from the Body.—­Small portions of tissue, such as the end of a finger, the tip of the nose or a portion of the external ear, accidentally separated from the body, if accurately replaced and fixed in position, occasionally adhere by primary union.