While the absolute restoration of complete and considerable portions of the body is scarcely possible in man, yet the body is by no means unprovided with powers by which injuries are repaired and loss of structures is made good. It may be said that the higher and more complex animals are endowed with greater ability to protect themselves from injury, and that their tissues possess the necessary powers of restoration in the case of those injuries to which they are specially liable. In the various processes here to be considered, it will be seen that what may be called the definite intention to attain a certain result which is shown in the process of development, is distinctly visible, and that the tissues have a remarkable power of meeting adverse conditions.
The power of repair is well seen in the various processes concerned in the healing of wounds. There are some wounds which heal by a process fitly designated Immediate union. That is to say, the surfaces are brought together and coalesce without any new-formed material being produced to serve as a bond of union. This occurs mostly in clean-cut wounds, which, shortly after their infliction, are closed so as to bring the cut surfaces into close contact. It is necessary for this process that all inflammation be avoided either at the time of infliction of the wound or afterwards. We are to think of the living tissues not as mere mechanical pieces of texture, but as possessed, by virtue of their vitality, of wonderful powers of adaptability to circumstances. When two living surfaces are brought into contact and all disturbing conditions are averted, then the blood-vessels form communications, the nerves by and by unite and become continuous, and the connective tissue coalesces. In all this there is a certain amount of new-formation by karyomitosis, and there may even be slight inflammatory exudation, but there is no permanent new tissue produced, and a wound in the skin, or even in muscle, may unite, and no trace of a bond of union or even of the line of union can be found after a few days. The epidermis probably does not unite so directly, and the wound is covered by new-formed epidermis.
Primary adhesion is a process of a more complicated kind. In it inflammation plays a part. When a wound is inflicted the mere mechanical injury, or exposure afterwards, frequently leads to a trivial but acute inflammation, resulting, as we have seen, in the coating of the cut surface with a fibrinous exudation, the so-called glaze. If two surfaces thus coated with fibrine be brought in contact they unite, the fibrine acting as a glue or cement. But the fibrine does not form a permanent bond of union, and if union is not effected by other methods, then the wound will subsequently gape by the breaking down of the tibrine. In order to effect a permanent union we must have formative cells produced, such as are concerned in the formation of connective tissue out of granulation tissue, and we must have a new-formation of blood-vessels. The uniting tissue is very trivial in amount; there is merely a limited production of formative cells which replace the thin layer of fibrine, and a budding of the blood-vessels till communications are formed between the two surfaces. This whole process may occupy only a day or two, and the permanent new-formed tissue forming the cicatrix is usually very small in amount.
The term Union by the first intention is commonly used so as to include both of the conditions described above, any case of union occurring upon application of the two surfaces soon after the infliction of the wound being so designated. (See more fully in Paget's "Surgical Pathology.")
Union by the second intention or by granulation is a name given to the closure of a wound by the .adhesion and coalescence of two layers of granulations. We have already seen how this comes about. The granulations ultimately develop into connective tissue which forms the cicatrix or permanent bond of union. In this case the cicatrix is a much more considerable one than in union by the first intention.