The manner in which the divided surfaces of a wound are united is not always the same. How this will be effected will depend upon a variety of circumstances, notably the time which elapses between the infliction of the wound and the readjustment or apposition of the parts, the manner in which they are brought together, the nature of the cut surfaces, the presence or absence of foreign matter, the extent of injury done to the surrounding tissues, etc. The time occupied in the process of healing, as most people know, may be very short or very long, according to the extent of the wound and the particular method by which healing is effected.

By careful observation and enquiry, it has been shown that the healing of wounds may take place in five different ways, viz.: - 1. By immediate union. 2. By 'primary adhesion, or union by the adhesive inflammation. 3. By blood-clot. 4. By granulation, or by the second intention. 5. By scarring under a scab. To the lay mind, these expressions do not convey much meaning, but by a little explanation they may be made just as intelligible as they appear to the professional reader.

Immediate Union

In healing by immediate union, the divided parts, on being brought together, are caused to adhere in the first instance by the sticky nature of the matter which then covers their surface, and in a short time - sometimes not more than twenty-four hours - a firm and perfect connection is re-established between the previously divided parts. Examples of this kind of healing are noticed when, on cutting the finger, the parts are brought together and tightly bound up, when, on removing the wrapping, complete repair is found to have been effected without inflammation or discharge, and with little or no pain or swelling.

In this case no scar remains to mark the seat of the injury, and no new tissue intervenes between the newly-united surfaces; they have simply grown together.

Primary Adhesion

Here, instead of the divided parts growing together directly, as in immediate union, without the intervention of new material, the two surfaces of the wound become covered over with a thin layer of cells incorporated with a quantity of adhesive matter which has exuded from the vessels. Some of the former resemble the round, colourless corpuscles of the blood, but they soon begin to change their form by lengthening out into thread-like bodies, and ultimately to be resolved into a layer of connective tissue by which the divided parts are firmly and permanently reunited. In the course of these changes, new blood-vessels from the old ones in the adjoining tissue shoot out into the uniting substance, and organization having been thus established, repair is completed by the growth of a layer of cuticle over the united parts. This mode of healing is unattended by any local or general disturbance, and illustrates what takes place in "healing by the first intention".

Healing By Blood-Clot

Healing by blood-clot differs but little from that just described. It occurs when, in consequence of the edges of a wound not being brought immediately into apposition with each other, the space between them becomes filled with blood-clot, into which white blood corpuscles and plasma cells soon penetrate.

By a process of development the latter are resolved into fibrous tissue, which, as in healing by the first intention, form the permanent bond of union between the divided parts. These two modes of union can only be effected where the lips of the wound are undisturbed and all causes of irritation are excluded.

Healing By Granulation

If, instead of healing by one of the three methods referred to above, an incised wound be left open, it is noticed that the surface soon becomes coated over with a pale, glairy substance composed of white blood cells in the midst of a quantity of sticky albuminoid matter, which together have passed out of the blood-vessels of the part. Sooner or later, according to the extent of damage the tissues have sustained, small, raised points begin to appear here and there above the glairy film, until the entire surface of the wound is dotted over with little, rounded, fleshy-looking granules, or, as they are termed, granulations. These by enlarging coalesce or run one into another, and having formed a continuous layer on the surface of the wound, begin again to throw out granulations as before, and by a repetition of the process the breach is filled in and repair is completed (fig. 410). As the granulations grow and mature they soon begin to shrink, and in doing so draw the sides of the gaping wound together. At the same time they lose their red colour, become dense and firm in consistence, and are finally resolved into a tough connective tissue, as indicated by the white, glistening scar which remains on the site of the wound.

Fig. 410. - Wound Healing by Granulation.

1, Discharge. 2, Margin of sore. 3, New epithelium at edge of sore. 4, Epithelium unaffected by the wound. 5, Bloodvessel. 6, Deep organizing layer beneath granulation. 7, Hair.

As to the nature and source of granulations, they are made up of little heaps of closely-packed cells permeated by minute blood-vessels. The former are derived from connective tissue cells which previously existed in the injured structures, and which, under the influence of irritation excited in the part as a result of the injury, have been caused to multiply, while the new vessels with which they are supplied are offshoots from the old ones. In this mode of healing, many of the cells intended for the purpose of tissue-formation die and are cast off as pus cells, and constitute the white, creamy discharge commonly termed "matter".

When, in the course of healing, granulations are in excess, and project beyond the wound as a fleshy excrescence, they are usually spoken of as "proud flesh".

The three forms of healing first referred to are unattended with danger or suffering to the patient; but in the more prolonged process of suppuration and granulation involved in the fourth, various complications may arise by which the general health is more or less seriously impaired, or life imperilled. These untoward results occur for the most part before the surface of the wound is completely covered with granulations, and are brought about by the entrance of bacteria into the wound, and their reproduction there, giving rise to what is commonly termed "blood-poisoning".

Besides contaminating the blood, bacteria also operate prejudicially against the healing of wounds. The chemical products they give out irritate and inflame the injured part, and produce a more or less copious outflow of pus, and delay the healing process.

Blood-poisoning presents several forms and degrees of severity, from a slight traumatic fever to a fatal collapse, or the development of a succession of abscesses in different parts of the body, extending over a more or less protracted period.

The fluids contained in and upon the surface of a wound serve as a suitable habitat or breeding-ground for a variety of micro-organisms, which not only undergo rapid multiplication, but in the course of their growth and development give off chemical products which in certain conditions of the wound become absorbed into the system, and produce that form of blood-poisoning known as septic intoxication. Large wounds are the more dangerous because of the large supply of this septic product they yield, and the greater extent of absorbing surface they present. The period of greatest danger is during the first six or seven days, or until the wound becomes covered with granulations, which form a barrier against the absorption of the poisonous bacterial products.

In addition to the poison they form in the wound, some microorganisms are themselves capable of entering the blood and tissues of the body. Here they grow and multiply, and add to the store of poison entering from without, and by their presence produce that condition of the body known as septicemia.

When the invading organisms enter the blood in large numbers, they may provoke it to coagulate around them, and the small clots so formed, by plugging the capillary vessels, form scattered centres of inflammatory action and pus formation in different organs and parts of the body, constituting another variety of blood-poisoning - pyemia.

The more seriously the tissues are damaged at the time when the wound is inflicted, the more vigorous is the growth and action of the invading organisms. The diminished vitality of the one having reduced their powers of resistance, renders them more amenable to the action of the other. It is for this reason that torn and contused wounds heal so much more slowly than others inflicted with a sharp instrument.

Although, as we have seen, the evil consequences resulting from bacteria and their products may be serious, and even fatal, their occurrence in the horse is, comparatively speaking, rare. Every day horses with deep, gaping wounds may be seen, having no protection of any kind, and little or no medical care, yet they pass through their trouble with little or no more discomfort or suffering than is occasioned by the prolonged period of healing resulting from the local inflammation which the micro-organisms and their products excite.

Still, notwithstanding the natural resistance which wounded flesh offers to the entrance of bacteria and their products into the system, it happens from time to time that the one proves too feeble to exclude the other, hence it has been found necessary in the treatment of wounds to resort to special methods by which to destroy and exclude from them all offending organisms. This mode of treatment, first devised and practised by Lord Lister, is known as the antiseptic system. It is based on the fact that such bacteria as prove inimical to wounds, and endanger life by poisoning the blood, are capable of being destroyed or inhibited in their action by the application of various dressings composed of certain chemical substances in the form of powder or solution. The more common agents employed for this purpose are carbolic acid, perchloride of mercury (corrosive sublimate), iodoform, boric acid, etc. See pp. 420, 421.

Healing Under A Scab

This is nature's method of repairing wounded parts, and in suitable circumstances the best that can be devised. It is well illustrated when, after the surface of the skin has been broken, the blood and other exuded matters are allowed to dry upon it, and form, as they will, a complete defensive covering or scab. In this condition all foreign substances which would irritate and inflame the wound are excluded from it, and so long as this state continues, healing proceeds rapidly without interruption. Anything, however, that inflames and provokes the formation of pus (matter) beneath the scab, interferes with the process and delays reparation.