REPAIR OF WOUNDS.
The process of repair is essentially the same, being modified only by the needs of the wound and the tissues involved, and by their environment. Whether soft tissues or bones are being repaired the differences are apparent rather than real, as bony tissue is temporarily decalcified, and then, as soon as the process permits, is once more stiffened by deposition of calcium salts.
The process of repair should be begun immediately after the cessation of the disturbance which has produced the wound, and as soon as the bleeding is checked. It may be materially influenced and retarded by the presence of bacteria or other foreign bodies, but its character remains unchanged. Healing has been described as occurring by primary union, or by “the first intention,” and by granulation, or the “second intention.”
Wounds which have been permitted to remain clean, with their edges brought together so that the surfaces are in contact, are healed with a minimum of waste of reparative material, the process being as follows: The small vessels are occluded with thrombi up to the first collateral branches; the leukocytes begin to penetrate the film of blood, which, having coagulated, serves as a cement to help hold the surfaces together. By their proliferation and more complete organization the gap between the surfaces is bridged with both fibrous and capillary bloodvessels, and within sixty or seventy hours the clot has become largely replaced by organized cells. Meantime from the endothelial cells of the vessels and vascular spaces, as well as from the fixed cells of the connective tissue, the so-called fibroblasts are formed, which are later converted into connective tissue. Many of the cells which have wandered to the scene of activity, or have been there reproduced in unnecessary numbers, disappear again, either into the circulation or they serve as food for the fibroblasts. Branching cells attach themselves more intimately, and thus the original clot is completely converted into fibrous and connective tissue, and this becomes a scar, which extends as deeply as did the original injury. New capillaries are rapidly formed by a budding process, and supply the pabulum required for nourishment of the new cells. By fusion or amalgamation of neighboring vascular buds complete new vessels are formed, extending through the new tissue from one side to the other, while around them the fibroblasts or connective-tissue elements arrange themselves. From this it will appear that the coagulum which forms within a wound is desirable as a scaffolding upon which the process of repair may be begun. But it is desirable that this coagulum should be small in amount, in order that these processes may not be too long delayed; hence the advisability of removing all clots within a wound when closing it, and preventing the formation or leaving of dead spaces in the tissues in which blood clots may collect.
The process of granulation is not dissimilar to that described above, save only in its gross appearances. Granulations consist of vascular buds surrounded by leukocytes and lightly covered by them, while around the base of each bud epithelioid and spindle cells arrange themselves, these fixed cells organizing themselves more and more, as the wound fills up, with the more superficial layers of granulations. In time they are converted into a dense fibrous tissue which forms later what is known as the scar. As before, also, the spaces between the young capillary loops are filled with large nucleated cells derived from the fixed cells of the tissue, and from the endothelial lining of the newly formed vessels. Thus fibroblasts are produced in each case, and are often more or less mingled with giant cells, especially if some foreign body, such as a silk ligature, be embodied in the tissues. The particular function of the leukocytes seems to be the removal of red corpuscles and fibrin from the original clot.
The granulation tissue thus constituted by capillary loops and proliferating cells constitutes the basis of all wound repair. Later this tissue assumes more of the fibrous and less of the cellular character, while the fibroblasts arrange themselves in accordance with the mechanical requirements of the tissues and the stress or strain placed upon them. This tissue is at first vascular, but as it condenses its capillaries become less numerous and smaller, and the final white fibrous scar is usually almost bloodless.
When there has been loss of skin, or when skin edges are not brought together, the deeper process of granulation needs an epithelial covering, which cannot be afforded by mesoblastic or endothelial cells. The formation of an epithelial or epidermal covering is a process peculiar to epithelial tissue alone, and takes place mainly from the cells of the rete Malpighii.
Epithelial elements of the skin will afford a large amount of covering, and yet even their activity sometimes is insufficient and has to be atoned for by skin grafting. Should the granulating surface be small, and so situated that the fluid upon its surface may dry by evaporation, there will result a crust or scab, which, while it conceals from observation what is going on beneath, serves as an admirable protection, beneath which proliferation of epithelium takes place. A spontaneous detachment of the scab may take place when this process is complete, and with the loosening of the crust it is apparent that repair has become complete. This is known as healing under a scab or under a crust.
Two clean and healthy granulating surfaces may be so placed in contact with each other as to blend together by exactly the same process as that by which granulations are first formed. This is called secondary adhesion, or by the older writers the “third intention.” Advantage is taken of this possibility in the application of what are called secondary sutures, which may be placed some days before they are utilized, with the intent to bring together surfaces so soon as they shall present granulations.
One of the most interesting of all healing processes is that by which severed tissues, when promptly replaced, often reëstablish vascular communication and grow again in a satisfactory manner. Thus a severed ear, nose, or finger-tip may be replaced, and, if carefully held in situ, the parts being kept at rest, will prevent disfigurement and the loss of important tissues. In these cases the severed tissue remains passive several days until it has become vascularized. Meantime its nutrition seems to be maintained through the medium of the living tissues to which it has been affixed, probably by absorption of their blood plasma.