In simple cells like the leucocytes, reproduction may take place by simple fission, thus: a constriction appears in the nucleus and in the body of the cell in the same line, and the two divide without any visible protoplasmic changes. Such a simple mode of division does not occur in the more highly specialized cells of various tissues. If the karyokinetic action be not very vigorous, the nucleus may divide, but the cell body remains intact, producing the cell with two or more nuclei so commonly observed. Every cell reproduces its kind, spindle cells producing connective tissue; epithelial cells epithelium; and bone cells producing bone.
Repair of Wounds and Healing by Apposition. When a wound occurs, the cut edges immediately retract on account of the elasticity of the tissues, and the gap fills with blood and serum. If no bacterial or chemical irritant is introduced, there are no true inflammatory changes. The divided blood vessels are soon plugged with coagulated blood, which extends into the cut vessels to the nearest branch. The capillaries around the seat of injury dilate slightly, the fixed cells of the tissues become active, dividing by karyokinesis as already described. The endothelial cells lining the divided blood vessels multiply and take an active part in the process. In spite of the congestion and the new cells produced, the reaction is much less than that of inflammation. The new cells invade the blood clot, consuming it and also any foreign matter, or any tissue which may have been killed by the injury. From the loops of the occluded capillaries, at the sides of the wound, spring buds of endothelial cells, becoming thicker and then hollow as they extend, blood cells forming in them and blood entering them also from behind. These advancing endothelial tubes join with those on the opposite side of the wound, and thus the new forming tissues are supplied with blood vessels.
It is said that new vessels are also formed by the pre-existing lymph-spaces and by independent cells. Meantime the connective tissue cells have been forming fibres across the clot and epithelial cells over its surface, if skin or mucous membrane be involved in the injury. The new vessels disappear, and the new connective tissue forms the scar. This is the process of primary union in a wound in which there is not a marked cavity or a loss of tissue on any of the exposed surfaces of the body, and no matter how closely the edges of such a wound may lie in contact, it can heal by no other method. Even the closest apposition of the sides of a wound cannot prevent the interposition of a thin layer of clot and the partial death and absorption of a very thin layer on its surfaces. This is also known as primary union.
Healing by Granulation. When a wide gap has been produced by retraction or by actual loss of tissue, healing takes place by granulation, as it is called, a process which differs from that just described merely in the fact that more tissue must be reproduced. The outpouring of blood and serum, occlusion of the vessels, congestion, multiplication of fixed cells, emigration of leucocytes, and production of vascular loops and buds, goes on as before. As the formative changes advance, small, round elevations of a rosy color appear on the new surface, making it look like velvet. These rounded elevations of the healing surface are called granulations.
They advance steadily on all sides, filling the gaping wound until the level of the original surface is reached, the new tissue organizing behind them, and contracting as it organizes, so that the space to be filled is daily made smaller by this contraction as well as by the production of new tissue. As the surface is reached, the epithelial cells on the edges of the granulating area slowly spread over it, the granulations generally projecting above the adjoining surface and the epithelium growing over them as they contract again to their proper level. The advancing line of epidermis is visible as a pink line, gradually whitening with time.
CHAPTER V
WOUNDS AND CONTUSIONS
A wound is a solution of continuity or division of the soft tissues produced by cutting, tearing, or compressing force. The classification of wounds according to their causation or nature is as follows: