Lymphocytes both large and small, as well as eosinophiles in small numbers, may also be present.

Eosinophile cells are actively ameboid and are able to escape from the blood-vessels. As a rule, they are not present in marked numbers except in certain subacute or chronic inflammations of the skin or mucous membranes.

The plasma cells probably originate from the connective tissue, but may be derived partly from the blood. They are rather large, and contain a pale, vesicular nucleus eccentrically placed and a finely granular basophilic protoplasm. These cells are usually most numerous in acute toxic conditions and are supposed to play some part in the formation of connective tissue.

The mast cells or basophilic leukocytes are large cells containing usually a trilobed vesicular nucleus and large granules in the cytoplasm. They are most common in inflammations of mucous membranes and in the neighborhood of tumors, especially if they have undergone mucoid changes.

The fibroblasts or epithelioid cells are formed by the proliferation of pre-existing connective-tissue cells.

Giant cells, those containing more than one nucleus, are frequently present. The formation of these cells probably takes place in one of two ways. If a single cell is not sufficiently powerful to remove the offending particle, several may coalesce, and in that way successfully make the attack. They may, however, form through a multiplication of the nuclei without division of the cytoplasm.

Fig. 25. - Granulation Tissue (Mallory).

a, Surface portion, composed chiefly of newly formed blood-vessels; very few fibroblasts; many polymorphonuclear leukocytes between the vessels and in the fibrin of the surface; blood-vessels and leukocytes separated by serum, b, Deeper portion; many lymphocytes between bloodvessels; young fibroblasts growing in horizontal arrangement at base.

In the process of repair there is formed what is called granulation tissue which acts as a strong barrier to absorption and infection. In it there is the formation of loops of new capillaries derived from the endothelial lining of pre-existing blood-vessels. The endothelial cell becomes larger, the nucleus divides by mitosis, and two cells are formed. These cells continue dividing until a sprout-like process extending into the surrounding tissue is formed. Adjoining sprouts unite, and, although at first solid, finally become hollowed out, thus allowing the circulation to be re-established. At the same time that this is taking place there is a multiplication of the fixed connective-tissue cells, which surround and act as a supporting framework to the loops of new-forming capillaries.

In the proliferation of the connective tissue there is first found a small round cell with a round or oval nucleus. As the tissue becomes older the cells tend to elongate and become spindle shaped. At first they are very close together, but gradually separate, and the homogeneous intercellular substance becomes fibrillar and supports the cells. Those cells concerned in the formation of the cicatrix are called fibroblasts.

In the new-formed tissue there is at first an overproduction of cells and blood-vessels, but eventually it becomes less vascular and cellular. This is brought about to a great extent by the contraction of the cicatrix, which, at first reddish and elevated, finally becomes pale and depressed.

According to surgeons, cicatrization may take place in one of two ways:

Union by first intention, or primary union. In this the edges of the wound are closely brought together and very little exudate escapes. In this narrow space the same processes take place as are seen in the formation of. granulation tissue, coagulation, fibrin formation, phagocytosis, and proliferation of capillaries and connective tissue, but to a much less extent. The epithelial surface is replaced by a proliferation of the neighboring epithelium.

Fig. 26. - Formation of New Blood-vessels, as Seen in the Tail of a Tadpole (Arnold).

Union By Second Intention

Union By Second Intention, secondary union, or union by granulation, takes place when the edges of the wound are far apart and there is a large amount of exudate present.

This process is the same as healing by first intention, except that in it there is supplied the material bridge over the gap.

If an epithelial surface is affected, the granulation tissue is gradually covered by proliferation of adjacent cells.

Regeneration

Regeneration, although commonly applied to the formation of cicatricial tissue, really refers to the power of individual tissues to reproduce their own kind.

Generally speaking, the more highly specialized the tissue, the less is its regenerative power. If such tissues are destroyed, they are generally replaced by fibrous tissue. A cell can give rise in regeneration only to a tissue that has the same blastodermic origin.

The fibrous connective tissue is probably the most active.

Epithelium Of The Surface

Epithelium Of The Surface variety is constantly and completely regenerating. Whether regeneration takes place in the more highly specialized epithelial organs, such as the kidney and liver, is rather improbable.

Muscular tissue is capable of regeneration to a slight degree, but the chief repair after injury to muscle takes place within the connective tissues surrounding the fibers.

Blood-vessels, as is seen in the formation of granulation tissue, are capable of marked multiplication. The new-formed vessels in regeneration are usually only temporary; existing only long enough for the tissue to receive its nutrition, then disappearing during the contraction of the cicatrix.

Bone, as is noticed in the repair of fractures, is able to undergo complete regeneration.

Cartilage is incapable of regeneration. In injuries it is replaced by fibrous connective tissue.

Nerve-cells of the highly specialized type, such as ganglion cells, cannot regenerate, but the neuroglia or nerve connective tissue can. The neuroglia differs from the ordinary fibrous tissue in that it is derived from the ectodermic layer of the blastoderm.

Fig. 27. - Heart. Organizing Pericarditis. Fibrin still Present Along Surface and Beneath it (Mallory).