Focal Necrosis is a condition in which minute areas of necrosis scarcely visible to the naked eye occur, particularly in the lymph-follicles and the liver in various forms of severe infection. They may be due to minute thrombi or to alterations in the endothelium of the capillaries.

Gangrene may be of two forms - dry and moist. The tissues involved are those that are exposed either directly or indirectly to the atmosphere.

Dry gangrene, or mummification, is the death of tissues with subsequent drying. It occurs particularly in the extremities of old people or of those who are much debilitated. Is generally due to some obstruction of slow formation of the arterial system, by a thrombus, an embolus, by disease of the walls, by a spasmodic contraction of the vessel, or by pressure from the outside. It is usually circumscribed, there is very little odor, the tissues become almost black and mummify through evaporation of the moisture.

Moist gangrene is the death of living tissues plus an infection by bacteria that are capable of producing putrefaction.

It occurs in those parts that are exposed to the air, either directly or indirectly.

It takes place in people who have previously been in good physical condition, usually being the result of extensive venous obstruction combined with a weak arterial supply.

The part involved undergoes necrosis and afterward becomes infected. It becomes greenish black, gas blebs appear on the skin or in the tissues, and an extremely offensive odor develops.

The cells break down completely, hemorrhage takes place as a result of destruction of the blood-vessels, and many toxic substances are formed. They resemble the alkaloids and may bring about marked disturbances of the organism. This form of gangrene may terminate in several ways.

The dead tissue, sphacelus or slough, gives rise to a zone of inflammation, which is known as the line of demarcation, at the point of contact with the healthy tissue. This zone, as a rule, indicates the limits of the gangrenous process. At this site there is a constantly increasing interval between the dead and living tissue. The tissues here break down and form the line of ulceration. It is an attempt of nature to throw off the foreign substance and at the same time to form new tissue. The process is known as exfoliation. If the necrotic tissue cannot be thrown off, as is the case when bone is involved, there will probably be a sequestrum formed. This is the result of new bone forming around the dead tissue before there has been time for it to exfoliate.

If the degenerated area cannot be discharged, as when the internal organs are involved, it frequently becomes surrounded by a capsule of connective tissue that protects the neighboring parts - process of encapsulation. Again, the necrotic tissue may disappear through absorption, may calcify, or undergo cicatrization or organization.

Fig. 14. - Senile Dry Gangrene of the Lower Extremity, Showing Line of Demarcation (Hektoen).

Fat necrosis is a peculiar type occurring usually in the fat within the abdominal cavity. In nearly all cases it seems to be dependent upon some disease of the pancreas, particularly hemorrhagic pancreatitis.

It is the result of the splitting of the fat molecule into its fatty acid and into glycerin. The fatty acids are deposited as crystals and unite with calcium to form salts.

These areas are generally about the size of a pea, whitish in color, soft or gritty. A zone of inflammation may or may not surround them.

Death is the cessation of life - meaning that all the component parts of the organism cease to live.

Up to a certain time the cells of the body are able to supply all the needs, but eventually the natural term of life is reached and the cells gradually fail to support the tissues. Such a condition would be termed physiologic death. If, however, it follows as a result of diseased processes, it would be pathologic.

The two, however, cannot be strictly separated, as in old age there are always conditions present that are not normal.

The conditions absolutely necessary for life are a continuation of circulation, respiration, and innervation.

There may be a destruction of certain portions of the body without death following, but a cessation of any of the above-mentioned functions brings about dissolution. This is known as somatic death, and, according to which function ceased, it is said to have taken place by syncope, asphyxia, or coma.