The changes which the dead tissues undergo vary considerably according to circumstances, and the resulting appearances are so different that special names are given and forms described according to the appearances presented. From what has gone before, it will appear that inflammation frequently goes along with necrosis; the two are sometimes produced simultaneously by the same cause, or the dead structures may in themselves, or by the products evolved by them, produce inflammation.

The circumstances which determine the form which the necrosis will assume are chiefly these - the position of the dead structure, whether internal or external, whether protected from the access of microbes or not; the presence or absence of acute inflammation; the bulk of the dead piece; its structure and chemical constitution. The most important circumstance is whether putrefactive changes occur or not. Dead pieces of tissue, like all dead animal matter, are liable to decomposition under the influence of microbes. In most cases of necrosis in external parts and in the lungs, the air finding access carries with it the microbes concerned in these processes, and putrefaction is the result. In this case the term gangrene is usually-applied.

In most cases of necrosis the process is characterized by the disappearance of the nuclei of the cells. Figs. 37 and 38 show this in the case of necrosis from embolism in the kidney. Fig. 37, is from the tissue outside the necrosed area, and it is seen that the nuclei of both the connective tissue and epithelium are preserved. Fig. 38 on the other hand, is from the infarction near its outer limits; here the nuclei have disappeared from the epithelium, but are preserved in the connective tissue.

Fig. 37. - Section of normal part of kidney. The nuclei of the epithelium of the tubules brought out by staining.

In Fig. 39 there is also disappearance of the nuclei of the renal epithelium in consequence of the presence of microbes in a capillary blood-vessel. This is probably from necrosis as a result of the action of the microbes, but as putrefaction also leads to disappearance of the nuclei, it may be an effect of the post-mortem growth of microbes, which however must have been planted during life.

A. Dry Gangrene Or Mummification

These terms are applied in the case of external parts, when the circumstances are such that there is little moisture in the parts. Hence it will not occur where acute inflammation precedes or accompanies the process. It is mostly met with in cases of embolism or other obstruction of the arteries, and is characteristic of senile gangrene and of Raynaud's disease. The part undergoes a gradual deepening of colour; at first merely livid, it passes into purple, deep blue, and even black. This is due to the fact that the blood-pigment is dissolved out and stains the tissues, which deepen in colour as the pigment becomes concentrated by the part drying.

Fig. 38. - Section of affected part of same kidney near border of infarction. The tubular epithelium necrosed, the nuclei invisible. The interstitial tissue with many nuclei.

The cuticle generally gets raised by the accumulation of a red fluid beneath it, and bullae are formed. If the cuticle separates, evaporation is accelerated. The part, which is usually the extremity of the lower limb, gradually shrinks, and is converted into a hard black mass, often with a mouldy smell. In dry gangrene there is putrid decomposition, but as there is a deficiency of fluid this occurs to a subordinate degree.

B. Moist Gangrene, Sphacelus, Sloughing

In the conditions designated by these names, putrid decomposition plays a prominent part, and the tissues are separated in a softened condition. Moist or ordinary gangrene is the condition where a considerable portion of the body has died and is undergoing separation. It is well seen in cases where a portion of the leg has died in consequence of injury. The parts which were at first hot, red, and painful, become mottled with brown, blue, and black, and the surface often presents blisters. The part becomes cold and darker except at the margin, where a dusky red line of demarcating inflammation appears. The tissues are universally stained with blood-pigment, the cuticle gives way, and putrescence advances, causing breaking down of the tissues, which are separated in all stages of softening, the more resistant tissues such as bone and cartilage retaining their form. The part exhales a strong odour, and its juices contain fluid fat, phosphates, extractives, as well as multitudes of microbes.

Fig. 39. - Capillary blood-vessel (6) in kidney filled with micrococci, from a case of pyaemia. There is a tubule on either side (a, a), the nuclei of which are visible except in the neighbourhood of the capillary x650.

Sloughing is a similar process where smaller parts of the soft tisssue are separated in a state of putrid decomposition.

C. Desiccation Without Putrescence

This is somewhat similar to dry gangrene, except that, occurring in internal parts, there is no decomposition. The part simply dries in and shrivels. The most striking example of this is afforded by extra-uterine pregnancy, in which the foetus after its death is retained in the abdomen of the mother. The foetus may remain for many years, simply drying in and getting encased in a capsule, which becomes impregnated with lime salts. Virchow in a case of this kind found muscle, connective tissue, and vessels still recognizable after twenty years. In twin pregnancies also, one of the foetuses may die at an early period, but, being retained, it is born along with the other in the form of a dry flattened object.

D. Softening Without Putrescence, Colliquefaction

This is almost peculiar to necrosis of the nervous system in consequence of obstruction of arteries. The process is accompanied by fatty degeneration.

E. Coagulation-Necrosis

This term was introduced by Cohnheim, and the subject has been elaborated by Weigert. When necrosis occurs in internal parts which are rich in cells, then the tissue frequently becomes converted into a solid, firmly compacted mass. Weigert compares the process to the coagulation of the blood, and asserts that the cells in dying enter into combination with the fibrinogen contained in the fluid which permeates the tissue. In order to this process the tissue must be a very cellular one, and an abundant supply of fluid must be present. The embolic infarction in the spleen and kidney forms the most typical example. The infarction forms a stiff, firm wedge which may be pale or may contain blood. The process is characterized by the disappearance of the nuclei from the affected structures.

A peculiar change which occurs in voluntary muscle is also regarded by Weigert and others as a form of coagulation-necrosis. This is the process described by Zenker as waxy degeneration of muscle, which is also designated colloid and hyaline degeneration. It is brought about by direct injury to the living muscle. It also occurs in certain febrile states where the temperature runs high, especially in typhoid fever and phthisis pulmonalis, and is sometimes seen in paralyzed muscles. It may be produced artificially in a living animal by freezing the muscle, or immediately after death, by injuring the muscle before the occurrence of post-mortem rigidity. The change is met with, in the human subject, chiefly in the diaphragm, the rectus abdominis, and the adductors of the arms. It consists in a coagulation of the contractile substance, the fibre being converted into a homogeneous translucent cylinder. The affected fibres are brittle, and the cylinders get broken up, often presenting transverse cleavages, or giving origin to oval clumps, as in Fig. 40.

Fig. 40. - Coagulation-necrosis in muscle. The sarcolemma surrounds the coagu lated clump, drawing together at the ends, x 350.

In atheroma of arteries there are often translucent structureless pieces in the walls, which Weigert also claims as examples of coagulation-necrosis. Similarly some tube-casts in the kidney are supposed to originate by necrosis and coagulation of the epithelium.

Hyaline degeneration, as used by Recklinghausen, to a large extent covers the same ground as the coagulation-necrosis of Cohnheim and Weigert. (See further on).

F. Caseous Necrosis

This is a condition closely allied to coagulation-necrosis. In the latter the necrosis occurs for the most part suddenly in structures hitherto unaltered, whereas caseous necrosis is always part of another pathological process. It is highly characteristic of tuberculosis, so much so that the term is nearly equivalent to the older designation tuberculization, and caseous matter is virtually the same as crude or yellow tubercle. It is also frequent in syphilitic new-formations. In all cases there is a great abundance of cells, which are not the normal cells, but are produced by exudation or new-formation. The necrosed tissue undergoes a process of condensation or coagulation, accompanied by fatty degeneration, the result being a somewhat dense brittle matter, which has been compared to cheese.

In its microscopic details the process involves a complete obscuration of structure. The nuclei disappear as in coagulation-necrosis, and the presence of abundant fine fat granules renders the structure very opaque and homogeneous. As the necrosis involves not only the new-formed cells, but the tissue in which they are seated, the result is a •disappearance of the details of the tissue, sometimes to such an extent as to render the structure difficult of identification.

G. Molecular Necrosis

This term is used chiefly in regard to lesions of a surface in which there is a gradual extension of necrosis, and the dead structures are removed in a finely disintegrated condition without solid sloughing. The result of this is the formation of an ulcer, and as there is a gradual extension of the necrosis there is usually a progressive Ulceration. The process is usually the result of infective agents. Thus tuberculosis of a mucous surface produces a progressive ulceration by necrosis of the tissue and gradual removal of the dead tissue. The necrosis here, as mentioned above, is caseous necrosis.