It may be said that the whole of the modern activity in regard to infection has its foundation on septic infection. The terms sepsis and septic (III Septic Infection 137 = I putrefy) designate the contamination of the animal body with the products and agents of putrefaction. It is, of course, with microbes that we have here to do, and more especially those which, contaminating wounds, are liable to produce suppurative inflammation. The specific forms of these microbes will be considered under Bacteriology; here more the processes which result in the tissues will be reviewed.

The pyogenic micrococci are the principal forms of septic infective agents, but to them must be added the bacillus coli communis, a microbe which seems capable of assuming various forms, and of presenting very varying degrees of virulence. Its exact pathological position is by no means fully defined, and the determination of this position is rendered difficult by the fact that it multiplies with extraordinary rapidity after death of the individual, and, penetrating from the alimentary canal, may be found in distant organs, such as the liver and heart, within a few hours of death.

Septic infection is induced in various ways. The microbes are present in the air, in water, and on the cutaneous and mucous surfaces of the body. These surfaces are furnished with layers of cells which prevent the intrusion of infective agents, but in the case of wounds, they are ever ready to find access, and they may without wounds be carried into mucous canals or cavities in which they are not normally present, and which have no sufficient provision for their extrusion. This is the case with the urinary bladder for example. A breach of sin face is usually necessary for septic infection, but not always.

A recent wound forms the most favourable opportunity for septic infection. The interstitial spaces of the body are laid open, in the recesses of which the microbes may lodge, and there is no such blocking of the passage with new-formed cells as ultimately occurs. The larger and more intricate the spaces to which the infective agents find access, the more considerable and ineradicable is usually the infection. Hence a compound fracture in which the tissues are usually torn in a complicated fashion, if it become septic, is commonly one of the most serious of conditions.

The most direct effect of the septic invasion is acute inflammation, in which the emigration of leucocytes from the blood plays a prominent part. Hence suppuration is a feature of most septic infections, and may indeed be almost regarded as the principal effect and criterion of such infection. Besides the inflammation, which is of greater or less intensity, there is, in many cases, a necrosis of tissue, which takes the form of a gradual solution without the production of considerable sloughs. Taking the contamination of a wound, such as a compound fracture as an example, it may be said that the complicated cavity produced, being planted with the septic microbes, becomes the seat of their rapid multiplication. This implies an abundant production of their special toxines, which again have their local and general effects. The local effect is an acute inflammation, suppurative in character. Under the influence of pressure or gravitation the pus, containing the septic agents, is liable to filter into the spaces in the loose connective tissue which forms the sheaths of muscles and the subcutaneous tissue. The suppuration extends to these spaces, and, as the dividing septa undergo solution, larger spaces or cavities are formed. It is thus that Purulent Infiltration may lead the way to the formation of Abscess. In other situations abscess may occur, as in the midst of an organ (brain, liver, kidney), by a gradual necrosis and accumulation of pus, the septic agent having been implanted and undergoing multiplication in situ. It is consistent with what has been already noted in regard to inflammation that the tissues around the inflamed centre show active new-formation, chiefly by division of the fixed cells of the connective tissue. Hence a layer of granulation tissue mostly forms around the abscess. This layer of cells was, under a mistaken idea, called the pyogenic membrane, as if it were the means of secreting pus. No doubt leucocytes exude from its vessels as from those of the tissue around to which the toxine extends with sufficient concentration, but the cellular layer is really a barrier to the extension of the process, controlling the penetration of the microbes, and shutting off communicating passages in the connective tissue around. An abscess which has acquired a complete cellular membrane is thus more under control than one in which purulent infiltration is still possible.

Septic infection of serous cavities or of surfaces other than mucous has a somewhat different course to that described above. The septic agent may reach the peritoneal cavity directly from the cutaneous surface, the intestine, the genital canal in females, or the urinary organs. The pericardium may, by circuitous routes, become contaminated. The meninges may directly through the skull or by indirect routes be infected. The result is acute inflammation, going on to suppuration, but without necrosis of tissue. It is seldom that abscess formation occurs in the loose connective tissue under the serous membranes, unless as a part of the original infection to which that of the serous cavity is itself secondary. In the serous cavity there is for the most part a fibrinous exudation which goes on to suppuration. The toxine which has the power of causing solution of the connective tissue has a similar power on the fibrine, and a solution of it occurs, in some cases very rapidly, so that the more solid exudation gives place to fluid pus.

In a local septic infection, however, the blood-vessels may be involved, more particularly the thin-walled veins. The infection may extend through the wall, or it may find entrance through the severed ends of these vessels in cases of wounds or injuries. The •consequence of such an extension is an inflammation of the wall of the vein, which is accompanied by coagulation of the blood, already stagnant, as a rule, in the vessel. Hence the term Thrombo-phlebitis, so familiar in older medical literature, means a septic and ultimately a, suppurative inflammation of a vein. This condition is not now common, except in connection with the veins of the skull and meninges, arising in these regions chiefly from septic infection of the complicated structures of the middle ear. The veins of the leg, in cases of septic infection in connection with compound fracture, used to be sometimes found full of pus, and with the walls of the vessel infiltrated with pus. It was to such a condition as this that the term Pyaemia was given under the idea that the presence of pus in the blood-vessels was the serious and essential element. The admission of septic microbes into the veins frequently gives rise to their transportation to other parts, and the septic infection may become distributed, not in the form of a general infection of the blood, but in that of multiple infections at a distance from the primary seat. This extension occurs by a process of embolism, and the embolic obstruction does not arise merely from the microbes, in which case it would be capillary, but the softened thrombi from the veins are the obstructing agents, and as they carry with them the septic agents, they become the vehicles of infection. As the infection of the vein is the primary condition, it is the lungs which are most frequently the seat of such infective emboli, although the infective particles may extend through the lungs and be distributed by the systemic arteries. In this case the organs most frequently affected are the kidneys, but we may have also the liver, heart, intestine and other organs involved.

It will be seen that pyaemia is a septic infection in which dissemination has occurred by embolism. The result is, in every place where the septic material is implanted, the occurrenoe of an acute inflammation with necrosis. Fig. 122 gives a general view of such a lesion in the kidney. It is an abscess in miniature, the central parts are necrosed, as evidenced by the entire absence of nuclear staining, whilst around this is a great congregation of leucocytes. It is to such lesions at a distance from the original seat that the name Metastatic Abscesses is given.

Pywmic abscess of kidney: a, necrosed central part; b, arta of inflammation.

Fig. 122. - Pywmic abscess of kidney: a, necrosed central part; b, arta of inflammation, the tissue packed with pus corpuscles; c, capillaries filled with micrococci; d, Malpighian glomeruli; e, uriniferous tubules, x 10.

It may be added that septic microbes may apparently reach the blood without the intermediation of a wound of any kind. Probably these fine particles in minute numbers obtrude themselves into the vessels in the lungs and alimentary canal, but are readily disposed of as a general rule. They may, however, from exceptional local circumstances or from some special virulence in themselves, be planted in the internal structures of the body. This takes place most frequently in the bones (see Acute Infective Ostitis) and on the endocardium (see Malignant Endocarditis). In the latter case the situation is such that metastatic abscesses in the systemic circulation are peculiarly liable to occur.

Septic infection is commonly associated with extension of the toxines into the blood with the result of Septic Intoxication or septicaemia. The principal feature of this condition is fever. The occurrence of Rigor followed by febrile rise of temperature is often the signal of the establishment of a septic infection or of its further extension. The fever is not generally homogeneously continuous, but has the irregular character of hectic fever. It cannot be said to what extent this septic intoxication may be a complication of conditions, not primarily septic, such as scarlet fever, diphtheria, tuberculosis.

The septic toxine may also lead to organic change at the seat of its excretion. It seems probable that the somewhat frequent inflammation of the kidneys in tuberculosis and certain forms of inflammation'of the kidneys in scarlet fever are due to this cause.