We have left the injured area with an increased amount of fluid and cells within it, with the blood vessels dilated and with both cells and fluid streaming through their walls, and the cells belonging to the area actively repairing damages and multiplying.  The process will continue as long as the cause which produces the injury continues to act, and will gradually cease with the discontinuance of this action, and this may be brought about in various ways.  A foreign body may be mechanically removed, as when a thorn is plucked out; or bacteria may be destroyed by the leucocytes; or a poison, such as the sting of an insect, may be diluted by the exudate until it be no longer injurious, or it may be neutralized.  Even without the removal of the cause the power of adaptation will enable the life of the affected part to go on, less perfectly perhaps, in the new environment.  The excess of fluid is removed by the outflow exceeding the inflow, or it may pass to some one of the surfaces of the body, or in other cases an incision favors its escape.  The excess of cells is in part removed with the fluid, in part they disappear by undergoing solution and in part they are devoured by other cells.  With the diminishing cell activity the blood vessels resume their usual calibre, and when the newly formed vessels become redundant they disappear by undergoing atrophy in the same way as other tissues which have become useless.

When these changes take place rapidly the inflammation is said to be acute, and chronic when they take place slowly.  Chronic inflammation is more complex than is the acute, and there is more variation in the single conditions.  The chronicity may be due to a number of conditions, as the persistence of a cause, or to incompleteness of repair which renders the part once affected more vulnerable, to such a degree even that the ordinary conditions to which it is subjected become injurious.  A chronic inflammation may be little more than an almost continuous series of acute inflammations, with repair continuously less perfect.  Chronic imflammations are a prerogative of the old as compared with the young, of the weak rather than the strong.

FOOTNOTES:  [1] The term exudation is used to designate the passing of cells and fluid from the vessels in inflammation; the material is the exudate.

[2] By transudation is meant the constant interchange between the blood and the tissue fluid.

CHAPTER V

INFECTIOUS DISEASES.—­THE HISTORICAL IMPORTANCE OF EPIDEMICS OF DISEASE.—­THE LOSSES IN BATTLE CONTRASTED WITH THE LOSSES IN ARMIES PRODUCED BY—­INFECTIOUS DISEASES.—­THE DEVELOPMENT OF KNOWLEDGE OF EPIDEMICS.—­THE VIEWS OF HIPPOCRATES AND ARISTOTLE.—­SPORADIC AND EPIDEMIC DISEASES.—­THE THEORY OF THE EPIDEMIC CONSTITUTION.—­THEORY THAT THE CONTAGIOUS MATERIAL IS LIVING.—­THE DISCOVERY OF BACTERIA BY LOEWENHOECK IN 1675.—­THE RELATION OF CONTAGION TO THE THEORY OF SPONTANEOUS GENERATION.—­NEEDHAM AND SPALLANZANI.—­THE DISCOVERY OF THE COMPOUND MICROSCOPE IN 1605.—­THE PROOF THAT A LIVING ORGANISM IS THE CAUSE OF A DISEASE.—­ANTHRAX.—­THE DISCOVERY OF THE ANTHRAX BACILLUS IN 1851.—­THE CULTIVATION OF THE BACILLUS BY KOCH.—­THE MODE OF INFECTION.—­THE WORK OF PASTEUR ON ANTHRAX.—­THE IMPORTANCE OF THE DISEASE.