TUBERCULOSIS. Tuberculosis is a disease of man and animals caused by the tubercle bacillus, which belongs to the "acid-fast" group of bacteria, some of which are pathogenic and others quite harmless. The most important pathogenic varieties are the "human," the "bovine" and the "avian." Whether these are to be regarded as distinct organisms or merely as mutations is not yet settled but they are easily distinguished by their selective effects on test animals.
Fundamentally, tuberculosis consists in an inflammatory reac tion of any particular tissue to the invading bacilli, and since tubercle bacilli are relatively little virulent, this tissue reaction is subacute in character. It consists in the formation round the bacilli of a microscopic agglomeration of cells constituting the so-called "tubercle." In a typical tubercle there is usually a very large or "giant" multi-nucleated cell in the centre, surrounded by smaller epithelioid cells, and outside these again a zone of lympho cytes. Blood vessels are completely absent, and the bacilli may be contained within the giant cell or scattered among the cells. Though in the earliest stage, the tubercle is microscopic, when several are formed close together they become visible to the naked eye, and constitute the condition known as miliary tubercle from their supposed resemblance to millet seeds. Almost as soon as formed the tubercle undergoes central degeneration (casea tion), believed to be directly caused by a toxin produced by the bacilli, aided by the avascular character of the tubercle itself. The further progress of the disease varies greatly according to the resisting power of the individual and the presence or absence of secondary, usually pyogenic, infection of the tuberculous focus. If resistance be low, and especially if pyogenic micrococci gain access, the focus becomes merely a specialized variety of abscess and behaves like other abscesses, even, at times, to the extent of invading the blood stream and leading to a generalized tubercu losis. If, on the contrary, resistance be high the inflammatory reaction terminates in a reparative formation of fibrous (scar) tissue, often with local deposition of calcium salts. Between these extremes are innumerable varieties.
Tuberculosis, when it has reached the stage at which it is clinically recognizable, may be regarded as the end-result of a slow progressive and long unrecognized bacterial invasion. It
should never be forgotten that, in the majority of infected per sons, this clinically recognizable end-stage is never reached; in other words, only a relatively small proportion of the number of infected persons actually becomes clinically tuberculous.
With human tuberculosis, it is where men are herded together in the centres of industry that the infection becomes so common as to be almost universal. That this statement is true is proved by the post-mortem findings of reliable pathologists in Europe and America who have investigated long series of cadavers of persons dying from all diseases. In the words of M. Fishberg "no matter what the cause of death may have been, whether the persons knew they had tuberculosis or not, between 5o and i00% of people living in large cities show active, quiescent or healed tuberculous lesions in some organ of their bodies." These findings bring into sharp relief the importance of the dis tinction between infection and disease. The results of tuberculin tests applied to healthy persons (Pollak, Vienna, 19ii ; Mantoux, Paris, 1909 and i 91o, Fishberg, New York, 1914; Calmette, Lille, 1911 and others) show that, by the age of 15 years, nearly every individual living under urban conditions has already become infected with the tubercle bacillus.