_The Relation of the Bacillus to the Disease._ All four of Koch's postulates have been fulfilled in the case of _Bacillus tuberculosis_. Hence we are dealing with the specific cause of the disease. Yet, whilst this is so, we may usefully ask ourselves: _How_ does the bacillus set up the changes in normal tissues which result in tubercular nodules? In arriving at a solution of this problem we are materially aided if we bear in mind the fact that such an organism in healthy tissues has a double effect. First, there is an ordinary inflammatory irritation, and secondly, there is a specific change set up by the toxins of the bacillus. Directly the invading bacilli find themselves in a favourable nidus they commence multiplication. In three or four days this acts as an irritant upon the surrounding connective-tissue cells, which proliferate, and become changed into large cells known as _epithelioid cells_. At the periphery of this collection of epithelioid cells we have a congested area. This change has been accomplished by the presence of the bacilli themselves. The production of their specific poisons changes the epithelioid cells in the centre of the nodule, some of which become fused together, whilst others expand and undergo division of nucleus. By this means we obtain a series of large multi-nucleated cells named _giant cells_. If the disease is very active, these soon caseate and break down in the centre. In a limb we get a discharge; in a lung we get an expectoration. Both discharge and expectoration arise from a breaking down of the new cell formation. Previously to breaking down we have in a fully developed nodule healthy tissue, inflammatory zone, epithelioid cells, giant cells, containing nuclei and bacilli. The sputum or the discharge will, during the acute stage of the disease at all events, contain countless numbers of the bacilli, which may thus be readily detected, and their presence used as evidence of the disease. It is obvious that if the centre of the nodule degenerates and comes away as discharge a cavity will be left behind. By degrees this small cavity may become a very large one, as is frequently the case in the lung, which particularly lends itself to such a condition. Hence, though at the outset a tubercular lung is solid, at the end it is hollow. [Illustration: BACILLUS TUBERCULOSIS (In sputum from a case of phthisis, "consumption" of the lungs) × 1000 _By permission of the Scientific Press, Limited_] [Illustration: BACILLUS TUBERCULOSIS (The bacilli are arranged within the giant cell) × 1000] [Illustration: STREPTOCOCCUS PYOGENES (From broth culture) × 1000 _By permission of the Scientific Press, Limited_] [Illustration: BACILLUS ANTHRACIS (From splenic blood of cow) × 1000 _By permission of the Scientific Press, Limited_]