1794. Baillie drew attention to grey miliary nodules occurring in tuberculosis, and called them "tubercles."
1834. Laennec described four varieties: miliary; crude; granular; encysted.
1843. Klencke produced tuberculosis by intravenous injection of tubercular giant cells.
1865. Villemin demonstrated infectivity of tubercular matter by inoculation of discharges; Connheim, Armanni, Burdon Sanderson, Wilson Fox, and others showed that nothing but tubercular matter could produce tuberculosis.
1877. Living cells were found in tubercles, "micrococci" (Klebs, Toussaint, Schiller).
1882. Koch isolated and described the specific bacillus, and obtained pure cultivations (1884).
The Bacillus of Koch, 1882. Delicate cylindrical rods, measuring 1.5–4 micromillimetres in length and about .2 µ in breadth; non-motile. Many are straight with rounded ends; others are slightly curved. They are usually solitary, but may occur in pairs, lying side by side or in small masses. They are chiefly found in fresh tubercles, more sparingly in older ones. Some lie within the giant cells; others lie outside; shorter in tissue sections of bovine tuberculosis, but longer in the milk (Crookshank).
When stained they appear to be composed of irregular cubical or spherical granules within a faintly stained sheath. In recent lesions the protoplasm appears more homogeneous, and takes on the segmented or beaded character only in old lesions, pus, or sputum.
Morphological differences are found under different circumstances, and within limits variation occurs according to the environment.