The tubercle possesses in many cases a special structure, and certain cell-forms frequently occur in it and give it a characteristic appearance. The central part of the tubercle usually contains _giant cells_ with numerous nuclei. The uninuclear cells are partly lymphoid, partly large epithelial or endothelial cells; these are called _epithelioid_ cells. It was not till 1865 that the specific nature of tuberculosis was asserted by Villemin. Burdon Sanderson (1868-69) in England confirmed his work, and it was extended by Connheim, who a few years later laid down the principle that all is tubercular which by transference to properly constituted animals is capable of inducing tuberculosis, and nothing is tubercular unless it has this capability. Klebs (1877) and Max Schiller (1880) described masses of living cells or micrococci in many tuberculous nodules in the diseased synovial membrane and in lupus skin. In 1881 Toussaint declared he had cultivated from the blood of tubercular animals and from tubercles an organism which was evidently a micrococcus, and in the same year Aufrecht stated that the centre of a tubercle contained small micrococci, diplococci, and some rods. But it was not till the following year, 1882, that Koch discovered and demonstrated beyond question the specific _Bacillus tuberculosis_. It is now held to be absolutely proved that the introduction of the bacillus, or its spores or products, is the one and only essential agent in the production of tuberculosis. Its recognised manifestations are as follows: Tuberculosis in the lungs = _acute or chronic phthisis_; " in the skin = _lupus_[1]; " in the mesenteric glands = _Tabes mesenterica_; " in the brain = _hydrocephalus_; " in lymphatic glands = _Scrofula_.[91] The disease may occur generally throughout the body or locally in the suprarenal capsules, prostate, intestine, larynx, membranes of the heart, bones, ovaries, pleura, kidneys, spleen, testicles, Fallopian tubes, uterus, etc. We may summarise the history of the pathology of tubercle thus: 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).