The exact period of giant-cell formation depends on the rapidity of the formative processes. Thus different conditions occur. Inside the giant cells the bacilli are arranged in relation to the nuclei in one of three ways: (_a_) polar, (_b_) zonal, or (_c_) mixed. The breaking down of the nodule is partly due to the cell-poisons, and partly because the nodule is non-vascular, owing to the fact that new capillaries cannot grow into the dense nodule, and the old ones are all occluded by the growth of the nodule. From the local foci of disease the tubercle process spreads chiefly by three channels: (_a_) _By the lymphatics_, affecting particularly the glands. Thus we get tuberculosis set up in the bronchial, tracheal, mediastinal, and mesenteric glands, and it is so frequently present as to be a characteristic of the disease. This is the common method of dissemination in the body. (_b_) _By the blood-vessels_, by means of which bacilli may be carried to distant organs. (_c_) _By continuity of tissues_, infective giant-cell systems encroaching upon neighbouring tissues, or discharge from lungs or bronchial glands obtaining entrance to the gullet and thus setting up intestinal disease also. It has been abundantly proved that the respiratory and digestive systems are principally affected by Koch's bacillus. Wherever the bacilli are arrested, they excite formation of granulations or miliary tubercular nodules, which increase and eventually coalesce. The lymphatic glands which collect the lymph from the affected region are the earliest affected, always the nearest first, and then the disease appears to be appreciably stopped on its invading march. Each lymphatic gland acts as a temporary barrier to progress until the disease has broken its structure down. It remains local, in spite of increase in number and importance of the foci of disease, as long as the bacilli have not gained access to the blood stream. _Toxins and Tuberculin._ Koch, Crookshank, and Herroun, Hunter, and others have isolated products from pure cultures of the tubercle bacillus. These have comprised chiefly _albumoses_, _alkaloids_, and various _extractives_. Koch's observations led him to suppose that in pure cultures of tubercle a substance appeared having healing action on tuberculosis, and an extract of this in glycerine he termed "tuberculin." It was made as follows: A veal broth containing peptone and glycerine was inoculated with a pure culture of the bacillus and incubated at 38° C. for six or eight weeks. An abundant growth with copious film formation appeared. The culture was then concentrated by evaporation over a water-bath until reduced to about one-tenth of its volume. [Illustration: FLASK USED IN THE PREPARATION OF TUBERCULIN]