21. _As a rule we can regard as points of primary infection polynuclear leucocytes in the blood and lymphatic receptive apparatus; next in order are the muscular elements in the walls of the smallest blood-vessels._ Endothelium and epithelium may become carriers of Tb. through the action of polynuclear leucocytes which have wandered into these tissues. 22. In studying the results of an infection with tubercular virus, aside from the degree of virulence, from the dosage, the single or repeated Tb. inoculation, and the primary point of attack (regional and cellular), we have particularly to consider the age at which the primary infection occurs. I distinguish four periods: (_a_) Infantile period of infection; (_b_) Puerile ” ” ” (_c_) Virile ” ” ” (_d_) Senile ” ” ” 23. It seems to me that in the epidemiological origin of pulmonary consumption the infantile Tb. infection, followed by latent or manifest scrofula in the puerile period, is of great significance, so that we can formulate the following doctrine: “_An infantile tubercular infection predisposes to tubercular pulmonary consumption_.” Under “scrofula” I here include the alteration in the muscle of the blood-vessels, caused by the Tb. infection, which finds its expression in the increased sensitiveness to tuberculin, and which in general is equivalent to “scrofulous diathesis” of the older authors. 24. The primary infection from the mouth or nose with tubercular virus derived from food or even from inhalation, in the small quantities that under ordinary conditions of life are concerned, is followed after the bacilli have entered the circulation, by alterations in the walls of the smallest vessels. These manifest themselves as follows: (_a_) Microscopically, by a loosening of the vessel wall, between whose elements, shortly after the infection, tubercle bacilli can be found. These bacilli, brought here by the wandering cells, are set free on the destruction of the cells. (_b_) In primary functional disturbances which can be recognized by the temperature curve and the heart action. (_c_) In secondary tuberculin hypersensitiveness. 25. Following mild infections the alterations in the vessel walls may retrogress with a suppression [Beseitigung] of the tubercle bacilli. Without exception, however, the hypersensitiveness to tuberculin remains, varying in degree and length of time according to the virulence of the infecting Tb. and to their more or less locally limited action on the vascular system.