The examination of two hundred and twenty-five children of the average age of five years who had died of diphtheria showed tuberculous infection in one-fifth of the cases and the frequency of infection increases with age.

The defence on the part of the body is chiefly by the formation of dense masses of cicatricial tissue which walls off the affected area and in which the bacilli do not find favorable conditions for growth.

This mode of defence, which is probably combined with the production of substances antagonistic to the toxines produced by the bacilli, is so efficacious that in the great majority of cases no further extension of the process takes place.

In certain cases, however, the growth of the bacilli in the focus is unchecked, the tissue about them is killed and becomes converted into a soft semi-fluid material; further extension then takes place.

All parts of the enormous surface of the lungs are connected by means of the system of air tubes or bronchi, and the bacilli have favorable opportunity for distribution, which is facilitated by sudden movements of the air currents in the lung produced by coughing.