1. The production of lobar pneumonia in rabbits is dependent upon the introduction of organisms into the alveoli themselves.
2. In order to accomplish this the catheter through which they are injected must be inserted as deeply into a bronchus as possible and the culture fluid injected with considerable force.
3. Large numbers of organisms injected into the trachea just beyond the larynx set up no great changes in the lungs, even though the injection be forcible. This fact suggests the presence of a protective mechanism in the upper air passages, which, under normal conditions, prevents the penetration of organisms into the lungs.
4. If animals be subjected to cold, alcohol, and the inhalation of irritating gases, the so called secondary factors in the etiology of lobar pneumonia in man, then the injection of pneumococci into the trachea causes inflammatory changes of the upper respiratory tract and occasionally pneumonia.
5. The vagi prevent foreign material in the pharynx and upper respiratory tract from reaching the lungs. Section of one vagus may be followed by pneumonia, while section of both invariably leads to this result.
6. It is possible that the secondary factors mentioned above owe their action to their influence upon the vagus control of the upper respiratory tract.