INFLUENZA BACILLUS. 325 
resistance is chosen for the inoculation—viz., the brain, 
upon which the toxic products of the bacillus influenzz 
acts most powerfully. 
The cell bodies of the bacilli seem to possess consid- 
erable pyogenic action. 
Immunity. Possibly an immunity for a short period 
against the influenza poison may be established after an 
attack. At least in three experiments made by Pfeiffer 
on monkeys, these animals, after recovering from an 
inoculation with bacilli, seemed to be much less suscep- 
tible to a second injection. 
In patients suffering from influenza the bacilli are 
found chiefly in the nasal and bronchial secretions. 
In acute uncomplicated cases they may be observed 
microscopically in large masses and often in absolutely 
pure culture; the green, purulent sputum derived from ° 
the bronchial tubes is especially suitable for examina- 
tion. The older the process is the fewer bacilli will be 
found, and the more frequently will they be seen lying 
swilhitn the pus-cells instead of being embedded free in 
. the secretion as at first. At the same time they stain 
less readily and present more irregular and swollen 
forms. Very frequently, perhaps almost invariably 
(Finkler), the influenza process invades portions of the 
lung tissue. In severe cases a form of pneumonia is 
the result, which is lobular and purulent in character, 
and accompanied by symptoms almost identical with 
bronchopneumonia due to the pneumococeus. The walls 
of the bronchioles and alveolar septa become densely 
infiltrated with leucocytes, and the lumina of the bron- 
chial tubes and alveoli are similarly filled. The pus- 
cells are found to contain more or less influenza bacilli. 
There may be partial softening of the tissues, or even 
