554 
human types, and that tubercle bacilli of the so-called “ bovine ” type 
are, as a general rule, more virulent than those of the human type for 
all animals, including manlike apes, and the conclusion is almost 
forced upon us that the tuberculous dairy cow is, to say the very least, 
one of the most important sources of tubercle bacilli with which we 
have to deal. 
The commoner occurrence of tuberculosis in the lung than in other 
parts of the body should not encourage us to undervalue tubercle 
bacilli concealed in articles of food, as it has been shown that infection 
may penetrate to the lung as easily by the way of the intestine as 
directly through the trachea and bronchi ; in fact, a critical considera- 
tion of the two modes of infection, inhalation and ingestion, shows 
that the latter is in better harmony with known facts than the former. 
As there is a lack of clearness about the popular conception of the 
channels through which disease germs may penerate into and injure 
the lung, it may be well to devote a few additional paragraphs to 
this subject. 
The inhalation of infectious material directly into the lung re- * 
quires, first, that the infectious material must be suspended in the 
air, and, second, that the infectious material must remain in suspen- 
sion while the air j^asses through a long, narrow, tortuous, moist- 
walled system of channels. We have seen how difficult it is to pul- 
verize the tuberculous material, sputum, from which fine tubercu- 
lous dust is supposed to arise, and we have seen how rapidly tubercle 
bacilli are destroyed when they are exposed to light and drying; 
hence we may conclude that dust charged with live, virulent tubercle 
bacilli is by no means plentiful. But even if tuberculous dust was | 
abundantly suspended in the air, its penetration into the finer bron- j 
chial tubes of the upper portions of the lung, where tuberculous proc- 
esses most commonly begin, would necessitate a suspension of the laws 
that govern the relatiA’e movements of substances of higher and lower 
specific gravity actuated by the same force. 
AMien a moving fluid holds solid particles of relatively higher 
specific gravity in suspension, every change in the direction of the 
movement will cause the heavier, solid particles to move somewhat 
more tangentially than the lighter fluid. If the movement occurs in 
a tube, the heavier particles will be thrown with more or less force 
against the wall of the tube. Alien the heavier particles are a dry 
dust and the fluid is a dry gas like air and the movement occurs in 
moist-walled channels like the air tubes, the dust will be thrown 
against and adhere to the moist walls, and the air will be thoroughly 
purified long before the number of turns or changes of direction have ^ 
been made that occur in the air passages from the exterior of the 
body to the finer bronchial tubes. Now, the larger air tubes on • 
which dust may be deposited are covered with a ciliated epithelium 
