300 BACTERIOLOGY. 
all facts tend to show that the great majority of cases, 
in adults at least, come from human infection. The 
cases where fairly strong proof of bovine infection has 
been obtained are certainly rare. 
Further, we have the undoubted fact that constant 
sojourn in one species of animal tends to increase the 
virulence of the germ for that animal and to lessen it 
for others. 
Theobald Smith has made the interesting discovery 
that there is a wide difference between the culture 
growth of the average bovine bacillus and the average 
one from human sources, the bovine bacilli being shorter 
and straighter, and growing less luxuriently than those 
from man; and, further, that the bovine bacilli are 
much more virulent for rabbits. He has found these 
differences persist for long periods, and believes that 
the simple passage through a single person in a case of 
human tnberculosis would not be sufficient to change 
these characteristics. He has not yet had a chance to 
examine the bacilli of any case in young children where 
milk infection was strongly suspected, but in adults 
not one of some half a dozen cultures showed the 
bovine characteristics. 
At present it seems fair to assume that bovine bacilli 
are capable of infecting only those that are very sus- 
ceptible, such as young children. This question is in 
great need of further study, and unless proof is 
brought to show that bovine bacilli never infect human 
beings, no cattle which are shown to be tubercular 
should be allowed to furnish milk, or at least none un- 
sterilized should be used for drinking purposes. The flesh 
is less harmful, as muscular tissue is seldom infected. 
Bird (Avian) Tuberculosis. Tuberculosis is very com- 
