CONGRESS ON TUBERCULOSIS. 
459 
bacilli are as a rule distinctly more virulent for cattle and other 
domesticated animals than human bacilli, or that the results of 
experiments indicate that in natural circumstances there is little 
danger of cattle becoming infected from human beings. But it 
cannot be admitted that the low virulence of human bacilli for 
cattle proves, or even makes it probable, that bovine bacilli have 
onlv a feeble pathogenic power for man. That might have 
been held to be probable if it had been shown that bovine bacilli 
were very virulent only for cattle, but since it is well established 
that these bacilli are highly dangerous for such diverse species 
as the rabbit, horse, dog, pig, and sheep, and, in short, for almost 
every quadruped on which they have been tried, it appears to 
be highly probable that they are also dangerous to man. At 
any rate it is impossible to cite any ascertained fact relating to 
other bacterial diseases that makes the contrary conclusion 
probable. It is well known that the majority of disease-exciting 
bacteria are harmful to only one or two species, but all those 
that are common to all the domesticated animals are also patho¬ 
genic to man. 
With regard to the view that the difference between human 
and bovine bacilli in respect of virulence for cattle is of such a 
fixed and constant character that it may be relied upon to dis¬ 
tinguish the one from the other, it need only be said that that 
is very far from proved. It appears to be quite possible that 
what may be called the normal or average virulence of bovine 
bacilli for cattle may be reduced by passage through the human 
subject. Besides, there are very great differences in the viru¬ 
lence of tubercle bacilli found in animals of the same species, 
and if a low degree of virulence for cattle is to be taken as the 
distinguishing feature of human bacilli, there will be no diffi¬ 
culty in proving that the human disease is sometimes trans¬ 
mitted to the lower animals. 
The third proposition in Dr. Koch’s argument is the only 
one which is really germane to the point at issue, viz., that only 
cases of primary intestinal tuberculosis can possibly have had 
their origin in infected milk or meat, and that “ such cases are 
extremely rare.” Dr. Koch refers to several large series of post¬ 
mortem observations that appear to justify this statement, and 
adds that he could have cited many more pointing to the same 
conclusion. Now, if it were a fact that all the statistics relat¬ 
ing to this point were unanimous, it would have to be admitted 
that primary intestinal tuberculosis is rare in the human sub¬ 
ject, and that cases of infection through milk are still rarer, 
