jected into them, yet, as a matter of
historical fact, the actual number of instances in which children or
other human beings have been definitely proved to have contracted the
disease from the milk of a tuberculous cow is still exceedingly and
encouragingly small. A careful study of the entire literature of the
past twenty years, some three years ago, revealed _only thirty-seven
cases_; and of these thirty-seven Koch's careful investigations have
since disproved the validity of nine.

On the other hand, it is anything but safe to accept Koch's practical
dictum and neglect the meat and milk of cattle as a source of danger in
tuberculosis. First, because the degree of our immunity against the
bovine bacilli is still far from settled; and, second, because, while
bacteriologists are fairly agreed that the _avian_, the _bovine_, and
the _human_ represent three distinct and different variations, if not
species, of the bacillus, they are almost equally agreed that they are
probably the descendants of one common species, which may possibly be a
bacillus commonly found upon meadow grasses, particularly the well-known
timothy, and hence very frequently in the excreta of cattle, and known
as the _grass bacillus_ or _dung bacillus_ of M[oe]ller. This bacillus
has all the staining, morphological, and even growth characteristics of
the tubercle bacillus except that it produces only local irritation and
little nodular masses, if injected into animals. Our knowledge of its
existence is, however, of great practical importance, inasmuch as it
warned us that in our earlier studies of the bacilli contained in milk
and butter we have been mistaking this organism for a genuine tubercle
bacillus. As a consequence, of late years our tests for the presence of
tubercle bacilli in milk are made not only by searching for the organism
with the microscope, but also by injecting the centrifugated sediment of
the infected milk into guinea pigs, to see if it proves infectious. Many
of our earlier statements as to the presence of tubercle bacilli in milk
and butter are now invalidated on this account.

Not only are the three varieties of tubercle bacilli probably of common
origin, but they may, under certain peculiar conditions, be transformed
into one another, or, at least, enabled to live under the conditions
favorable to one another. This was shown nearly fifteen years ago by the
ingenious experiments of Nocard, the great veterinary pathologist. He