st every civilized country have demonstrated
that in the majority of cases tuberculin is an excellent means for
diagnosing the existence or nonexistence of the disease, but giving us no
positive information as to the extent to which the disease has
progressed. When tuberculin produces a typical reaction we may be
almost sure that there exists in the body of the animal a tubercular
process. The cases in which a careful examiner has not succeeded in
finding it are very rare, and I am led to believe that when,
notwithstanding all the pains taken, it has escaped discovery, the reason
is that it is located in a portion of the body that is particularly
inaccessible. Nevertheless, it is not to be denied that a fever, entirely
accidental and of short duration, may in some rare cases have simulated a
reaction. However this may be, the error committed in wrongly condemning
an occasional animal for tuberculosis is of no practical consequence.

A worse aspect of the case is that there are some diseased animals in
which tuberculin fails to discover the existence of tuberculosis. In
most of these, no doubt, the deposits are old, insignificant, and
generally calcified, or they are cases where the disease is arrested and
perhaps in process of recovery, and which are possibly incapable of
disseminating the contagion. But it is known that there are cases, not
altogether rare, where tuberculin fails to cause a reaction in a highly
tuberculous animal, and consequently one in which the disease exists in
an extremely contagious form. For this reason a clinical examination
should always be made of an animal which does not give a reaction but
which shows symptoms indicating that, notwithstanding the test, it may
suffer from tuberculosis.

Nocard, of Paris, wrote also in 1898 as follows:

The degree of certainty of the indications furnished may be stated in
precise terms. _The observation of a clear reaction to tuberculin is
unequivocal; the animal is tuberculous._ The pretended errors imputed to
the method are explained by the extreme sensitiveness of the reagent,
which is capable of detecting the smallest lesion. It often requires
prolonged and minute researches in the depths of all the tissues to
discover the few miliary centers, the presence of which has been
revealed. The reaction is absolutely specific. In those cases where it is
observed with animals which show lesion