204 TUBERCULOSIS 
tion and are wholly uninjured by it. The farmer may, therefore, 
have his herd tested’ with the confidence that his healthy animals 
will not suffer by the test. On the other hand, the animals that 
have become infected with the disease will show a rise in tem- 
perature, and the test will thus make it possible to separate the 
affected animals from those that are yet in health. 
The accuracy of the test has been the subject of much dispute. 
It has been found subject to some error. If animals are tested 
under abnormal conditions, as, for example, when in new barns, or 
if taken from a cattle car and tested at once, even healthy animals 
may respond. But when the animals are in normal conditions the 
healthy animals probably never respond, or at all events so rarely 
as not to interfere with the accuracy of the test. Secondly, some 
animals very far advanced in the disease fail to respond. These 
cases are of little importance since they are commonly detected 
readily by clinical symptoms. Thirdly, all animals which are 
moderately attacked, and all of the very incipient cases of tuber- 
culosis, are detected by the tuberculin. Even a single minute 
tuberculosis gland is sufficient to cause a positive reaction to the 
test. 
This last fact forms at once the strength and the weakness of the 
tuberculin test. Tuberculin does pick out with great accuracy all 
mild cases, and clinical symptoms will pick out the rest. But this 
test fails to distinguish between severe and mild forms, putting in 
one class the animal that may have a small tuberculous gland, 
which may heal in a short time, and the animal with a severe case 
of intestinal tuberculosis which is scattering bacilli to the great 
danger of the rest of the herd. Experience has shown that, of the 
animals responding to the test, some run down rapidly and require 
slaughtering in a few weeks, while others wholly recover, live sev- 
eral years of useful life, and after death show, by postmortem 
examinations, that the original tubercle has been healed and the 
animals have come again into normal condition. There is thus a 
great difference between clinical tuberculosis and tuberculin tuber- 
