482 IOWA DEPARTMENT OF AGRICULTURE 
periments and comparative studies of Tlieobald Smith, however, which 
attracted special attention to the difference in virolence shown by tubercle 
bacilli from human and bovine sources when inoculated upon cattle. 
Smith mentioned also certain morphological and cultural differences in 
bacilli from these two sources, and in the location and histology of the 
lesions in cattle produced bj^ such bacilli. He did not conclude, however, 
that bovine bacilli could not produce disease in the human subject, but 
said: 
"It seems to me that, accepting the clinical evidence on hand, bovinq 
tuberculosis may be transmitted to children when the body is over- 
powered by large number of bacilli, as in udder tuberculosis, or when 
certain unknown favorable conditions exist." 
Koch, however, in his address at the British Congress on Tuberculosis, 
went far beyond this and maintained that "human tuberculosis differs 
from bovine and can not be transmitted to cattle." As to the suscepti- 
bility of man to bovine tuberculosis, he said it vras not yet absolutely 
decided, but one was "nevertheless already at liberty to say that, if such 
susceptibility really exists, the infection of human beings is but a very 
rare occurrence." He emphasized this view in the following language: 
"I should estimate the extent of infection by the milk and flesh of 
tubercular cattle and the butter made of their milk as hardly greater than 
that of hereditary transmission, and I therefore do not deem it advisable 
to take any measures against it." 
This conclusion was so radically different from the views of most ex- 
perimenters and so out of harmony with facts which had apparently been 
demonstrated by others that it at once aroused opposition in the congress, 
followed by the adoption of dissenting resolutions, and led to numerous 
investigations in various countries. Koch's conclusions were based upon 
his failure to produce tuberculosis in cattle and other animals by inoculat- 
ing them with tubercular material of human origin, and his success in 
causing progessive and fatal tuberculosis in the same kinds of animals 
when inoculated with tubercular material of bovine origin. With such 
positiveness did he hold to the constant and specific difference between 
the human and bovine bacillus that he promulgated an experimental 
method of discriminating between them. Speaking of the etiology of 
intestinal tuberculosis in man, he said: 
"Hitherto nobody could decide with certainty in such a case whether 
the tuberculosis of the intestine was of human or of animal origin. Now 
we can diagnose them. All that is necessary is to cultivate in pure cul- 
ture the tubercle bacilli found in the tubercular material, and so ascer- 
tain whether they belong to bovine tuberculosis by inoculating cattle with 
them. For this purpose I recommend subcutaneous injection, which yields 
quite specially characteristic and convincing results." 
These important and comprehensive conclusions followed from a com- 
paratively few experiments upon animals, and apparently no effort had 
been made to learn to what extent human tubercle bacilli might differ 
in their virulence for cattle or what grades of virulence there might be 
among bacilli of bovine origin. Vagedes had already shown that bacilli 
were sometimes present in human lesions v.-hich were as virulent as 
bovine bacilli, but his work was wholly ignored by Koch. 
