TUBERCULOSIS 381 



noticed long ago that there are certain differences between 

 the bacilli of human and of bovine tuberculosis, the latter 

 tending to be shorter and thicker and less readily culti- 

 vated than the former ; also, human tuberculous material 

 injected into a rabbit generally produces small discrete 

 lesions which tend to retrogress, while bovine material 

 induces a progressive disease with large caseating masses. 1 

 These differences were regarded as being due to variations 

 in the bacilli as a result of growing upon a different soil 

 and not to any fundamental difference between the two 

 strains. In 1901, however, Koch stated 2 that young 

 cattle and swine cannot be infected with human tuber- 

 culous material, and he therefore concluded that human 

 and mammalian tubercle bacilli are essentially distinct. 

 As a result of his experiments he made the statement 

 that "though the important question whether man is 

 susceptible to bovine tuberculosis at all is not yet abso- 

 lutely decided, if such a susceptibility really exists, the 

 infection of human beings is but a very rare occurrence." 

 This view met with considerable opposition, and a 

 second Royal Commission was appointed to investigate 

 the question, and the following summarises the results 

 obtained, from which it will be gathered that, while there 

 is no justification for assuming that man is infected from 

 human sources alone, infection from human sources is 

 probably vastly more frequent than from any other. 

 Thirty different viruses isolated from cases of tuber- 

 culosis occurring spontaneously in bo vines were studied, 

 and the results of introducing them into a number of 

 different animals by feeding and inoculation are recorded. 

 In calves, inoculation usually results in generalised pro- 

 gressive tuberculosis, but the effect is somewhat dependent 



1 The bacilli derived from tuberculosis of the sheep, pig, and horse (pul- 

 monary lesions) are also of the bovine type. 



2 See Brit. Med. Journ., 1901, vol. ii, p. 189. 



