to 



VI. TUBERCULOSIS OF THE COW 143 



on clinical and pathological evidence. In his opinion, such 

 diseases as Tabes mesenterica, tuberculous glands in the neck, 

 etc., -were of essentially bovine origin. 



The communicability of the bovine bacillus has been also 

 shown by numerous other investigations. The valuable work 

 of Hamilton and Young in this country may be specially 

 mentioned. Additional evidence is also forthcoming from 

 the numerous recorded cases in which there has been direct 

 evidence of infection of man from bovine sources, generally 

 from accidental inoculation. 



From the numerous investigations it has now been estab- 

 lished that there are two chief types of tubercle bacilli — a 

 bovine type and a human type — which are differentiated by 

 their cultural characters, and especially by the different patho- 

 genic properties. In brief, the bovine type grows with a 

 scantier growth and with greater difficulty upon artificial 

 culture media, but is much more pathogenic, causing local and 

 generalised tuberculosis when inoculated into bovines, and 

 acute tuberculosis when intravenously injected into rabbits. 

 On the other hand, the human type does not, as a rule, cause 

 tuberculosis in rabbits, and only a localised and retrogressive 

 tuberculosis when inoculated into bovines. These differences 

 are set out in greater detail in Chapter XIII. 



How far the two types are simply varieties evolved by 

 environment cannot be said to be yet established, but from 

 the occurrence of intermediate types, gradations being met 

 with between the extremes of the two types, from the fact 

 that the differences are of degree rather than kind, and from 

 the fact that some investigators have been able to consider- 

 ably modify the human type by passage through animals so 

 that it acquires many of the characters of bovine bacilli, the 

 evidence is in favour of a common origin and an essential 

 unity of the two types. 



The Tuberculosis Commissioners sum up the matter as 

 one concerning which there is room for difference of opinion, 

 and add : " We prefer to regard these two types as varieties 

 of the same bacillus, and the lesions which they produce, 

 whether in man or in other mammals, as manifestations of 

 the same disease." 



From the available evidence it may be accepted that a 



