358 BACTEmoLOGY. 



conspicuous differences that may be observed between 

 the bacilli obtained from human and those from bovine 

 tuberculosis ; and in a series of inoculation experiments 

 Ravenel has shown that for a large number of species 

 tubercle bacilli of bovine origin were constantly more 

 virulent than those from human sources. 



Anatomical lesions very suggestive of, though not 

 identical with, those produced by bacillus tuberculosis, 

 have also from time to time been observed in mice, rats, 

 guinea-pigs,, rabbits, cats, goats, bovines, hogs, and man. 

 They do not aj)pear to be of a specific nature as regards 

 etiology, for the reason that different authors have 

 described different organisms as the causative agents. 

 These affections are usually classed under the name 

 pseud o tuberculo sis. 



Susceptibility of Animals to Tuberculosis. — 

 The animals that are known to be susceptible to tuber- 

 culosis are man, apes, cattle, horses, sheep, guinea-pigs, 

 pigeons, rabbits, cats, and field-mice. White mice, 

 dogs, and rats possess immunity from the disease. 



Tuberculin. — The filtered products of growth from 

 old fluid cultures of the tubercle bacillus represent what 

 is known as tuberculin — a group of proteid substances 

 possessing most interesting properties. When injected 

 subcutaneously into healthy subjects tuberculin has no 

 effect ; but when introduced into the body of a tuber- 

 culous person or animal a pronounced systemic reaction 

 results, consisting of sudden but temporary elevation of 

 temperature, with, at the same time, the occurrence 

 of marked hyperemia about the tuberculous focus, 

 a change histologically analogous to that seen in the 

 primary stages of acute inflammation. This zone of 



