IMMUNITY IN TUBERCULOSIS 231 



apparent between bacilli from different sources. But although I 

 devoted the greatest attention to this point, I could find nothing of the 

 kind. In all the cultures, whether taken from miliary tubercles, lupus 

 or perlsucht, the tubercle bacilli behaved exactly the same." 



Our knowledge of the nature of the tubercle bacillus has been 

 increased until at this time several distinct kinds are recognized. These 

 may conveniently be classified according to their chief sources into 

 human, bovine and avian tubercle bacilli, and into so-called tubercle 

 bacilli of cold-blooded animals. This last group of bacilli, which will 

 detain us only a short time, differs greatly from the other varieties, as 

 can readily be seen when the fact is recalled that the high temperatures 

 — temperatures approaching blood heat — which are required for the 

 growth of the mammalian and avian bacilli, quite preclude their multi- 

 plication under conditions of ordinary external nature. Hence they 

 are not adapted to a life outside the living body except as cultivated 

 artificially at this relatively high temperature. In man's conflict with 

 tuberculosis this fact is of the greatest service, since by reason of it he 

 is enabled to disregard the danger of any increase in tubercle bacilli 

 outside the animal body. The relatively low temperatures at which 

 the tubercle bacilli of cold-blooded animals develop adapt them, indeed, 

 to an independent existence; but, as they are wholly devoid of power 

 to cause disease in warm-blooded animals and as they would appear 

 to have a restricted dissemination even among cold-blooded species, 

 they are of comparatively small importance. 



Of far greater consequence is the question whether the disparity 

 which exists between the several kinds of tubercle bacilli derived from 

 warm-blooded animals is a wide one. This question, which at first 

 sight may appear to be chiefly of academic interest, has, in reality, 

 far-reaching practical significance. The close relationship which man 

 bears to domestic animals makes every fact of animal disease of high 

 value to him. And in the case of no animal disease are facts of greater 

 moment than in tuberculosis. Not only is the human race, by reason 

 of its dependence upon the animal kingdom for food, work, etc., exposed 

 to the diseases of animals which are transmissible to man, but domestic 

 animals are also exposed to diseases of human beings. This correlative 

 susceptibility may, therefore, cooperate to produce a vicious circle of 

 events by which infection or the dangers of infection are kept alive 

 and threatening. Hence it is that an effective solution of the problem 

 of limitation of tuberculosis, whether by suppression outright or by 

 suppression through the induction of immunity, must take into account 

 the degree to which tuberculous animals of different species, through 

 direct or more remote association, are a source of danger to one another. 



There is no longer any doubt that the avian tubercle bacillus de- 

 parts considerably from the human and from the bovine types of bacilli. 



