TUBERCULOSIS AMONG ANIMALS 55 



others to *oi milligramme when given with food. The sensitive- 

 ness of guinea-pigs is so marked that inoculation of them by 

 a suspected material may be used as a test for the presence 

 of the bacilli when the latter are too few in number to be 

 detected by microscopic examination. 



In the second series of experiments — those made with 

 tubercle bacilli of human origin — the viruses employed were 

 divided into* two groups, those of Group I. being much more 

 virulent than those of Group II. The same species of animals 

 were used, and the results obtained were very similar to those 

 observed with bovine material. Rats and mice again displayed 

 high powers of resistance. Infection did not cause visible 

 tuberculous lesions ; but after the introduction of large doses, 

 multiplication of the bacilli took place, and death resulted from 

 a condition which has been called " tuberculous septicaemia." 



Dogs showed only slight traces of the disease when inoculated 

 or fed with the bacilli of Group II. The single experiment 

 made with Group I. caused death from general tuberculosis. 



The few observations made upon cats showed that the 

 species was highly resistant to the bacilli of Group II., but was 

 more susceptible to those of Group I. 



The goat, pig, ox, and rabbit were highly susceptible to the 

 viruses belonging to Group I., but in most cases only showed 

 slight disease after infection by Group II. Rabbits, however, 

 were less resistant than calves, and with some viruses of 

 Group II. showed progressive generalised tuberculosis. 



Chimpanzees, monkeys, and guinea-pigs were readily infected 

 by the bacilli of either group, and this in very small doses. 



Combining the results from these two sets of experiments, 

 and sinking small differences, we find that the order into which 

 the various classes of animals fall when arranged according to 

 their degrees of susceptibility to tuberculosis is as follows : 



Rats and Mice. 



Dogs. 



Cats. 



Pigs, Goats, Bovines. 



Rabbits. 



Guinea-pigs, Monkeys, Apes. 

 What explanations can be suggested to account for the 

 enormous differences between the first and last of this series ? 

 In the first place, it seems clear that the degree of resistance 



