386 A MANUAL OF BACTERIOLOGY 



adult, however, infection with the bovine type is rela- 

 tively uncommon, and in pulmonary tuberculosis the 

 bacillus is invariably of the human type. 1 



Although Eber 2 and others claim to have infected 

 calves with the human bacillus and to have transformed 

 it by a series of passages through calves into the bovine 

 type, the human and bovine types of bacilli, as a rule, 

 maintain their characters and must be regarded as being 

 essentially distinct. The same may be said to be the 

 case with the avian bacillus. 



The channels of infection in tuberculosis are varied, 

 and in some localities of the body are still the subject of 

 dispute. Direct infection is possible in the case of the 

 skin, the alimentary tract and the genito-urinary tract. 

 In the last-named, however, infection by way of the 

 blood-stream is far commoner than by the direct route, 

 and the same may be said of infection in other regions. 



The channel of infection in primary pulmonary tubercu- 

 losis has been the subject of considerable controversy. 

 Three routes suggest themselves, directly by the air, or 

 indirectly by way of the lymphatics or of the blood- stream. 

 Direct infection by inspired air carrying tubercle bacilli 

 is undoubtedly possible, but it seems to be a rare occur- 

 rence. In the dry state (e.g. tuberculous sputum dried 

 and powdered), tubercle bacilli in the air seem to be 

 carried to the alveoli only exceptionally, when, for 

 instance, they are present experimentally in large num- 

 bers. Under natural conditions they would probably 

 never be numerous enough to gain access to the alveoli. 

 In the moist state, e.g. in droplets of tuberculous sputum, 

 it is much easier to obtain a pulmonary infection experi- 

 mentally, but under natural conditions this mode of 

 infection is probably also exceptional. 



1 Bulloch, " Horace Dobell Lecture," 1910. 



2 Centr.f. Bakt., Abt. I (Orig.), lix. 1911, p. 193. 



