630 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
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. The early observations of the Italian investigators, Rivolta 
and Mafucci, have been confirmed and so extended as to give us a 
fairly comprehensive knowledge of the capacities for pathogenic 
action, upon different animal species, of the avian bacilli. At the 
same time painstaking studies of the degree to which birds are sub- 
ject to inoculation with pure cultures of tubercle bacilli of human 
origin support the view of diversity in type of bacilli and suscepti- 
bility of species. And yet, while fowl react only with shght local 
lesions, as a rule, to inoculations of tubercle bacilli of human origin, 
certain mammals have proved themselves fairly subject to experi- 
mental inoculation with avian bacilli. While the guinea pig, other- 
wise so sensitive to inoculation tuberculosis with the mammalian 
bacilli, is relatively resistant to the avian variety, the rabbit, which 
exhibits a marked degree of refractoriness to the human bacilli, suc- 
cumbs quite readily to the avian bacilli. It is, however, worth noting 
that the reactions in the rabbit which avian tubercle bacilli call forth 
do not conform to those observed in tuberculosis in general; there 
is absence of typical tubercles and caseation, and the chief patho- 
logical alterations observed are found in connection with the enlarged 
spleen. 
The literature on tuberculosis contains a small number of refer- 
ences to the cultivation from human subjects of the avian tubercle 
bacillus. From our present knowledge it may be postulated that 
avian tubercle bacilli occur rarely in man. Rabinowitsch has, in- 
deed, recently emphasized the occasional occurrence of the avian 
bacilli in cattle, swine, horses, and monkeys; but they constitute a 
small source of danger in the spread of tuberculous disease among 
mammals. The parrot, because of its use as a pet and of its sus- 
ceptibility to the avian bacillus, on the one hand, and of the human 
bacillus, on the other, is a greater menace to public welfare. 
The subject of bovine tuberculosis and of bovine tubercle bacilli 
is among the most important of all the questions relating to the sup- 
pression of tuberculosis. The admirable studies of Theobald Smith 
established the distinction in type subsisting between certain bacilli 
of human and of bovine origin. We have come now to regard these 
types as separate and not to be transmuted, at least not readily under 
