STUDY OF DFLAYED OR « LATENT » TUBERCULOUS INFECTION 603 
invasion much in the same way as tbe Meriones resists the 
action of bacilli of normal pathogenic power. 
For tbe purpose of eslimating Ibe effect wbich various 
agents hâve upon the virulence of a given strain of tubercle 
bacilli, the cobaye is eminently suitable, because Ibis animal 
reacts readily and in a very uniform manner to tubercle bacilli 
obtainable from human, canine, bovine, equine, porcine and 
other mammalian tuberculous lésions, and in a certain pro¬ 
portion of cases, can also be infected with avian tuberculous 
products. 
Subcutaneous inoculation on tbe inner aspect of the bind leg 
at tbe level of tbe fémoro-tibial articulation with a pure cul¬ 
ture or matter containing an extremely small number of bacilli 
produces in tbe cobaye typical lésions, tbe extentand distribu¬ 
tion of which are of gréai value in determining the number 
and virulence of tbe bacilli. Subcutaneous inoculations in 
other parts of tbe body, or intraperitoneal injections yield 
much less comparable results. 
As lésions appear in new organs, tbe older lésions increase 
in extent, passing through stages of necrosis, caséation, 
fibrosis, etc. At first the organs atïected increase considerably 
in sizTe, but after a time they show a tendency to contraction 
owing to tbe production of fibrous tissue and absorption of 
degenerated products. 
The extent and distribution of the tuberculous lésions in an 
inoculated animal are determined by six factors at least : 
1. The number of bacilli, 
2. The virulence of the bacilli, 
3. The time which lias elapsed since infection, 
4. The résistance of the animal to infection, 
5. The seat of inoculation, and 
6. The amount of damage done to the tissues at the time 
of inoculation. 
For purposes of comparison some of tliese factors mav be 
more or less successfully reduced to constants by suitable pré¬ 
cautions. 
The différences produced by time can be eliminated by exa- 
mining the animais after a constant interval of time. The 
resislance of the animal can be made as constant as possible, 
