LOCATION OF THE BACILLI IN THE TISSUES. 275 



to prevent it, the sputum becomes dried, is ground into 

 dust, and sets free in the atmosphere the spores of tu- 

 bercle bacilli which came with it from the lungs. The 

 frequency of pulmonary tuberculosis points to this as 

 one of the commonest sources and modes of infection. 



LOCATION OF THE BACILLI IN THE TISSUES. The 

 bacilli will be found to be most numerous in those tissues 

 which are in the active stage of the process. 



In the very initial stage of the disease the bacilli will 

 be fewer in number than later. At this time only here 

 and there single rods may be found ; later they will be 

 more numerous, and, finally, when the process has ad- 

 vanced to a stage easily recognizable by the naked eye, 

 they will be found in the granulation zones in clumps 

 and scattered about in large numbers. 



In the central necrotic masses, which consist of cell 

 detritus, it is rare that the organisms can be demon- 

 strated microscopically. It is at the periphery of these 

 areas and in the progressing granular zone that they are 

 most frequently to be seen. 



This apparent absence of the bacilli from the central 

 necrotic area must not be taken, however, as evidence 

 that this tissue does not contain them. As bacilli, they 

 are difficult to demonstrate here because the probabili- 

 ties are that in this locality, owing to conditions unfa- 

 vorable to their further growth, they are in the spore 

 stage, a stage in which it is as yet impossible, with our 

 present methods of staining, to render them visible. 

 The fact that this tissue is infective, and with it the 

 disease can be reproduced in susceptible animals, speaks 

 for the accuracy of this assumption. A conspicuous 

 example of this condition is seen in old scrofulous 

 glands. These glands usually present a slow process, 



