298 SA GTERIOL OGY. 



sumably highly resistant and not destroyed by drying, 

 are thrown off from the lungs in the sputum of tuber- 

 culous patients in large numbers, and unless special 

 precautions be taken to prevent it the sputum becomes 

 dried, is ground into dust, and sets free in the atmos- 

 phere the spores of tubercle bacilli which came with it 

 from the lungs. The frequency of 'pulmonary tuber- 

 culosis 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 

 probabilities are that in this locality, owing to con- 

 ditions unfavorable to their further growth, they are 

 in the spore-stage, a stage in which it is as yet impos- 

 sible, with our present methods of staining, to render 

 them visible. The fact that this tissue is infective. 



