218 TUBERCULOSIS. 



the circumstance that young capillaries cannot grow into a 

 part where tubercle bacilli are active, and that the already 

 existing capillaries become thrombosed, owing to the action 

 of the bacillary products on their walls, and ultimately 

 disappear. At the periphery of tubercular lesions there 

 may be considerable vascularity and new formation of 

 capillaries. 



Presence and Distribution of the Bacilli. A few facts 

 may be stated regarding the presence of bacilli, and the 

 numbers in which they are likely to be found in tubercular 

 lesions. On the one hand, they may be very few in 

 number and difficult to find, and on the other hand, they 

 may be present in very large numbers, sometimes forming 

 masses which are easily visible under the low power of the 

 microscope. 



They are usually very few in number in chronic lesions, 

 whether the latter are tubercle nodules with much con- 

 nective tissue formation or old caseous collections. In 

 caseous material one can sometimes see a few bacilli faintly 

 stained, along with very minute unequally stained granular 

 points, some of which may be spores of the bacilli. 

 Whether they are spores or not, the important fact has been 

 established that tubercular material in which no bacilli 

 can be found microscopically, may be proved, on experi- 

 mental inoculation into animals, to be still virulent. In 

 such cases the bacilli may be present in numbers so small 

 as to escape observation, or their spores only may be 

 present. In subacute lesions, with well -formed tubercle 

 follicles and little caseation, the bacilli are generally scanty. 

 They are most numerous in acute tubercular lesions, especi- 

 ally where caseation is rapidly spreading, for example, in 

 such conditions as caseous catarrhal pneumonia (Fig. 59), 

 acute tuberculosis of the spleen in children, which is often 

 attended with a good deal of rapid caseous change, etc. In 

 acute miliary tuberculosis a few bacilli can generally be found 

 in the centre of the follicles ; but here they are often much 

 more scanty than one would expect. The tubercle bacillus 

 is one which not only has comparatively slow growth, but 



