ACTION ON THE TISSUES 



275 



still virulent. In subacute lesions, .with well-formed tubercle 

 follicles and little caseation, the bacilli are generally scanty. 

 They are most numerous in acute lesions, especially where 

 caseation is rapidly spreading, for example, in such conditions as 

 caseous catarrhal pneumonia (Fig. 76), acute tuberculosis of the 

 spleen in children, which is often attended -with a good deal of 

 rapid caseous change, etc. ; in such conditions they often form 





4fcr > ¥ •".-«■&- 



Fig. 7ti. — Tubercle bacilli in section of human lung in acute phthisis. 

 The bacilli are seen lying singly, and also in large masses to left of 

 Held. The pale background is formed by caseous material. 



Stained with carbol-fuchsin and Bismarck-brown, x 1000. 



large masses which are easily seen under a low power of the 

 microscope. 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 retains its form and staining power for a much 

 longer period than most organisms. As a rule the bacilli are 

 extra-cellular in position. Occasionally they occur within the. 

 giant-cells, in which they may be arranged in a somewhat radiate 



