244 



TUBERCULOSIS 



not, the important fact has been established that tubercular 

 material in which no bacilli can be found microscopically, may 

 be proved, on experimental inoculation into animals, to be still 

 virulent. In such cases the bacilli may be present in numbers so 

 small as to escape observation, or it may be that their spores only 

 are present. In subacute lesions, with well-formed tubercle 

 follicles and little caseation, the bacilli are generally scanty. 



FIG. 85. Tubercle bacilli in section of human lung in acute phthisis. The 

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

 pale background is formed by caseous material. 



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



They are most numerous in acute lesions, especially where 

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

 caseous catarrhal pneumonia (Fig. 85), 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 retains its form and staining power for a much 



