ACTION ON THE TISSUES 



281 



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. 81. 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-fuchsiu and Bismarck-brown, x 1000. 



They are most numerous in acute lesions, j especially where 

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

 caseous catarrhal pneumonia (Fig. 81), "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 

 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 mbre scanty than one would expect. The 



