216 BACTERIA. 



to modify the bacilli by introducing them into different 

 animals whose normal temperatures and other general 

 metabolic conditions are different. This modification has 

 more than nominal value, for it has been proved experi- 

 mentally that, although the organisms in human and in 

 bovine tuberculosis are morphologically identical, they are 

 not absolutely the same in all their vital and pathogenic 

 characteristics. For instance, tubercle bacilli taken from 

 a phthisical patient and introduced into the tissues of a cow 

 will soon set up an acute general tuberculosis, whilst bacilli 

 taken from a case of perlsucht, or ordinary bovine tuber- 

 culosis, almost invariably give rise to the perlsucht form of 

 tuberculous disease, and rarely, or never, to the acute 

 generalized form. It would appear that in these cases the 

 microbe becomes adapted to the special conditions present 

 in each host, and consequently becomes less suited to the con- 

 ditions in others. It might be objected that in the case of the 

 fowl the bacilli are present in enormous numbers, and that, 

 therefore, their action should be more virulent ; but, in 

 answer to this, it may be pointed out that increased activity 

 of growth is not necessarily always associated with increased 

 virulence. In illustration of this fact, it may be pointed out 

 that, after many experiments, Nocard and Roux were able 

 to obtain most luxuriant cultivations of the tubercle bacillus 

 on agar-agar or on blood serum, to which 6-8 per cent, of 

 glycerine had been added. The organisms on these media 

 grow so rapidly that they are quite visible at the end of four 

 days, and at the end of twenty days, and not four weeks, as 

 on ordinary blood serum, the growth seems to have reached 

 its maximum, when it appears as a pale grey, thick, mamil- 

 lated or reticulated, mass. In the same way luxuriant 

 growths may be obtained in bouillon to which a similar 

 proportion of glycerine has been added, small opaque flakes 

 or flocculi first making their appearance at the surface, and 

 then sinking to the bottom, where they remain. This 

 growth in glycerine broth may take place at a comparatively 

 low temperature, i8-2O C., though it then goes on very 

 slowly. Earlier generations of such cultivations produce 

 typical tubercle nodules that grow with extreme and charac- 

 teristic rapidity when inoculated, but after several generations 

 of such pure cultivations have been made in these glycerine 

 media, the virulence may become distinctly diminished, 



