304 A MANUAL OF BACTERIOLOGY 



glycerin gelatin at 22 C. Gelatin and blood-serum are not 

 liquefied. On glycerin agar the growth forms a dry, crinkled 

 and wrinkled, cream-coloured or brownish-yellow film, which 

 has been well described as resembling the patches of lichen 

 met with on trees (Fig. 37). The growth, 

 however, varies considerably, both in 

 colour and in the amount of wrinkling, 

 though retaining more or less the char- 

 acteristics just mentioned. In broth it 

 forms soft, cream-coloured, flaky masses, 

 which increase slowly both in size and 

 number, the broth remaining perfectly 

 bright and clear. Sometimes a dry crink- 

 led film forms on the surface of the broth, 

 and may spread all over it, and tends to 

 creep up the sides of the vessel. This film 

 formation seems to be essential for the 

 preparation of a satisfactory old tuber- 

 culin, but it is necessary in order to start 

 it that some of the inoculated particles 

 should float and form nuclei from which 

 FIG. 37. Tubercle the film spreads. The virulent organism 

 bacillus Glycerin- f rom t h e pr i mary cultivations is difficult 



agar culture three , . 



months old. to grow on anything but glycermated 



potato or serum, or brain agar. 



TUBERCULINS. Extracts of, and suspensions of tritu- 

 rated, tubercle bacilli, human or bovine, are employed 

 in treatment and for the diagnosis of tuberculosis. The 

 preparations are known as tuberculins. 



Old tuberculin, T.A. This is prepared by growing the 

 tubercle bacillus in glycerin veal broth in a shallow layer 

 in flat flasks (Fig. 38), so that there is a free supply of 

 oxygen. After some weeks an abundant growth with 

 copious film formation develops ; the latter seems to be 

 essential, but it does not appear to matter whether the 



