THE TUBERCLE BACILLUS. 209 



having a special tendency to soften and break down into a 

 pus-like fluid. 



Tuberculosis in fowls (avian tuberculosis) is a common 

 and very infectious disease, nearly all the birds in the 

 poultry -yard being sometimes affected. It is considered 

 by some that the bacillus in avian tuberculosis is different 

 from that in the mammalian form. This question will be 

 discussed below. Further statements with regard to 

 tuberculosis artificially produced will be given later. 



From these statements it will be seen that the disease 

 in animals presents great variations in character, and 

 may differ in many respects from that met with in the 

 human subject. The tubercle nodules may be of so 

 large a size, e.g., in the horse and ox, as to be described 

 as sarcoma-like ; they may be tough and firm, with little 

 or no caseation, or they may be softened in the centre, 

 more resembling abscesses, or again there may be an erup- 

 tion of very minute granulations. However different their 

 naked-eye appearances may be, they are built up histologic- 

 ally on the same plan, ^ -i - , 



and of greater im- f x' 



portance still is the \ 



fact that they are all ^ 



produced by the /| 



tubercle bacillus. - ^^^\^^\ 



Tubercle Bacillus " 



Microscopical Char- 

 acters. Tubercle 

 bacilli are minute y ^ j ^ 

 rods which usually 

 measure 2.5 to 3.5 //, \ 



in length, and .3 p in ^ 



thickness, i.e. in pro- s\ 



portion to their length FIG. 56. Tubercle bacilli, from a pure 



they are comparatively culture on glycerine agar. 



thin organisms (Figs. 



56 and 57). Sometimes, however, longer forms, up to 5 /x 



or more in length, are met with, both in cultures and 



