THE TUBERCLE BACILLUS 237 



Tuberculosis in fowls (avian tuberculosis) is a common and very 

 infectious disease, nearly all the birds in a poultry-yard being sometimes 

 affected. 



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 

 relation of the different forms of tuberculosis is discussed below. 



Tubercle Bacillus Microscopical Characters. Tubercle 

 bacilli are minute rods which usually measure 2 '5 to 3*5 //, in 

 length, and '3 //, in thickness, i.e. in proportion to their length 

 they are comparatively thin organisms (Figs. 82 and 83). Some- 

 times, however, longer 



forms, up to 5 /A or more ^r~gr~ t i 



in length, are met with, f s 



both in cultures and in the \ if 



tissues. They are straight * \ 



or slightly curved, and are jj 

 of uniform thickness, or / i *"" \ ^v x 

 may show slight swelling /.-_ " {} _ . 



at their extremities. When ^ 



stained they appear uni- ^ viX / 



formly coloured, or may \$ f v_ * </ 



present small uncoloured \ 



spots along their course, 



with darkly -stained parts \ 



between. In such a min- \ (^ 



ute organism it is ex- ^-"' 



tremely difficult to deter^ FlG 82 . Tubercle bacilli, from a pure 



mine the exact nature of culture on glycerin agar. 



the unstained points. Ac- Stained with carbol-fuchsin. x 1000. 



cordingly, we find that 



some observers consider these to be spores, while others find that 



it is impossible to stain them by any means whatever, and con- 



sider that they are really of the nature of vacuoles. Against their 



being spores is also the fact that many occur in one bacillus. 



Others again hold that some of the condensed and highly-stained 



particles are spores. It is impossible to speak definitely on the 



question at present. We can only say that the younger bacilli 



stain uniformly, and that in the older forms inequality in staining 



is met with ; this latter condition is, however, not associated with 



greater powers of resistance. 



The bacilli in the tissues occur scattered irregularly or in 

 little masses. They are usually single, or two are attached end 

 to end and often form in such a case an obtuse angle. True 



