512 DISEASE IN WILD MAMMALS AND BIRDS 
The avian tubercle as it rests in the tissue seems like a 
sharply outlined almost encapsulated body. This is in 
part due to the homogeneity of its structure and in part 
to the fibrocellular condensation around the caseous part 
of the growths. Examined from without inward, there is 
a round cell mantle, between the elements of which course 
fine but easily perceptible fibrils ; elastic tissue has been 
seen among them. The small cells continue more deeply 
than the fibres, to be succeeded in prominence by epithe- 
lioid cells of rather dense character, the nuclei especially 
seeming quite rich in chromatin and round. The 
rotundity of the nuclei remains not only in the single 
separate cells forming the middle zone of the cellular 
cortex, but can be found in the nuclei of the compound or 
giant cells which comprise the internal layer lying upon 
the necrotic centre. These giant cells are characteristic 
for the avian tubercle in assuming a form like syncytia 
with nuclei arranged in irregular radiating columns. This 
internal large cell area may completely surround the 
central necrosis or it may be interrupted by the large 
single cells. Tubercle bacilli are more common in and 
between single cells than multinuclear ones. Within the 
cellular zone lies the necrotic centre, often, indeed usually, 
full of chromatic debris. This centre is commonly quite 
amorphous but occasionally one will see what is probably 
the remains of a coarse coagulum. Between the necrosis 
and the cells one usually finds a split, a sort of separation 
of the gangrenous from the living part. Old tubercles 
with denser fibrous capsule retain this microanatomy 
in part, but the cellular zone gradually becomes thinner 
and thinner until all that remains is a narrow cortex of 
round cells and imperfectly retained multinuclear cells. 
Tuberculous granulation tissue without definite milia 
consists entirely of the round cells with small vacuoles 
and a fine but definite fibrosis. Tuberculomata consist of 
cells of varying sizes with small round nuclei. Interstitial 
fibrosis is delicate and barely visible unless especially 
