90 DISEASES OF DOMESTICATED BIRDS 
die of the disease. The muscles of the breast may be represented by 
barely a trace. 
Avian tubercles do not differ greatly in gross appearance from 
those of the mammalian type. The smallest ones consist of nodules 
of tissue in which central caseation has not yet begun. In larger 
ones varying degree of caseation is illustrated, but calcification 
is not frequently observed. Avian tuberculosis differs from mam- 
malian tuberculosis markedly in that the visceral organs primarily 
are involved and lesions of lymph nodes are of subordinate im- 
portance. 
The liver is usually studded throughout with nodules which are 
easily separated from the hepatic tissue. There may be several sizes 
of nodules, suggesting that several corresponding invasions of bac- 
teria have occurred. The smaller tubercles are white or grayish 
white, while the larger, older ones are more yellow in color. The 
presence of the tubercles brings about secondary degenerative 
changes in the liver, as a result of which the tissue becomes pale and 
friable. As a consequence, fatal hemorrhages may occur. ‘The or- 
gan is enlarged in advanced cases, and in exceptional cases may con- 
stitute one-fourth of the weight of the bird. 
The spleen, like the liver, is found involved in practically all 
advanced cases. The relative enlargement is greater, for this organ 
may attain the size of a walnut. The small amount of functional 
tissue remaining is often striking. 
The smallest tubercles of the intestine are located in the deeper 
layers of the mucous membrane. Slightly larger ones are in inti- 
mate contact with the intestinal wall but project somewhat. Still 
larger ones, as much as an inch in diameter, are pedunculated and 
in most cases the interior of such tubercles communicates with the 
lumen of the intestine. Lesions of the intestine also occur in the 
form of numerous but isolated ulcers on the mucous membrane. 
The kidneys, ovary, mesentery, and lungs are involved less often, 
while cases of tuberculosis of the pancreas, gizzard, and skin are 
rare in fowls. In the kidneys, the tubercles are discrete, hard, and 
glassy. These organs alone may be affected. Small tubercles are 
observed on the mesentery associated with tubercles on the intestine. 
Involvement of the lung apparently is associated with advanced de- 
velopment of the disease and then it occurs comparatively rarely. 
The tubercles may be isolated or aggregated into extensive areas. 
Skin lesions may consist of single spherical masses, each surround- 
ing the root of a feather, or more often, consist of larger masses. 
