TUBERCULOSIS 157 
stream and lodged in any part of the body, such as the brain, kidneys, 
spleen, testes, ovaries, bones, joints, and subcutaneous and inter- 
muscular glands and serous membranes. The evidence at hand, how- 
ever, seems to show that in a large majority of cases the primary 
lesions are located in one of the following organs: (1) in the 
lungs or the lymphatic glands draining them; (2) in the lymphatic 
glands about the head; (3) in the mesenteric glands and intestines; 
(4) in the portal glands or liver substance itself; and (5) in the genera- 
tive organs and udder. 
It not infrequently happens that the apparent primary lesions 
occur on the pleura, peritoneum, meninges or synovial membranes 
while the organs remain freéYrom disease. In such cases the lesions 
consist of many tubercles varying from one to ten or more millimeters 
in diameter or of bunches of closely set tubercles which are more or 
less flattened or irregular in shape, owing to their mutual pressure. 
Sometimes these tubercles are attached to the serous membrane by a 
small, tough, fibrous pedicle: frequently, however, this is absent and 
the nodules rest bodily upon the membrane. 
The structure of the tubercle consists in the beginning of a few cells 
surrounding the invading specific organisms. These are soon encased 
by a zone of epithelioid cells and giant cells which is soon surrounded 
by an outer layer of round or lymphoid cells. The central portion 
becomes necrosed and as the nodule enlarges the central necrotic por- 
tion becomes correspondingly large. 
The histological structure of the tubercle is typically illustrated in 
the beginning avian tubercle. In cattle there is a strong tendency 
for the necrotic tissue to become infiltrated with- lime salts. In 
certain species a deposit of fibrous tissue in the. outer zone of the 
tubercle has been observed. In the smaller and more susceptible 
experimental animals such as the guinea pig and rabbit and frequently 
in swine, the lesions are of a more diffuse nature infiltrating the inter- 
stitial tissue with the tuberculous mass and gradually encroaching 
upon the parenchyma. Circumscribed tubercles may also be present. 
In secondary or generalized tuberculosis one or more of the organs, 
such as the omentum, serous membranes, or lymphatic system, may 
become more or less thickly sprinkled with minute grayish nodules 
about the size of a millet seed. These tubercles are at first almost the 
color of mother-of-pearl but later as the central caseous degeneration 
begins they become grayish. Giant cells are usually numerous. 
