394 CHRONIC INFECTIOUS DISEASES 



tion of the lesion may depend upon the kind of animal, mode 

 of infection, and whether the disease is primary (local), or 

 secondary (generalized). In the ox, tuberculosis is usually 

 confined to the lungs, serous membranes, and lymph glands. 

 In swine the digestive tract with corresponding lymph glands 

 is most frequently involved. In the horse the lymph glands' 

 (mesenteric, retroperitoneal) are generally elected. How- 

 ever, exceptions to this rule are frequently noted; in gener- 

 alized (spread via blood) tuberculosis the nodules may occur 

 in any organ, even being found in muscle. The influence of 

 the mode of infection is difficult to determine, since it has 

 been proved that subcutaneous inoculations in calves (even 

 at the tail tip) with tubercle bacilli were followed by pul- 

 monary lesions. 



The most characteristic lesion in tuberculosis is the 

 tubercle which has undergone caseous degeneration. The 

 lesion may vary in size from a small millet seed (miliary 

 tubercle) to a cheesy mass larger than a human head, due to 

 the confluence of numbers of smaller foci. While the indi- 

 vidual tubercle is at first of a translucent, gray appearance, 

 later from the degeneration, which begins in its centre, it 

 assumes a yellow color. The formation of nodules tending to 

 caseate, particularly if corresponding lymph glands are simi- 

 larly affected, is characteristic of tuberculosis. 



Ox. — As noted, the lungs, serous membranes, lymph glands, 

 especially the bronchial and mediastinal, are most commonly 

 involved. 



Lungs. — In the lungs nodules or nodes of varied size, of 

 firm to fluctuating consistency, with usually well-defined out- 

 line, invade the tissue. On cut surface the dry, yellow, 

 friable caseation surrounded by a thick capsule is found, or, 

 on the other hand, the contents are soft, puriform, thick- 

 fluid, covered by a thin connective-tissue layer. The color is 

 grayish yellow to pronounced yellow. The size will vary 

 from that of a millet seed to a clenched fist, or, by confluence, 

 a whole lobe of lung tissue may be found changed to a case- 

 ous mass. In old cases calcification of the tubercle occurs, 

 whereby it grits under the knife when cut through. Usually 

 in the neighborhood of a larger node small tubercles are pres- 



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