160 TUBERCULOSIS 
directly from the primary focus. This may be purulent, caseous or 
calcareous. The color may be whitish, gray or of a yellowish tinge. 
(2) The lesions may consist of miliary tubercles. In later stages these 
nodules, more or less translucent, may become yellowish, caseated and 
calcareous in their centers. Large tubercular nodules are frequently 
formed by the massing of several of these minute tubercles. 
Fic. 34. TuBerctLosis oF THE MUCOUS MEMBRANE OF THE TRACHEA, COW. 
When the lungs are primarily attacked the caudal (principal) lobes 
are most frequently involved. Smith considers the seeming predilec- 
tion for the larger lobes to be due to mechanical conditions. The 
writer has found, however, that in certain herds which have been killed 
after the tuberculin test. the primary and only lung lesions were in the 
