ONTFECTIOUS DISEASES 223 



tubercle the central portion is caseous and slightly yellow, 

 or fibrous and white. Sometimes there is a combination 

 of both conditions, and occasionally the tubercles contain 

 calcareous granules. The central portion of porcine 

 tubercles rarely contains liquefied necrotic tissue. 



Microscopically, porcine tubercular lesions are always 

 cellular in the beginning, the cellular tubercles being 

 fairly constant in structure, regardless of the tissue in 

 which they occur. The center is at first represented by 

 a mass of lymphoid cells, the other cellular elements 

 occurring as the tubercle develops. 



Necrosis, or fibrosis, succeeds the cellular stage in 

 the porcine tubercular lesion. Necrosis of tubercles is 

 probably the result of the activity of virulent bacteria or 

 the low resistance of the infected animal. The necrotic 

 center may be surrounded by a cellular zone (lymphoid 

 and endothelial ells), or it may be surrounded by fibro- 

 blasts. The necrotic material is invariably caseated and 

 later becomes calcified. 



Fibrous lesions vary from the formation of small 

 quantities of fibrous tissue to dense fibrous centers. 

 They are probably produced by bacteria of low virulence, 

 or occur in animals having a marked resistance. The 

 central portion of the fibrous lesion may become calcified. 



The so-called arbor vitse gland is a fibrous center in 

 which the fibrous tissue is arranged similarly to the trunk 

 and branches of a tree, hence the name. In the hog, 

 this lesion is observed in the cervical lymph nodes. The 

 Bacterium tuberculosis has been demonstrated in about 

 thirty per cent of arbor vitse glands. 



The lesions of porcine tuberculosis are in brief either 

 cellular, necrotic, and calcified tubercles, or cellular, 

 fibrous, and calcified tubercles. The lesion is always 

 non vascular, as in other animals. 



Tubercular lesions are most common in the digestive 

 viscera. The liver and spleen frequently contain caseous 

 nodules. 



Pulmonary porcine tuberculosis manifests itself either 

 by a few or many cheesy foci or by a few large centers. 



