INFECTIOUS DISEASES OF CATTLE. 421 



only the serous membranes. At the same time the lymphatic glands 

 near the diseased organs are usually involved. Other organs, such as 

 the liver, not infrequently contain tubercles. Though the disease 

 may remain restricted to a single organ, it now and then is found 

 generalized, affecting all organs of the body. 



In the lungs (PI. XXXIV) the changes observed vaiy according 

 to the age and intensity of the disease process. They usually begin 

 with the appearance of very minute tubercles. These may appear in 

 large numbers on the surface of the lungs or within the lung tissue. 

 Later the contents become cheesy and partly calcified. "When these 

 tubercles are sufficiently numerous to become confluent, large masses 

 may be foi-med, which undergo the same retrogressive changes of 

 caseation and calcification. In addition to the formation of tubercles 

 in the lung tissue, certain other changes take place. There is usually 

 present bronchitis with abundant catarrhal secretion. This plugs 

 up the smaller ?.ir tubes, and the lung tissue supplied with air by 

 these tubes collapses. Subsequently it becomes filled with yellowish, 

 cheesy matter, which greatly distends the small air tubes and air 

 vesicles (bronchopneumonia). The connective tissue between the 

 lung lobules, around the tubercleSj and around the air tubes becomes 

 thickened and indurated. In the larynx and the bronchi tubercles 

 may vegetate upon the mucous membi-ane, and ulcers may result 

 from their breaking down. The infiammatory irritation which the 

 growth of the tubercles on the surface of the lungs arouses gives rise 

 to adhesion of the lungs to the ribs and diaphragm. This adhesion 

 is sometimes so firm and extensive that the lungs appear grown to 

 the chest wall. 



When, therefore, the lungs in advanced stages of the disease are 

 cut open we obsene large, yellowish masses, from one-quarter to 

 three-quarters of an inch in diameter, of a cheesy texture, in which 

 calcified, gritty particles are embedded and which are surrounded by 

 very firm connective tissue. The neighboring lung tissue, when col- 

 lapsed and involved in bronchopneumonia, has the color and con- 

 sistency of pale-red flesh. The air tubes, large and small, stand out 

 prominently on the cut surface. They are distended with a pasty, 

 yellowish, cheesy mass, surrounded and enveloped in thick mucus, 

 and their walls greatly thickened. The larger bronchi may be sac- 

 culated, owing to the distention produced bv the cheesy contents. 



The disease usually attacks the bronchial glands, which are situated 

 on the trachea and bronchial tubes at the bifurcation. The changes 

 in the glands are the same as those going on in the lung tissue, and 

 they frequently reach an enormous size. 



The tubercle formation on the serous membranes covering the lungs 

 and chest wall (PL XXXIX, fig. 2), which may go on at the same 

 time with the lung disease or independent of it, has been called 



