414 DISEASES OF CATTLE. 



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

 generalized, affecting all organs of the bod3^ 



In the lungs (PL XXXIV) the changes observed vary according 

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

 with the appearance of veiy 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, "\^^len these 

 tubercles are sufficiently numerous to become confluent, large masses 

 may be formed, 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 

 bronchitis with abundant catarrhal secretion ; this plugs up the 

 smaller air tubes, and the lung tissue supplied with air by the tubes 

 collapses. Subsequently it becomes filled with yellowish, cheesy 

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

 (bronchop)neumonia) . The connective tissue between the lung lobules, 

 around the tubercles, and around the air tubes becomes thickened 

 and indurated. In the larynx and the bronchi tubercles may vegetate 

 upon the mucous membrane, and ulcers may result from their break- 

 ing down. The inflammatory 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 observe 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 by 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 w^all (PI. XXXVII, fig. 2), which may go on at the same 

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

 " pearly disease," on account of the peculiar appearance of the tu- 

 bercles. These begin as very minute, grayish nodules, which give 

 the originally smooth, lustrous membrane a roughened appearance. 

 These minute tubercles enlarge, become confluent, and project above 

 the surface of the membrane as wartlike masses, attaining the size 

 of peas. In this stage their attachment to the membrane is by means 

 of delicate fibers. The attachment is loose, so that the tubercle hangs 



