COXTAGIOUS PLElTBOPNEUMOiSriA. 373 



duces the firm, liepatized condition so characteristic of this disease. 

 If we bear in mind that the veins in diilVrent parts of tlie hmg tissue 

 are plugged at different times, and that, therefore, the affected re- 

 gions are in different stages of disease, it will be easily understood 

 how the different shades of color from dark red to grayish or 

 yellowish red are produced. 



The complete plugging of the veins may lead to the death of cir- 

 cumscribed masses of lung tissue. A line of separation forms between 

 the living and the dead tissue and a thick cyst wall of fibrous tissue 

 forms around the latter. The dead tissue for a time preserves the 

 appearance of lung tissue, then undergoes disintegration and li(iue- 

 faction. The softened mass is finally absorbed, and the walls of the 

 cyst, or capsule around it, gradually collapse and form a cicatrix. 

 This favorable termination takes place only when the dead mass is 

 not too large. It may, however, involve over half of one of the 

 large lobes. Under such circumstances recovery is improbable. A 

 more favorable termination is the abundant growth of fibrous tissue 

 around and into the hepatized masses. The formation of fibrous 

 tissue may extend to the pleura, or lung covering, and cause firm 

 adhesion of the lungs to the chest wall and to the pericardium, or 

 heart case. 



The same peculiar, inflammatory changes which take place between 

 the lobules of the lung and around the bronchi and vessels may invade 

 the pleural cavity, cause extensive membranous and spongy deposits 

 on the pleura and firm deposits around the heart and large arteries, 

 the gullet, and windpipe. 



These are the main features of the lung disease caused by con- 

 tagious pleuropneumonia. In the typical, acute cases there are a 

 a sufficient number of peculiarities to enable us to make a positive 

 diagnosis. There are, however, many cases in which the disease is 

 restricted to small areas, or to the interlobular tissue, or in which 

 the changes are still imperfectly developed, or else so far advanced 

 that doubts may arise as to the true nature of the affection. In 

 such cases all obtainable facts, including the history of the case, the 

 symptoms during life, and the pathological changes observed on 

 post-mortem examination must be taken into consideration. Only 

 one who has made a careful studjr of the disease is fitted to decide 

 in such cases. 



Other kinds of lung disease, because of certain features common 

 to most lung diseases of cattle, may be confounded with pleuro- 

 pneumonia. The inflammation of the connective tissue between the 

 lobules is not infrequently observed in so-called interstitial pneu- 

 monia and may lead to the formation of whitish bands intersecting 

 the lung tissues in various directions. On the cut surface these 

 bands mav give rise to a decidedly marbled appearance. Again, in 



