384 DISEASES OF CATTLE. 



laries around the air vesicles, enters the latter, and produces the firm 

 hepatized condition so characteristic of this disease. It will be easily 

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

 or yellowish red are produced if we bear in mind that the veins in dif- 

 ferent parts of the lung tissue are plugged at different times, and that, 

 therefore, the affected regions are in different stages of disease. 



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 lique- 

 iaction. 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. This may, however, involve over a 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 contagious 

 pleuro-pneumonia. In the typical acute cases there are 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 

 as yet imperfectly developed, or else so far advanced that doubts may 

 arise as to the true nature of the affection. In such cases all obtain- 

 able 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 

 study of the disease is fitted to decide in such cases. 



Other kinds of lung disease may be confounded with pleuro-pneu- 

 monia because of certain features common to most lung diseases of 

 cattle. The inflammation of the connective tissue between the lobules 

 is not infrequently observed in so-called interstitial pneumonia and may 

 lead to the formation of whitish bands intersecting the lung tissue in 

 various directions. On the cut surface these bands may give rise to a 

 decidedly " marbled " appearance. Again, in traumatic pneumonia, due 

 as its name implies to the entrance of foreign bodies into the lung 

 tissue, generally from the paunch, the connective tissue around the 

 place of disease becomes inflamed and thickened and the disease itself 



