INFECTIOUS DISEASES OF CATTLE. 



371 



roughened, not smooth to the eye. Other areas equally firm may be 

 more grayish yellow in color and still others may be blackish. (PL 

 XXXIII.) Besides these areas which represent solidified (hepatized) 

 lung tissue there may be others which approach the normal lung tis- 

 sue in color and which are soft and float in water. From these a 

 milky purulent fluid may often be expressed. These different shades 

 are represented in Plate XXXII, fig. 2, within a small compass. Some 

 authorities are inclined to consider these variations in color on the 

 same cut surface as the so-called marbling of pleuro-pneumonia. It 

 matters not whether we regard the bands between the lobules or the 

 varying shades of the lobules themselves as the marbling, provided 

 either or both are peculiar to contagious pleuro-pneumonia. If we 

 examine the blood vessels appearing on such cut surface they will 

 usually be found plugged within the firmly hepatized regions. The 

 artery contains a dark, soft, removable clot, the vein a grayish pink, 

 granular, fragile plug (thrombus), which adheres firmly to the wall of 

 the vein, and if this be slit open indications of a diseased condition 

 of the inner coat will be readily detected. When large regions of the 

 lung tissue are hepatized the main air tube and its branches are 

 usually filled with grayish, cylindrical branched masses of fibrin easily 

 removed, as they do not adhere to the mucous membrane. 



The views of pathologists differ as to the nature of the earliest 

 changes in pleuro-pneumonia, and it is not within the scope of this 

 work to present imperfectly developed or controverted theories. In 

 the foregoing description we have taken as a type the acute pleuro- 

 pneumonia in its fully developed phase, which can scarcely be 

 mistaken for any other disease. We have seen that there is an inflam- 

 matory condition of the connective tissue between the lobules, result- 

 ing in the exudation of coagulable lymph. This inflammation is 

 equally marked around the blood vessels and air tubes. It leads to 

 inflammatory changes in the inner wall of the veins, and these cause 

 the deposition of thrombi or plugs in the vessels, which prevent the 

 return of the blood. The blood pumped into the lung tissue through 

 the artery, but unable to get out by way of the vein, leaves the mesh- 

 work of capillaries around the air vesicles, enters the latter, and pro- 

 duces 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 different parts of the lung tissue are plugged at dif- 

 ferent 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- 



