450 CONTAGIOUS PLEURO-PNEUMONIA 



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 a deposition of thrombi or plugs in the vessels, 

 which prevent the return of the blood. The blood pumped 

 into the lung tissue leaves the meshwork of capillaries around 

 the air vesicles, enters the latter and produces the firm hepa- 

 tized 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 different times and that the affected regions are in 

 •different stages of the disease. 



One of the most conspicuous features in a microscopical 

 ■section from a lung affected with acute contagious pleuro- 

 pneumonia is the presence of intensely stained foci and lines. 

 These lines, to which Welch seems to have been the first to 

 ■call attention, are visible to the naked eye and when viewed 

 with a hand lens suggest by their peculiar curves the contour 

 lines of a map. They are situated at the margin of and 

 within the inflamed connective tissue which surrounds the 

 large vessels and separates the lobules from one another. A 

 ■closer examination ot these lines indicates that they coincide 

 with the boundaries of the lobules and of the individual lymph 

 spaces of the interlobular tissue. Under a high power of the 

 microscope they are resolved into dense masses of leucocytes in 

 various stages of degeneration. These dense bands are pre- 

 sumably attracted to the connective tissue boundary of the 

 lobules and to the walls of lymph spaces within the connective 

 tissue by the unknown cause of the inflammation, presumably 

 the cause of the disease itself. The space between the lines is 

 filled with fibrin, in which very few leucocytes are found. 



§ 356. Differential diagnosis. It is extremely difficult 

 to form an exact diagnosis of the disease during its develop- 

 jnent, because the symptoms which are present are few in 



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